[PATCH v5 0/8] Add SCMI Virtio & Clock atomic support

[PATCH v5 0/8] Add SCMI Virtio & Clock atomic support

[Cristian Marussi]

Hi,

This small series is the tail-subset of the previous V8 series about atomic
support in SCMI [1], whose 8-patches head-subset has now been queued on
[2]; as such, it is based on [2] on top of tag scmi-updates-5.17:

commit 94d0cd1da14a ("firmware: arm_scmi: Add new parameter to
		     mark_txdone")

Patch [1/8] substitute virtio-scmi ready flag and lock with a reference
counter to keep track of vio channels lifetime while removing the need of
a wide spinlocked section (that would cause issues with introduction of
virtio polling support)

Patch [2/8] adds a few helpers to handle the TX free_list and a dedicated
spinlock to reduce the reliance on the main one.

Patch [3/8] adds polling mode to SCMI VirtIO transport in order to support
atomic operations on such transport.

Patches [4,5/8] introduce a new optional SCMI binding, atomic-threshold-us,
to configure a platform specific time threshold used in the following
patches to select with a finer grain which SCMI resources should be
eligible for atomic operations when requested.

Patch [6/8] exposes new SCMI Clock protocol operations to allow an SCMI
user to request atomic mode on clock enable commands.

Patch [7/8] adds support to SCMI Clock protocol for a new clock attributes
field which advertises typical enable latency for a specific resource.

Finally patch [8/8] add support for atomic operations to the SCMI clock
driver; the underlying logic here is that we register with the Clock
framework atomic-capable clock resources if and only if the backing SCMI
transport is capable of atomic operations AND the specific clock resource
has been advertised by the SCMI platform as having:

	clock_enable_latency <= atomic-threshold-us

The idea is to avoid costly atomic busy-waiting for resources that have
been advertised as 'slow' to operate upon. (i.e. a PLL vs a gating clock)

To ease testing the whole series can be find at [3].

Any feedback/testing welcome as usual.

Thanks,
Cristian

[1]: https://lore.kernel.org/linux-arm-kernel/20211220195646.44498-1-cristian.marussi@arm.com/
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/sudeep.holla/linux.git/tag/?h=scmi-updates-5.17
[3]: https://gitlab.arm.com/linux-arm/linux-cm/-/commits/scmi_atomic_clk_virtio_V5/

---
v4 --> v5
- dt_bindings: fixed example and removed dtschema warnings/errors
- dt_bindings: added 'default: 0' clause
- introduced vio_msg refcounts and helpers to avoid premature reuse of
  freed messages when both poling and IRQ path are active on a buffer
- better handling of timed out polled messages on late replies using
  new VIO_MSG_POLL_TIMEOUT state
- fixed comments on locks
- removed unneeded virtqueue re-enable when fail to acquire channel in
  complete_cb

V3 --> V4
- renamed optional DT property to atomic-threshold-us

V2 --> V3
 - split out virtio_ring RFC patch into a distinct series
 - calling virtqueue_broke_device when cleaning up channel
 - removed RFC tags from CLK related patches

V1 --> V2
 - added vio channel refcount support patch
 - reviewed free_list support and usage
 - added virtio_ring RFC patch
 - shrinked spinlocked section within virtio_poll_done to exclude
   virtqueue_poll call
 - removed poll_lock
 - use vio channel refcount acquire/release logic when polling
 - using new free_list accessors
 - added new dedicated pending_lock to access pending_cmds_list
 - fixed a few comments

Cristian Marussi (8):
  firmware: arm_scmi: Add a virtio channel refcount
  firmware: arm_scmi: Review virtio free_list handling
  firmware: arm_scmi: Add atomic mode support to virtio transport
  dt-bindings: firmware: arm,scmi: Add atomic-threshold-us optional
    property
  firmware: arm_scmi: Support optional system wide atomic-threshold-us
  firmware: arm_scmi: Add atomic support to clock protocol
  firmware: arm_scmi: Add support for clock_enable_latency
  clk: scmi: Support atomic clock enable/disable API

 .../bindings/firmware/arm,scmi.yaml           |  10 +
 drivers/clk/clk-scmi.c                        |  71 ++-
 drivers/firmware/arm_scmi/Kconfig             |  15 +
 drivers/firmware/arm_scmi/clock.c             |  34 +-
 drivers/firmware/arm_scmi/driver.c            |  33 +-
 drivers/firmware/arm_scmi/virtio.c            | 591 +++++++++++++++---
 include/linux/scmi_protocol.h                 |   9 +-
 7 files changed, 655 insertions(+), 108 deletions(-)

-- 
2.17.1

[PATCH v5 1/8] firmware: arm_scmi: Add a virtio channel refcount

[Cristian Marussi]

Currently SCMI VirtIO channels are marked with a ready flag and related
lock to track channel lifetime and support proper synchronization at
shutdown when virtqueues have to be stopped.

This leads to some extended spinlocked sections with IRQs off on the RX
path to keep hold of the ready flag and does not scale well especially when
SCMI VirtIO polling mode will be introduced.

Add an SCMI VirtIO channel dedicated refcount to track active users on both
the TX and the RX path and properly enforce synchronization and cleanup at
shutdown, inhibiting further usage of the channel once freed.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
Cc: Peter Hilber <peter.hilber@opensynergy.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v4 --> v5
- removed unneeded virtqueue re-enable when fail to acquire channel in
  complete_cb
v2 --> v3
- Break virtio device at shutdown while cleaning up SCMI channel
---
 drivers/firmware/arm_scmi/virtio.c | 143 +++++++++++++++++++----------
 1 file changed, 92 insertions(+), 51 deletions(-)

diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
index fd0f6f91fc0b..ed00b072e981 100644
--- a/drivers/firmware/arm_scmi/virtio.c
+++ b/drivers/firmware/arm_scmi/virtio.c
@@ -17,7 +17,9 @@
  * virtqueue. Access to each virtqueue is protected by spinlocks.
  */
 
+#include <linux/completion.h>
 #include <linux/errno.h>
+#include <linux/refcount.h>
 #include <linux/slab.h>
 #include <linux/virtio.h>
 #include <linux/virtio_config.h>
@@ -27,6 +29,7 @@
 
 #include "common.h"
 
+#define VIRTIO_MAX_RX_TIMEOUT_MS	60000
 #define VIRTIO_SCMI_MAX_MSG_SIZE 128 /* Value may be increased. */
 #define VIRTIO_SCMI_MAX_PDU_SIZE \
 	(VIRTIO_SCMI_MAX_MSG_SIZE + SCMI_MSG_MAX_PROT_OVERHEAD)
@@ -39,23 +42,21 @@
  * @cinfo: SCMI Tx or Rx channel
  * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
  * @is_rx: Whether channel is an Rx channel
- * @ready: Whether transport user is ready to hear about channel
  * @max_msg: Maximum number of pending messages for this channel.
- * @lock: Protects access to all members except ready.
- * @ready_lock: Protects access to ready. If required, it must be taken before
- *              lock.
+ * @lock: Protects access to all members except users.
+ * @shutdown_done: A reference to a completion used when freeing this channel.
+ * @users: A reference count to currently active users of this channel.
  */
 struct scmi_vio_channel {
 	struct virtqueue *vqueue;
 	struct scmi_chan_info *cinfo;
 	struct list_head free_list;
 	bool is_rx;
-	bool ready;
 	unsigned int max_msg;
-	/* lock to protect access to all members except ready. */
+	/* lock to protect access to all members except users. */
 	spinlock_t lock;
-	/* lock to rotects access to ready flag. */
-	spinlock_t ready_lock;
+	struct completion *shutdown_done;
+	refcount_t users;
 };
 
 /**
@@ -76,6 +77,63 @@ struct scmi_vio_msg {
 /* Only one SCMI VirtIO device can possibly exist */
 static struct virtio_device *scmi_vdev;
 
+static void scmi_vio_channel_ready(struct scmi_vio_channel *vioch,
+				   struct scmi_chan_info *cinfo)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&vioch->lock, flags);
+	cinfo->transport_info = vioch;
+	/* Indirectly setting channel not available any more */
+	vioch->cinfo = cinfo;
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	refcount_set(&vioch->users, 1);
+}
+
+static inline bool scmi_vio_channel_acquire(struct scmi_vio_channel *vioch)
+{
+	return refcount_inc_not_zero(&vioch->users);
+}
+
+static inline void scmi_vio_channel_release(struct scmi_vio_channel *vioch)
+{
+	if (refcount_dec_and_test(&vioch->users)) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&vioch->lock, flags);
+		if (vioch->shutdown_done) {
+			vioch->cinfo = NULL;
+			complete(vioch->shutdown_done);
+		}
+		spin_unlock_irqrestore(&vioch->lock, flags);
+	}
+}
+
+static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
+{
+	unsigned long flags;
+	DECLARE_COMPLETION_ONSTACK(vioch_shutdown_done);
+
+	/*
+	 * Prepare to wait for the last release if not already released
+	 * or in progress.
+	 */
+	spin_lock_irqsave(&vioch->lock, flags);
+	if (!vioch->cinfo || vioch->shutdown_done) {
+		spin_unlock_irqrestore(&vioch->lock, flags);
+		return;
+	}
+	vioch->shutdown_done = &vioch_shutdown_done;
+	virtio_break_device(vioch->vqueue->vdev);
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	scmi_vio_channel_release(vioch);
+
+	/* Let any possibly concurrent RX path release the channel */
+	wait_for_completion(vioch->shutdown_done);
+}
+
 static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
 {
 	return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
@@ -119,7 +177,7 @@ static void scmi_finalize_message(struct scmi_vio_channel *vioch,
 
 static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 {
-	unsigned long ready_flags;
+	unsigned long flags;
 	unsigned int length;
 	struct scmi_vio_channel *vioch;
 	struct scmi_vio_msg *msg;
@@ -130,27 +188,24 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 	vioch = &((struct scmi_vio_channel *)vqueue->vdev->priv)[vqueue->index];
 
 	for (;;) {
-		spin_lock_irqsave(&vioch->ready_lock, ready_flags);
-
-		if (!vioch->ready) {
-			if (!cb_enabled)
-				(void)virtqueue_enable_cb(vqueue);
-			goto unlock_ready_out;
-		}
+		if (!scmi_vio_channel_acquire(vioch))
+			return;
 
-		/* IRQs already disabled here no need to irqsave */
-		spin_lock(&vioch->lock);
+		spin_lock_irqsave(&vioch->lock, flags);
 		if (cb_enabled) {
 			virtqueue_disable_cb(vqueue);
 			cb_enabled = false;
 		}
 		msg = virtqueue_get_buf(vqueue, &length);
 		if (!msg) {
-			if (virtqueue_enable_cb(vqueue))
-				goto unlock_out;
+			if (virtqueue_enable_cb(vqueue)) {
+				spin_unlock_irqrestore(&vioch->lock, flags);
+				scmi_vio_channel_release(vioch);
+				return;
+			}
 			cb_enabled = true;
 		}
-		spin_unlock(&vioch->lock);
+		spin_unlock_irqrestore(&vioch->lock, flags);
 
 		if (msg) {
 			msg->rx_len = length;
@@ -161,19 +216,14 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 		}
 
 		/*
-		 * Release ready_lock and re-enable IRQs between loop iterations
-		 * to allow virtio_chan_free() to possibly kick in and set the
-		 * flag vioch->ready to false even in between processing of
-		 * messages, so as to force outstanding messages to be ignored
-		 * when system is shutting down.
+		 * Release vio channel between loop iterations to allow
+		 * virtio_chan_free() to eventually fully release it when
+		 * shutting down; in such a case, any outstanding message will
+		 * be ignored since this loop will bail out at the next
+		 * iteration.
 		 */
-		spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
+		scmi_vio_channel_release(vioch);
 	}
-
-unlock_out:
-	spin_unlock(&vioch->lock);
-unlock_ready_out:
-	spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
 }
 
 static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
@@ -273,35 +323,20 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
 		}
 	}
 
-	spin_lock_irqsave(&vioch->lock, flags);
-	cinfo->transport_info = vioch;
-	/* Indirectly setting channel not available any more */
-	vioch->cinfo = cinfo;
-	spin_unlock_irqrestore(&vioch->lock, flags);
-
-	spin_lock_irqsave(&vioch->ready_lock, flags);
-	vioch->ready = true;
-	spin_unlock_irqrestore(&vioch->ready_lock, flags);
+	scmi_vio_channel_ready(vioch, cinfo);
 
 	return 0;
 }
 
 static int virtio_chan_free(int id, void *p, void *data)
 {
-	unsigned long flags;
 	struct scmi_chan_info *cinfo = p;
 	struct scmi_vio_channel *vioch = cinfo->transport_info;
 
-	spin_lock_irqsave(&vioch->ready_lock, flags);
-	vioch->ready = false;
-	spin_unlock_irqrestore(&vioch->ready_lock, flags);
+	scmi_vio_channel_cleanup_sync(vioch);
 
 	scmi_free_channel(cinfo, data, id);
 
-	spin_lock_irqsave(&vioch->lock, flags);
-	vioch->cinfo = NULL;
-	spin_unlock_irqrestore(&vioch->lock, flags);
-
 	return 0;
 }
 
@@ -316,10 +351,14 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
 	int rc;
 	struct scmi_vio_msg *msg;
 
+	if (!scmi_vio_channel_acquire(vioch))
+		return -EINVAL;
+
 	spin_lock_irqsave(&vioch->lock, flags);
 
 	if (list_empty(&vioch->free_list)) {
 		spin_unlock_irqrestore(&vioch->lock, flags);
+		scmi_vio_channel_release(vioch);
 		return -EBUSY;
 	}
 
@@ -342,6 +381,8 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
 
 	spin_unlock_irqrestore(&vioch->lock, flags);
 
+	scmi_vio_channel_release(vioch);
+
 	return rc;
 }
 
@@ -416,7 +457,6 @@ static int scmi_vio_probe(struct virtio_device *vdev)
 		unsigned int sz;
 
 		spin_lock_init(&channels[i].lock);
-		spin_lock_init(&channels[i].ready_lock);
 		INIT_LIST_HEAD(&channels[i].free_list);
 		channels[i].vqueue = vqs[i];
 
@@ -503,7 +543,8 @@ const struct scmi_desc scmi_virtio_desc = {
 	.transport_init = virtio_scmi_init,
 	.transport_exit = virtio_scmi_exit,
 	.ops = &scmi_virtio_ops,
-	.max_rx_timeout_ms = 60000, /* for non-realtime virtio devices */
+	/* for non-realtime virtio devices */
+	.max_rx_timeout_ms = VIRTIO_MAX_RX_TIMEOUT_MS,
 	.max_msg = 0, /* overridden by virtio_get_max_msg() */
 	.max_msg_size = VIRTIO_SCMI_MAX_MSG_SIZE,
 };
-- 
2.17.1

Re: [PATCH v5 1/8] firmware: arm_scmi: Add a virtio channel refcount

[Peter Hilber]

On 17.02.22 14:12, Cristian Marussi wrote:
> Currently SCMI VirtIO channels are marked with a ready flag and related
> lock to track channel lifetime and support proper synchronization at
> shutdown when virtqueues have to be stopped.
> 
> This leads to some extended spinlocked sections with IRQs off on the RX
> path to keep hold of the ready flag and does not scale well especially when
> SCMI VirtIO polling mode will be introduced.
> 
> Add an SCMI VirtIO channel dedicated refcount to track active users on both
> the TX and the RX path and properly enforce synchronization and cleanup at
> shutdown, inhibiting further usage of the channel once freed.
> 
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
> Cc: Peter Hilber <peter.hilber@opensynergy.com>
> Cc: virtualization@lists.linux-foundation.org
> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>

Acked-by: Peter Hilber <peter.hilber@opensynergy.com>

> ---
> v4 --> v5
> - removed unneeded virtqueue re-enable when fail to acquire channel in
>   complete_cb
> v2 --> v3
> - Break virtio device at shutdown while cleaning up SCMI channel
> ---
>  drivers/firmware/arm_scmi/virtio.c | 143 +++++++++++++++++++----------
>  1 file changed, 92 insertions(+), 51 deletions(-)
> 
> diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
> index fd0f6f91fc0b..ed00b072e981 100644
> --- a/drivers/firmware/arm_scmi/virtio.c
> +++ b/drivers/firmware/arm_scmi/virtio.c
> @@ -17,7 +17,9 @@
>   * virtqueue. Access to each virtqueue is protected by spinlocks.
>   */
>  
> +#include <linux/completion.h>
>  #include <linux/errno.h>
> +#include <linux/refcount.h>
>  #include <linux/slab.h>
>  #include <linux/virtio.h>
>  #include <linux/virtio_config.h>
> @@ -27,6 +29,7 @@
>  
>  #include "common.h"
>  
> +#define VIRTIO_MAX_RX_TIMEOUT_MS	60000
>  #define VIRTIO_SCMI_MAX_MSG_SIZE 128 /* Value may be increased. */
>  #define VIRTIO_SCMI_MAX_PDU_SIZE \
>  	(VIRTIO_SCMI_MAX_MSG_SIZE + SCMI_MSG_MAX_PROT_OVERHEAD)
> @@ -39,23 +42,21 @@
>   * @cinfo: SCMI Tx or Rx channel
>   * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
>   * @is_rx: Whether channel is an Rx channel
> - * @ready: Whether transport user is ready to hear about channel
>   * @max_msg: Maximum number of pending messages for this channel.
> - * @lock: Protects access to all members except ready.
> - * @ready_lock: Protects access to ready. If required, it must be taken before
> - *              lock.
> + * @lock: Protects access to all members except users.
> + * @shutdown_done: A reference to a completion used when freeing this channel.
> + * @users: A reference count to currently active users of this channel.
>   */
>  struct scmi_vio_channel {
>  	struct virtqueue *vqueue;
>  	struct scmi_chan_info *cinfo;
>  	struct list_head free_list;
>  	bool is_rx;
> -	bool ready;
>  	unsigned int max_msg;
> -	/* lock to protect access to all members except ready. */
> +	/* lock to protect access to all members except users. */
>  	spinlock_t lock;
> -	/* lock to rotects access to ready flag. */
> -	spinlock_t ready_lock;
> +	struct completion *shutdown_done;
> +	refcount_t users;
>  };
>  
>  /**
> @@ -76,6 +77,63 @@ struct scmi_vio_msg {
>  /* Only one SCMI VirtIO device can possibly exist */
>  static struct virtio_device *scmi_vdev;
>  
> +static void scmi_vio_channel_ready(struct scmi_vio_channel *vioch,
> +				   struct scmi_chan_info *cinfo)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&vioch->lock, flags);
> +	cinfo->transport_info = vioch;
> +	/* Indirectly setting channel not available any more */
> +	vioch->cinfo = cinfo;
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	refcount_set(&vioch->users, 1);
> +}
> +
> +static inline bool scmi_vio_channel_acquire(struct scmi_vio_channel *vioch)
> +{
> +	return refcount_inc_not_zero(&vioch->users);
> +}
> +
> +static inline void scmi_vio_channel_release(struct scmi_vio_channel *vioch)
> +{
> +	if (refcount_dec_and_test(&vioch->users)) {
> +		unsigned long flags;
> +
> +		spin_lock_irqsave(&vioch->lock, flags);
> +		if (vioch->shutdown_done) {
> +			vioch->cinfo = NULL;
> +			complete(vioch->shutdown_done);
> +		}
> +		spin_unlock_irqrestore(&vioch->lock, flags);
> +	}
> +}
> +
> +static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
> +{
> +	unsigned long flags;
> +	DECLARE_COMPLETION_ONSTACK(vioch_shutdown_done);
> +
> +	/*
> +	 * Prepare to wait for the last release if not already released
> +	 * or in progress.
> +	 */
> +	spin_lock_irqsave(&vioch->lock, flags);
> +	if (!vioch->cinfo || vioch->shutdown_done) {
> +		spin_unlock_irqrestore(&vioch->lock, flags);
> +		return;
> +	}
> +	vioch->shutdown_done = &vioch_shutdown_done;
> +	virtio_break_device(vioch->vqueue->vdev);
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	scmi_vio_channel_release(vioch);
> +
> +	/* Let any possibly concurrent RX path release the channel */
> +	wait_for_completion(vioch->shutdown_done);
> +}
> +
>  static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
>  {
>  	return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
> @@ -119,7 +177,7 @@ static void scmi_finalize_message(struct scmi_vio_channel *vioch,
>  
>  static void scmi_vio_complete_cb(struct virtqueue *vqueue)
>  {
> -	unsigned long ready_flags;
> +	unsigned long flags;
>  	unsigned int length;
>  	struct scmi_vio_channel *vioch;
>  	struct scmi_vio_msg *msg;
> @@ -130,27 +188,24 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
>  	vioch = &((struct scmi_vio_channel *)vqueue->vdev->priv)[vqueue->index];
>  
>  	for (;;) {
> -		spin_lock_irqsave(&vioch->ready_lock, ready_flags);
> -
> -		if (!vioch->ready) {
> -			if (!cb_enabled)
> -				(void)virtqueue_enable_cb(vqueue);
> -			goto unlock_ready_out;
> -		}
> +		if (!scmi_vio_channel_acquire(vioch))
> +			return;
>  
> -		/* IRQs already disabled here no need to irqsave */
> -		spin_lock(&vioch->lock);
> +		spin_lock_irqsave(&vioch->lock, flags);
>  		if (cb_enabled) {
>  			virtqueue_disable_cb(vqueue);
>  			cb_enabled = false;
>  		}
>  		msg = virtqueue_get_buf(vqueue, &length);
>  		if (!msg) {
> -			if (virtqueue_enable_cb(vqueue))
> -				goto unlock_out;
> +			if (virtqueue_enable_cb(vqueue)) {
> +				spin_unlock_irqrestore(&vioch->lock, flags);
> +				scmi_vio_channel_release(vioch);
> +				return;
> +			}
>  			cb_enabled = true;
>  		}
> -		spin_unlock(&vioch->lock);
> +		spin_unlock_irqrestore(&vioch->lock, flags);
>  
>  		if (msg) {
>  			msg->rx_len = length;
> @@ -161,19 +216,14 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
>  		}
>  
>  		/*
> -		 * Release ready_lock and re-enable IRQs between loop iterations
> -		 * to allow virtio_chan_free() to possibly kick in and set the
> -		 * flag vioch->ready to false even in between processing of
> -		 * messages, so as to force outstanding messages to be ignored
> -		 * when system is shutting down.
> +		 * Release vio channel between loop iterations to allow
> +		 * virtio_chan_free() to eventually fully release it when
> +		 * shutting down; in such a case, any outstanding message will
> +		 * be ignored since this loop will bail out at the next
> +		 * iteration.
>  		 */
> -		spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
> +		scmi_vio_channel_release(vioch);
>  	}
> -
> -unlock_out:
> -	spin_unlock(&vioch->lock);
> -unlock_ready_out:
> -	spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
>  }
>  
>  static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
> @@ -273,35 +323,20 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
>  		}
>  	}
>  
> -	spin_lock_irqsave(&vioch->lock, flags);
> -	cinfo->transport_info = vioch;
> -	/* Indirectly setting channel not available any more */
> -	vioch->cinfo = cinfo;
> -	spin_unlock_irqrestore(&vioch->lock, flags);
> -
> -	spin_lock_irqsave(&vioch->ready_lock, flags);
> -	vioch->ready = true;
> -	spin_unlock_irqrestore(&vioch->ready_lock, flags);
> +	scmi_vio_channel_ready(vioch, cinfo);
>  
>  	return 0;
>  }
>  
>  static int virtio_chan_free(int id, void *p, void *data)
>  {
> -	unsigned long flags;
>  	struct scmi_chan_info *cinfo = p;
>  	struct scmi_vio_channel *vioch = cinfo->transport_info;
>  
> -	spin_lock_irqsave(&vioch->ready_lock, flags);
> -	vioch->ready = false;
> -	spin_unlock_irqrestore(&vioch->ready_lock, flags);
> +	scmi_vio_channel_cleanup_sync(vioch);
>  
>  	scmi_free_channel(cinfo, data, id);
>  
> -	spin_lock_irqsave(&vioch->lock, flags);
> -	vioch->cinfo = NULL;
> -	spin_unlock_irqrestore(&vioch->lock, flags);
> -
>  	return 0;
>  }
>  
> @@ -316,10 +351,14 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
>  	int rc;
>  	struct scmi_vio_msg *msg;
>  
> +	if (!scmi_vio_channel_acquire(vioch))
> +		return -EINVAL;
> +
>  	spin_lock_irqsave(&vioch->lock, flags);
>  
>  	if (list_empty(&vioch->free_list)) {
>  		spin_unlock_irqrestore(&vioch->lock, flags);
> +		scmi_vio_channel_release(vioch);
>  		return -EBUSY;
>  	}
>  
> @@ -342,6 +381,8 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
>  
>  	spin_unlock_irqrestore(&vioch->lock, flags);
>  
> +	scmi_vio_channel_release(vioch);
> +
>  	return rc;
>  }
>  
> @@ -416,7 +457,6 @@ static int scmi_vio_probe(struct virtio_device *vdev)
>  		unsigned int sz;
>  
>  		spin_lock_init(&channels[i].lock);
> -		spin_lock_init(&channels[i].ready_lock);
>  		INIT_LIST_HEAD(&channels[i].free_list);
>  		channels[i].vqueue = vqs[i];
>  
> @@ -503,7 +543,8 @@ const struct scmi_desc scmi_virtio_desc = {
>  	.transport_init = virtio_scmi_init,
>  	.transport_exit = virtio_scmi_exit,
>  	.ops = &scmi_virtio_ops,
> -	.max_rx_timeout_ms = 60000, /* for non-realtime virtio devices */
> +	/* for non-realtime virtio devices */
> +	.max_rx_timeout_ms = VIRTIO_MAX_RX_TIMEOUT_MS,
>  	.max_msg = 0, /* overridden by virtio_get_max_msg() */
>  	.max_msg_size = VIRTIO_SCMI_MAX_MSG_SIZE,
>  };

[PATCH v5 2/8] firmware: arm_scmi: Review virtio free_list handling

[Cristian Marussi]

Add a new spinlock dedicated to the access of the TX free list and a couple
of helpers to get and put messages back and forth from the free_list.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
Cc: Peter Hilber <peter.hilber@opensynergy.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
 drivers/firmware/arm_scmi/virtio.c | 88 +++++++++++++++++++-----------
 1 file changed, 57 insertions(+), 31 deletions(-)

diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
index ed00b072e981..7ec4085cee7e 100644
--- a/drivers/firmware/arm_scmi/virtio.c
+++ b/drivers/firmware/arm_scmi/virtio.c
@@ -40,20 +40,23 @@
  *
  * @vqueue: Associated virtqueue
  * @cinfo: SCMI Tx or Rx channel
+ * @free_lock: Protects access to the @free_list.
  * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
  * @is_rx: Whether channel is an Rx channel
  * @max_msg: Maximum number of pending messages for this channel.
- * @lock: Protects access to all members except users.
+ * @lock: Protects access to all members except users, free_list.
  * @shutdown_done: A reference to a completion used when freeing this channel.
  * @users: A reference count to currently active users of this channel.
  */
 struct scmi_vio_channel {
 	struct virtqueue *vqueue;
 	struct scmi_chan_info *cinfo;
+	/* lock to protect access to the free list. */
+	spinlock_t free_lock;
 	struct list_head free_list;
 	bool is_rx;
 	unsigned int max_msg;
-	/* lock to protect access to all members except users. */
+	/* lock to protect access to all members except users, free_list  */
 	spinlock_t lock;
 	struct completion *shutdown_done;
 	refcount_t users;
@@ -134,18 +137,49 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
 	wait_for_completion(vioch->shutdown_done);
 }
 
+/* Assumes to be called with vio channel acquired already */
+static struct scmi_vio_msg *
+scmi_virtio_get_free_msg(struct scmi_vio_channel *vioch)
+{
+	unsigned long flags;
+	struct scmi_vio_msg *msg;
+
+	spin_lock_irqsave(&vioch->free_lock, flags);
+	if (list_empty(&vioch->free_list)) {
+		spin_unlock_irqrestore(&vioch->free_lock, flags);
+		return NULL;
+	}
+
+	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
+	list_del_init(&msg->list);
+	spin_unlock_irqrestore(&vioch->free_lock, flags);
+
+	return msg;
+}
+
+/* Assumes to be called with vio channel acquired already */
+static void scmi_virtio_put_free_msg(struct scmi_vio_channel *vioch,
+				     struct scmi_vio_msg *msg)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&vioch->free_lock, flags);
+	list_add_tail(&msg->list, &vioch->free_list);
+	spin_unlock_irqrestore(&vioch->free_lock, flags);
+}
+
 static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
 {
 	return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
 }
 
 static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
-			       struct scmi_vio_msg *msg,
-			       struct device *dev)
+			       struct scmi_vio_msg *msg)
 {
 	struct scatterlist sg_in;
 	int rc;
 	unsigned long flags;
+	struct device *dev = &vioch->vqueue->vdev->dev;
 
 	sg_init_one(&sg_in, msg->input, VIRTIO_SCMI_MAX_PDU_SIZE);
 
@@ -162,17 +196,17 @@ static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
 	return rc;
 }
 
+/*
+ * Assume to be called with channel already acquired or not ready at all;
+ * vioch->lock MUST NOT have been already acquired.
+ */
 static void scmi_finalize_message(struct scmi_vio_channel *vioch,
 				  struct scmi_vio_msg *msg)
 {
-	if (vioch->is_rx) {
-		scmi_vio_feed_vq_rx(vioch, msg, vioch->cinfo->dev);
-	} else {
-		/* Here IRQs are assumed to be already disabled by the caller */
-		spin_lock(&vioch->lock);
-		list_add(&msg->list, &vioch->free_list);
-		spin_unlock(&vioch->lock);
-	}
+	if (vioch->is_rx)
+		scmi_vio_feed_vq_rx(vioch, msg);
+	else
+		scmi_virtio_put_free_msg(vioch, msg);
 }
 
 static void scmi_vio_complete_cb(struct virtqueue *vqueue)
@@ -284,7 +318,6 @@ static bool virtio_chan_available(struct device *dev, int idx)
 static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
 			     bool tx)
 {
-	unsigned long flags;
 	struct scmi_vio_channel *vioch;
 	int index = tx ? VIRTIO_SCMI_VQ_TX : VIRTIO_SCMI_VQ_RX;
 	int i;
@@ -314,13 +347,7 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
 		if (!msg->input)
 			return -ENOMEM;
 
-		if (tx) {
-			spin_lock_irqsave(&vioch->lock, flags);
-			list_add_tail(&msg->list, &vioch->free_list);
-			spin_unlock_irqrestore(&vioch->lock, flags);
-		} else {
-			scmi_vio_feed_vq_rx(vioch, msg, cinfo->dev);
-		}
+		scmi_finalize_message(vioch, msg);
 	}
 
 	scmi_vio_channel_ready(vioch, cinfo);
@@ -354,33 +381,31 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
 	if (!scmi_vio_channel_acquire(vioch))
 		return -EINVAL;
 
-	spin_lock_irqsave(&vioch->lock, flags);
-
-	if (list_empty(&vioch->free_list)) {
-		spin_unlock_irqrestore(&vioch->lock, flags);
+	msg = scmi_virtio_get_free_msg(vioch);
+	if (!msg) {
 		scmi_vio_channel_release(vioch);
 		return -EBUSY;
 	}
 
-	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
-	list_del(&msg->list);
-
 	msg_tx_prepare(msg->request, xfer);
 
 	sg_init_one(&sg_out, msg->request, msg_command_size(xfer));
 	sg_init_one(&sg_in, msg->input, msg_response_size(xfer));
 
+	spin_lock_irqsave(&vioch->lock, flags);
+
 	rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, msg, GFP_ATOMIC);
-	if (rc) {
-		list_add(&msg->list, &vioch->free_list);
+	if (rc)
 		dev_err(vioch->cinfo->dev,
 			"failed to add to TX virtqueue (%d)\n", rc);
-	} else {
+	else
 		virtqueue_kick(vioch->vqueue);
-	}
 
 	spin_unlock_irqrestore(&vioch->lock, flags);
 
+	if (rc)
+		scmi_virtio_put_free_msg(vioch, msg);
+
 	scmi_vio_channel_release(vioch);
 
 	return rc;
@@ -457,6 +482,7 @@ static int scmi_vio_probe(struct virtio_device *vdev)
 		unsigned int sz;
 
 		spin_lock_init(&channels[i].lock);
+		spin_lock_init(&channels[i].free_lock);
 		INIT_LIST_HEAD(&channels[i].free_list);
 		channels[i].vqueue = vqs[i];
 
-- 
2.17.1

Re: [PATCH v5 2/8] firmware: arm_scmi: Review virtio free_list handling

[Peter Hilber]

On 17.02.22 14:12, Cristian Marussi wrote:
> Add a new spinlock dedicated to the access of the TX free list and a couple
> of helpers to get and put messages back and forth from the free_list.
> 
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
> Cc: Peter Hilber <peter.hilber@opensynergy.com>
> Cc: virtualization@lists.linux-foundation.org
> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>

Acked-by: Peter Hilber <peter.hilber@opensynergy.com>

> ---
>  drivers/firmware/arm_scmi/virtio.c | 88 +++++++++++++++++++-----------
>  1 file changed, 57 insertions(+), 31 deletions(-)
> 
> diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
> index ed00b072e981..7ec4085cee7e 100644
> --- a/drivers/firmware/arm_scmi/virtio.c
> +++ b/drivers/firmware/arm_scmi/virtio.c
> @@ -40,20 +40,23 @@
>   *
>   * @vqueue: Associated virtqueue
>   * @cinfo: SCMI Tx or Rx channel
> + * @free_lock: Protects access to the @free_list.
>   * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
>   * @is_rx: Whether channel is an Rx channel
>   * @max_msg: Maximum number of pending messages for this channel.
> - * @lock: Protects access to all members except users.
> + * @lock: Protects access to all members except users, free_list.
>   * @shutdown_done: A reference to a completion used when freeing this channel.
>   * @users: A reference count to currently active users of this channel.
>   */
>  struct scmi_vio_channel {
>  	struct virtqueue *vqueue;
>  	struct scmi_chan_info *cinfo;
> +	/* lock to protect access to the free list. */
> +	spinlock_t free_lock;
>  	struct list_head free_list;
>  	bool is_rx;
>  	unsigned int max_msg;
> -	/* lock to protect access to all members except users. */
> +	/* lock to protect access to all members except users, free_list  */
>  	spinlock_t lock;
>  	struct completion *shutdown_done;
>  	refcount_t users;
> @@ -134,18 +137,49 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
>  	wait_for_completion(vioch->shutdown_done);
>  }
>  
> +/* Assumes to be called with vio channel acquired already */
> +static struct scmi_vio_msg *
> +scmi_virtio_get_free_msg(struct scmi_vio_channel *vioch)
> +{
> +	unsigned long flags;
> +	struct scmi_vio_msg *msg;
> +
> +	spin_lock_irqsave(&vioch->free_lock, flags);
> +	if (list_empty(&vioch->free_list)) {
> +		spin_unlock_irqrestore(&vioch->free_lock, flags);
> +		return NULL;
> +	}
> +
> +	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
> +	list_del_init(&msg->list);
> +	spin_unlock_irqrestore(&vioch->free_lock, flags);
> +
> +	return msg;
> +}
> +
> +/* Assumes to be called with vio channel acquired already */
> +static void scmi_virtio_put_free_msg(struct scmi_vio_channel *vioch,
> +				     struct scmi_vio_msg *msg)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&vioch->free_lock, flags);
> +	list_add_tail(&msg->list, &vioch->free_list);
> +	spin_unlock_irqrestore(&vioch->free_lock, flags);
> +}
> +
>  static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
>  {
>  	return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
>  }
>  
>  static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
> -			       struct scmi_vio_msg *msg,
> -			       struct device *dev)
> +			       struct scmi_vio_msg *msg)
>  {
>  	struct scatterlist sg_in;
>  	int rc;
>  	unsigned long flags;
> +	struct device *dev = &vioch->vqueue->vdev->dev;
>  
>  	sg_init_one(&sg_in, msg->input, VIRTIO_SCMI_MAX_PDU_SIZE);
>  
> @@ -162,17 +196,17 @@ static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
>  	return rc;
>  }
>  
> +/*
> + * Assume to be called with channel already acquired or not ready at all;
> + * vioch->lock MUST NOT have been already acquired.
> + */
>  static void scmi_finalize_message(struct scmi_vio_channel *vioch,
>  				  struct scmi_vio_msg *msg)
>  {
> -	if (vioch->is_rx) {
> -		scmi_vio_feed_vq_rx(vioch, msg, vioch->cinfo->dev);
> -	} else {
> -		/* Here IRQs are assumed to be already disabled by the caller */
> -		spin_lock(&vioch->lock);
> -		list_add(&msg->list, &vioch->free_list);
> -		spin_unlock(&vioch->lock);
> -	}
> +	if (vioch->is_rx)
> +		scmi_vio_feed_vq_rx(vioch, msg);
> +	else
> +		scmi_virtio_put_free_msg(vioch, msg);
>  }
>  
>  static void scmi_vio_complete_cb(struct virtqueue *vqueue)
> @@ -284,7 +318,6 @@ static bool virtio_chan_available(struct device *dev, int idx)
>  static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
>  			     bool tx)
>  {
> -	unsigned long flags;
>  	struct scmi_vio_channel *vioch;
>  	int index = tx ? VIRTIO_SCMI_VQ_TX : VIRTIO_SCMI_VQ_RX;
>  	int i;
> @@ -314,13 +347,7 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
>  		if (!msg->input)
>  			return -ENOMEM;
>  
> -		if (tx) {
> -			spin_lock_irqsave(&vioch->lock, flags);
> -			list_add_tail(&msg->list, &vioch->free_list);
> -			spin_unlock_irqrestore(&vioch->lock, flags);
> -		} else {
> -			scmi_vio_feed_vq_rx(vioch, msg, cinfo->dev);
> -		}
> +		scmi_finalize_message(vioch, msg);
>  	}
>  
>  	scmi_vio_channel_ready(vioch, cinfo);
> @@ -354,33 +381,31 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
>  	if (!scmi_vio_channel_acquire(vioch))
>  		return -EINVAL;
>  
> -	spin_lock_irqsave(&vioch->lock, flags);
> -
> -	if (list_empty(&vioch->free_list)) {
> -		spin_unlock_irqrestore(&vioch->lock, flags);
> +	msg = scmi_virtio_get_free_msg(vioch);
> +	if (!msg) {
>  		scmi_vio_channel_release(vioch);
>  		return -EBUSY;
>  	}
>  
> -	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
> -	list_del(&msg->list);
> -
>  	msg_tx_prepare(msg->request, xfer);
>  
>  	sg_init_one(&sg_out, msg->request, msg_command_size(xfer));
>  	sg_init_one(&sg_in, msg->input, msg_response_size(xfer));
>  
> +	spin_lock_irqsave(&vioch->lock, flags);
> +
>  	rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, msg, GFP_ATOMIC);
> -	if (rc) {
> -		list_add(&msg->list, &vioch->free_list);
> +	if (rc)
>  		dev_err(vioch->cinfo->dev,
>  			"failed to add to TX virtqueue (%d)\n", rc);
> -	} else {
> +	else
>  		virtqueue_kick(vioch->vqueue);
> -	}
>  
>  	spin_unlock_irqrestore(&vioch->lock, flags);
>  
> +	if (rc)
> +		scmi_virtio_put_free_msg(vioch, msg);
> +
>  	scmi_vio_channel_release(vioch);
>  
>  	return rc;
> @@ -457,6 +482,7 @@ static int scmi_vio_probe(struct virtio_device *vdev)
>  		unsigned int sz;
>  
>  		spin_lock_init(&channels[i].lock);
> +		spin_lock_init(&channels[i].free_lock);
>  		INIT_LIST_HEAD(&channels[i].free_list);
>  		channels[i].vqueue = vqs[i];
>  

[PATCH v5 3/8] firmware: arm_scmi: Add atomic mode support to virtio transport

[Cristian Marussi]

Add support for .mark_txdone and .poll_done transport operations to SCMI
VirtIO transport as pre-requisites to enable atomic operations.

Add a Kernel configuration option to enable SCMI VirtIO transport polling
and atomic mode for selected SCMI transactions while leaving it default
disabled.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
Cc: Peter Hilber <peter.hilber@opensynergy.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v4 --> v5
- introduced vio_msg refcounts and helpers to avoid premature reuse of
  freed messages when both poling and IRQ path are active on a buffer
- better handling of timed out polled messages on late replies using new
  VIO_MSG_POLL_TIMEOUT state
- fixed comments on locks

v1 --> v2
- shrinked spinlocked section within virtio_poll_done to exclude
  virtqueue_poll
- removed poll_lock
- use vio channel refcount acquire/release logic when polling
- using new free_list accessors
- added new dedicated pending_lock to access pending_cmds_list
- fixed a few comments

v0 --> v1
- check for deferred_wq existence before queueing work to avoid
  race at driver removal time
- changed mark_txdone decision-logic about message release
- fixed race while checking for msg polled from another thread
- using dedicated poll_status instead of poll_idx upper bits
- pick initial poll_idx earlier inside send_message to avoid missing
  early replies
- removed F_NOTIFY mention in comment
- clearing xfer->priv on the IRQ tx path once message has been fetched
- added some store barriers
- updated some comments
---
 drivers/firmware/arm_scmi/Kconfig  |  15 ++
 drivers/firmware/arm_scmi/driver.c |   6 +-
 drivers/firmware/arm_scmi/virtio.c | 390 +++++++++++++++++++++++++++--
 3 files changed, 391 insertions(+), 20 deletions(-)

diff --git a/drivers/firmware/arm_scmi/Kconfig b/drivers/firmware/arm_scmi/Kconfig
index d429326433d1..7794bd41eaa0 100644
--- a/drivers/firmware/arm_scmi/Kconfig
+++ b/drivers/firmware/arm_scmi/Kconfig
@@ -118,6 +118,21 @@ config ARM_SCMI_TRANSPORT_VIRTIO_VERSION1_COMPLIANCE
 	  the ones implemented by kvmtool) and let the core Kernel VirtIO layer
 	  take care of the needed conversions, say N.
 
+config ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+	bool "Enable atomic mode for SCMI VirtIO transport"
+	depends on ARM_SCMI_TRANSPORT_VIRTIO
+	help
+	  Enable support of atomic operation for SCMI VirtIO based transport.
+
+	  If you want the SCMI VirtIO based transport to operate in atomic
+	  mode, avoiding any kind of sleeping behaviour for selected
+	  transactions on the TX path, answer Y.
+
+	  Enabling atomic mode operations allows any SCMI driver using this
+	  transport to optionally ask for atomic SCMI transactions and operate
+	  in atomic context too, at the price of using a number of busy-waiting
+	  primitives all over instead. If unsure say N.
+
 endif #ARM_SCMI_PROTOCOL
 
 config ARM_SCMI_POWER_DOMAIN
diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index c2e7897ff56e..4fd5a35ffa2f 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -648,7 +648,8 @@ static void scmi_handle_notification(struct scmi_chan_info *cinfo,
 
 	unpack_scmi_header(msg_hdr, &xfer->hdr);
 	if (priv)
-		xfer->priv = priv;
+		/* Ensure order between xfer->priv store and following ops */
+		smp_store_mb(xfer->priv, priv);
 	info->desc->ops->fetch_notification(cinfo, info->desc->max_msg_size,
 					    xfer);
 	scmi_notify(cinfo->handle, xfer->hdr.protocol_id,
@@ -680,7 +681,8 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
 		xfer->rx.len = info->desc->max_msg_size;
 
 	if (priv)
-		xfer->priv = priv;
+		/* Ensure order between xfer->priv store and following ops */
+		smp_store_mb(xfer->priv, priv);
 	info->desc->ops->fetch_response(cinfo, xfer);
 
 	trace_scmi_rx_done(xfer->transfer_id, xfer->hdr.id,
diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
index 7ec4085cee7e..f69442e97389 100644
--- a/drivers/firmware/arm_scmi/virtio.c
+++ b/drivers/firmware/arm_scmi/virtio.c
@@ -3,8 +3,8 @@
  * Virtio Transport driver for Arm System Control and Management Interface
  * (SCMI).
  *
- * Copyright (C) 2020-2021 OpenSynergy.
- * Copyright (C) 2021 ARM Ltd.
+ * Copyright (C) 2020-2022 OpenSynergy.
+ * Copyright (C) 2021-2022 ARM Ltd.
  */
 
 /**
@@ -42,9 +42,14 @@
  * @cinfo: SCMI Tx or Rx channel
  * @free_lock: Protects access to the @free_list.
  * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
+ * @deferred_tx_work: Worker for TX deferred replies processing
+ * @deferred_tx_wq: Workqueue for TX deferred replies
+ * @pending_lock: Protects access to the @pending_cmds_list.
+ * @pending_cmds_list: List of pre-fetched commands queueud for later processing
  * @is_rx: Whether channel is an Rx channel
  * @max_msg: Maximum number of pending messages for this channel.
- * @lock: Protects access to all members except users, free_list.
+ * @lock: Protects access to all members except users, free_list and
+ *	  pending_cmds_list.
  * @shutdown_done: A reference to a completion used when freeing this channel.
  * @users: A reference count to currently active users of this channel.
  */
@@ -54,14 +59,29 @@ struct scmi_vio_channel {
 	/* lock to protect access to the free list. */
 	spinlock_t free_lock;
 	struct list_head free_list;
+	/* lock to protect access to the pending list. */
+	spinlock_t pending_lock;
+	struct list_head pending_cmds_list;
+	struct work_struct deferred_tx_work;
+	struct workqueue_struct *deferred_tx_wq;
 	bool is_rx;
 	unsigned int max_msg;
-	/* lock to protect access to all members except users, free_list  */
+	/*
+	 * Lock to protect access to all members except users, free_list and
+	 * pending_cmds_list
+	 */
 	spinlock_t lock;
 	struct completion *shutdown_done;
 	refcount_t users;
 };
 
+enum poll_states {
+	VIO_MSG_NOT_POLLED,
+	VIO_MSG_POLL_TIMEOUT,
+	VIO_MSG_POLLING,
+	VIO_MSG_POLL_DONE,
+};
+
 /**
  * struct scmi_vio_msg - Transport PDU information
  *
@@ -69,12 +89,23 @@ struct scmi_vio_channel {
  * @input: SDU used for (delayed) responses and notifications
  * @list: List which scmi_vio_msg may be part of
  * @rx_len: Input SDU size in bytes, once input has been received
+ * @poll_idx: Last used index registered for polling purposes if this message
+ *	      transaction reply was configured for polling.
+ * @poll_status: Polling state for this message.
+ * @poll_lock: A lock to protect @poll_status
+ * @users: A reference count to track this message users and avoid premature
+ *	   freeing (and reuse) when polling and IRQ execution paths interleave.
  */
 struct scmi_vio_msg {
 	struct scmi_msg_payld *request;
 	struct scmi_msg_payld *input;
 	struct list_head list;
 	unsigned int rx_len;
+	unsigned int poll_idx;
+	enum poll_states poll_status;
+	/* Lock to protect access to poll_status */
+	spinlock_t poll_lock;
+	refcount_t users;
 };
 
 /* Only one SCMI VirtIO device can possibly exist */
@@ -117,6 +148,7 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
 {
 	unsigned long flags;
 	DECLARE_COMPLETION_ONSTACK(vioch_shutdown_done);
+	void *deferred_wq = NULL;
 
 	/*
 	 * Prepare to wait for the last release if not already released
@@ -127,10 +159,19 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
 		spin_unlock_irqrestore(&vioch->lock, flags);
 		return;
 	}
+
 	vioch->shutdown_done = &vioch_shutdown_done;
 	virtio_break_device(vioch->vqueue->vdev);
+	if (!vioch->is_rx && vioch->deferred_tx_wq) {
+		deferred_wq = vioch->deferred_tx_wq;
+		/* Cannot be kicked anymore after this...*/
+		vioch->deferred_tx_wq = NULL;
+	}
 	spin_unlock_irqrestore(&vioch->lock, flags);
 
+	if (deferred_wq)
+		destroy_workqueue(deferred_wq);
+
 	scmi_vio_channel_release(vioch);
 
 	/* Let any possibly concurrent RX path release the channel */
@@ -154,18 +195,34 @@ scmi_virtio_get_free_msg(struct scmi_vio_channel *vioch)
 	list_del_init(&msg->list);
 	spin_unlock_irqrestore(&vioch->free_lock, flags);
 
+	/* Still no users, no need to acquire poll_lock */
+	msg->poll_status = VIO_MSG_NOT_POLLED;
+	refcount_set(&msg->users, 1);
+
 	return msg;
 }
 
+static inline bool scmi_vio_msg_acquire(struct scmi_vio_msg *msg)
+{
+	return refcount_inc_not_zero(&msg->users);
+}
+
 /* Assumes to be called with vio channel acquired already */
-static void scmi_virtio_put_free_msg(struct scmi_vio_channel *vioch,
-				     struct scmi_vio_msg *msg)
+static inline bool scmi_vio_msg_release(struct scmi_vio_channel *vioch,
+					struct scmi_vio_msg *msg)
 {
-	unsigned long flags;
+	bool ret;
 
-	spin_lock_irqsave(&vioch->free_lock, flags);
-	list_add_tail(&msg->list, &vioch->free_list);
-	spin_unlock_irqrestore(&vioch->free_lock, flags);
+	ret = refcount_dec_and_test(&msg->users);
+	if (ret) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&vioch->free_lock, flags);
+		list_add_tail(&msg->list, &vioch->free_list);
+		spin_unlock_irqrestore(&vioch->free_lock, flags);
+	}
+
+	return ret;
 }
 
 static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
@@ -206,7 +263,7 @@ static void scmi_finalize_message(struct scmi_vio_channel *vioch,
 	if (vioch->is_rx)
 		scmi_vio_feed_vq_rx(vioch, msg);
 	else
-		scmi_virtio_put_free_msg(vioch, msg);
+		scmi_vio_msg_release(vioch, msg);
 }
 
 static void scmi_vio_complete_cb(struct virtqueue *vqueue)
@@ -230,6 +287,7 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 			virtqueue_disable_cb(vqueue);
 			cb_enabled = false;
 		}
+
 		msg = virtqueue_get_buf(vqueue, &length);
 		if (!msg) {
 			if (virtqueue_enable_cb(vqueue)) {
@@ -260,6 +318,49 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 	}
 }
 
+static void scmi_vio_deferred_tx_worker(struct work_struct *work)
+{
+	unsigned long flags;
+	struct scmi_vio_channel *vioch;
+	struct scmi_vio_msg *msg, *tmp;
+
+	vioch = container_of(work, struct scmi_vio_channel, deferred_tx_work);
+
+	if (!scmi_vio_channel_acquire(vioch))
+		return;
+
+	/*
+	 * Process pre-fetched messages: these could be non-polled messages or
+	 * late timed-out replies to polled messages dequeued by chance while
+	 * polling for some other messages: this worker is in charge to process
+	 * the valid non-expired messages and anyway finally free all of them.
+	 */
+	spin_lock_irqsave(&vioch->pending_lock, flags);
+
+	/* Scan the list of possibly pre-fetched messages during polling. */
+	list_for_each_entry_safe(msg, tmp, &vioch->pending_cmds_list, list) {
+		list_del(&msg->list);
+
+		/*
+		 * Channel is acquired here (cannot vanish) and this message
+		 * is no more processed elsewhere so no poll_lock needed.
+		 */
+		if (msg->poll_status == VIO_MSG_NOT_POLLED)
+			scmi_rx_callback(vioch->cinfo,
+					 msg_read_header(msg->input), msg);
+
+		/* Free the processed message once done */
+		scmi_vio_msg_release(vioch, msg);
+	}
+
+	spin_unlock_irqrestore(&vioch->pending_lock, flags);
+
+	/* Process possibly still pending messages */
+	scmi_vio_complete_cb(vioch->vqueue);
+
+	scmi_vio_channel_release(vioch);
+}
+
 static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
 
 static vq_callback_t *scmi_vio_complete_callbacks[] = {
@@ -327,6 +428,19 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
 
 	vioch = &((struct scmi_vio_channel *)scmi_vdev->priv)[index];
 
+	/* Setup a deferred worker for polling. */
+	if (tx && !vioch->deferred_tx_wq) {
+		vioch->deferred_tx_wq =
+			alloc_workqueue(dev_name(&scmi_vdev->dev),
+					WQ_UNBOUND | WQ_FREEZABLE | WQ_SYSFS,
+					0);
+		if (!vioch->deferred_tx_wq)
+			return -ENOMEM;
+
+		INIT_WORK(&vioch->deferred_tx_work,
+			  scmi_vio_deferred_tx_worker);
+	}
+
 	for (i = 0; i < vioch->max_msg; i++) {
 		struct scmi_vio_msg *msg;
 
@@ -340,6 +454,8 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
 						    GFP_KERNEL);
 			if (!msg->request)
 				return -ENOMEM;
+			spin_lock_init(&msg->poll_lock);
+			refcount_set(&msg->users, 1);
 		}
 
 		msg->input = devm_kzalloc(cinfo->dev, VIRTIO_SCMI_MAX_PDU_SIZE,
@@ -394,6 +510,21 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
 
 	spin_lock_irqsave(&vioch->lock, flags);
 
+	/*
+	 * If polling was requested for this transaction:
+	 *  - retrieve last used index (will be used as polling reference)
+	 *  - bind the polled message to the xfer via .priv
+	 *  - grab an additional msg refcount for the poll-path
+	 */
+	if (xfer->hdr.poll_completion) {
+		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+		/* Still no users, no need to acquire poll_lock */
+		msg->poll_status = VIO_MSG_POLLING;
+		scmi_vio_msg_acquire(msg);
+		/* Ensure initialized msg is visibly bound to xfer */
+		smp_store_mb(xfer->priv, msg);
+	}
+
 	rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, msg, GFP_ATOMIC);
 	if (rc)
 		dev_err(vioch->cinfo->dev,
@@ -403,8 +534,13 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
 
 	spin_unlock_irqrestore(&vioch->lock, flags);
 
-	if (rc)
-		scmi_virtio_put_free_msg(vioch, msg);
+	if (rc) {
+		/* Ensure order between xfer->priv clear and vq feeding */
+		smp_store_mb(xfer->priv, NULL);
+		if (xfer->hdr.poll_completion)
+			scmi_vio_msg_release(vioch, msg);
+		scmi_vio_msg_release(vioch, msg);
+	}
 
 	scmi_vio_channel_release(vioch);
 
@@ -416,10 +552,8 @@ static void virtio_fetch_response(struct scmi_chan_info *cinfo,
 {
 	struct scmi_vio_msg *msg = xfer->priv;
 
-	if (msg) {
+	if (msg)
 		msg_fetch_response(msg->input, msg->rx_len, xfer);
-		xfer->priv = NULL;
-	}
 }
 
 static void virtio_fetch_notification(struct scmi_chan_info *cinfo,
@@ -427,10 +561,225 @@ static void virtio_fetch_notification(struct scmi_chan_info *cinfo,
 {
 	struct scmi_vio_msg *msg = xfer->priv;
 
-	if (msg) {
+	if (msg)
 		msg_fetch_notification(msg->input, msg->rx_len, max_len, xfer);
-		xfer->priv = NULL;
+}
+
+/**
+ * virtio_mark_txdone  - Mark transmission done
+ *
+ * Free only completed polling transfer messages.
+ *
+ * Note that in the SCMI VirtIO transport we never explicitly release still
+ * outstanding but timed-out messages by forcibly re-adding them to the
+ * free-list inside the TX code path; we instead let IRQ/RX callbacks, or the
+ * TX deferred worker, eventually clean up such messages once, finally, a late
+ * reply is received and discarded (if ever).
+ *
+ * This approach was deemed preferable since those pending timed-out buffers are
+ * still effectively owned by the SCMI platform VirtIO device even after timeout
+ * expiration: forcibly freeing and reusing them before they had been returned
+ * explicitly by the SCMI platform could lead to subtle bugs due to message
+ * corruption.
+ * An SCMI platform VirtIO device which never returns message buffers is
+ * anyway broken and it will quickly lead to exhaustion of available messages.
+ *
+ * For this same reason, here, we take care to free only the polled messages
+ * that had been somehow replied (only if not by chance already processed on the
+ * IRQ path - the initial scmi_vio_msg_release() takes care of this) and also
+ * any timed-out polled message if that indeed appears to have been at least
+ * dequeued from the virtqueues (VIO_MSG_POLL_DONE): this is needed since such
+ * messages won't be freed elsewhere. Any other polled message is marked as
+ * VIO_MSG_POLL_TIMEOUT.
+ *
+ * Possible late replies to timed-out polled messages will be eventually freed
+ * by RX callbacks if delivered on the IRQ path or by the deferred TX worker if
+ * dequeued on some other polling path.
+ *
+ * @cinfo: SCMI channel info
+ * @ret: Transmission return code
+ * @xfer: Transfer descriptor
+ */
+static void virtio_mark_txdone(struct scmi_chan_info *cinfo, int ret,
+			       struct scmi_xfer *xfer)
+{
+	unsigned long flags;
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+	struct scmi_vio_msg *msg = xfer->priv;
+
+	if (!msg || !scmi_vio_channel_acquire(vioch))
+		return;
+
+	/* Ensure msg is unbound from xfer anyway at this point */
+	smp_store_mb(xfer->priv, NULL);
+
+	/* Must be a polled xfer and not already freed on the IRQ path */
+	if (!xfer->hdr.poll_completion || scmi_vio_msg_release(vioch, msg)) {
+		scmi_vio_channel_release(vioch);
+		return;
 	}
+
+	spin_lock_irqsave(&msg->poll_lock, flags);
+	/* Do not free timedout polled messages only if still inflight */
+	if (ret != -ETIMEDOUT || msg->poll_status == VIO_MSG_POLL_DONE)
+		scmi_vio_msg_release(vioch, msg);
+	else if (msg->poll_status == VIO_MSG_POLLING)
+		msg->poll_status = VIO_MSG_POLL_TIMEOUT;
+	spin_unlock_irqrestore(&msg->poll_lock, flags);
+
+	scmi_vio_channel_release(vioch);
+}
+
+/**
+ * virtio_poll_done  - Provide polling support for VirtIO transport
+ *
+ * @cinfo: SCMI channel info
+ * @xfer: Reference to the transfer being poll for.
+ *
+ * VirtIO core provides a polling mechanism based only on last used indexes:
+ * this means that it is possible to poll the virtqueues waiting for something
+ * new to arrive from the host side, but the only way to check if the freshly
+ * arrived buffer was indeed what we were waiting for is to compare the newly
+ * arrived message descriptor with the one we are polling on.
+ *
+ * As a consequence it can happen to dequeue something different from the buffer
+ * we were poll-waiting for: if that is the case such early fetched buffers are
+ * then added to a the @pending_cmds_list list for later processing by a
+ * dedicated deferred worker.
+ *
+ * So, basically, once something new is spotted we proceed to de-queue all the
+ * freshly received used buffers until we found the one we were polling on, or,
+ * we have 'seemingly' emptied the virtqueue; if some buffers are still pending
+ * in the vqueue at the end of the polling loop (possible due to inherent races
+ * in virtqueues handling mechanisms), we similarly kick the deferred worker
+ * and let it process those, to avoid indefinitely looping in the .poll_done
+ * busy-waiting helper.
+ *
+ * Finally, we delegate to the deferred worker also the final free of any timed
+ * out reply to a polled message that we should dequeue.
+ *
+ * Note that, since we do NOT have per-message suppress notification mechanism,
+ * the message we are polling for could be alternatively delivered via usual
+ * IRQs callbacks on another core which happened to have IRQs enabled while we
+ * are actively polling for it here: in such a case it will be handled as such
+ * by scmi_rx_callback() and the polling loop in the SCMI Core TX path will be
+ * transparently terminated anyway.
+ *
+ * Return: True once polling has successfully completed.
+ */
+static bool virtio_poll_done(struct scmi_chan_info *cinfo,
+			     struct scmi_xfer *xfer)
+{
+	bool pending, found = false;
+	unsigned int length, any_prefetched = 0;
+	unsigned long flags;
+	struct scmi_vio_msg *next_msg, *msg = xfer->priv;
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+
+	if (!msg)
+		return true;
+
+	/*
+	 * Processed already by other polling loop on another CPU ?
+	 *
+	 * Note that this message is acquired on the poll path so cannot vanish
+	 * while inside this loop iteration even if concurrently processed on
+	 * the IRQ path.
+	 *
+	 * Avoid to acquire poll_lock since polled_status can be changed
+	 * in a relevant manner only later in this same thread of execution:
+	 * any other possible changes made concurrently by other polling loops
+	 * or by a reply delivered on the IRQ path have no meaningful impact on
+	 * this loop iteration: in other words it is harmless to allow this
+	 * possible race but let has avoid spinlocking with irqs off in this
+	 * initial part of the polling loop.
+	 */
+	if (msg->poll_status == VIO_MSG_POLL_DONE)
+		return true;
+
+	if (!scmi_vio_channel_acquire(vioch))
+		return true;
+
+	/* Has cmdq index moved at all ? */
+	pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+	if (!pending) {
+		scmi_vio_channel_release(vioch);
+		return false;
+	}
+
+	spin_lock_irqsave(&vioch->lock, flags);
+	virtqueue_disable_cb(vioch->vqueue);
+
+	/*
+	 * Process all new messages till the polled-for message is found OR
+	 * the vqueue is empty.
+	 */
+	while ((next_msg = virtqueue_get_buf(vioch->vqueue, &length))) {
+		bool next_msg_done = false;
+
+		/*
+		 * Mark any dequeued buffer message as VIO_MSG_POLL_DONE so
+		 * that can be properly freed even on timeout in mark_txdone.
+		 */
+		spin_lock(&next_msg->poll_lock);
+		if (next_msg->poll_status == VIO_MSG_POLLING) {
+			next_msg->poll_status = VIO_MSG_POLL_DONE;
+			next_msg_done = true;
+		}
+		spin_unlock(&next_msg->poll_lock);
+
+		next_msg->rx_len = length;
+		/* Is the message we were polling for ? */
+		if (next_msg == msg) {
+			found = true;
+			break;
+		} else if (next_msg_done) {
+			/* Skip the rest if this was another polled msg */
+			continue;
+		}
+
+		/*
+		 * Enqueue for later processing any non-polled message and any
+		 * timed-out polled one that we happen to have dequeued.
+		 */
+		spin_lock(&next_msg->poll_lock);
+		if (next_msg->poll_status == VIO_MSG_NOT_POLLED ||
+		    next_msg->poll_status == VIO_MSG_POLL_TIMEOUT) {
+			spin_unlock(&next_msg->poll_lock);
+
+			any_prefetched++;
+			spin_lock(&vioch->pending_lock);
+			list_add_tail(&next_msg->list,
+				      &vioch->pending_cmds_list);
+			spin_unlock(&vioch->pending_lock);
+		} else {
+			spin_unlock(&next_msg->poll_lock);
+		}
+	}
+
+	/*
+	 * When the polling loop has successfully terminated if something
+	 * else was queued in the meantime, it will be served by a deferred
+	 * worker OR by the normal IRQ/callback OR by other poll loops.
+	 *
+	 * If we are still looking for the polled reply, the polling index has
+	 * to be updated to the current vqueue last used index.
+	 */
+	if (found) {
+		pending = !virtqueue_enable_cb(vioch->vqueue);
+	} else {
+		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+		pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+	}
+
+	if (vioch->deferred_tx_wq && (any_prefetched || pending))
+		queue_work(vioch->deferred_tx_wq, &vioch->deferred_tx_work);
+
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	scmi_vio_channel_release(vioch);
+
+	return found;
 }
 
 static const struct scmi_transport_ops scmi_virtio_ops = {
@@ -442,6 +791,8 @@ static const struct scmi_transport_ops scmi_virtio_ops = {
 	.send_message = virtio_send_message,
 	.fetch_response = virtio_fetch_response,
 	.fetch_notification = virtio_fetch_notification,
+	.mark_txdone = virtio_mark_txdone,
+	.poll_done = virtio_poll_done,
 };
 
 static int scmi_vio_probe(struct virtio_device *vdev)
@@ -484,6 +835,8 @@ static int scmi_vio_probe(struct virtio_device *vdev)
 		spin_lock_init(&channels[i].lock);
 		spin_lock_init(&channels[i].free_lock);
 		INIT_LIST_HEAD(&channels[i].free_list);
+		spin_lock_init(&channels[i].pending_lock);
+		INIT_LIST_HEAD(&channels[i].pending_cmds_list);
 		channels[i].vqueue = vqs[i];
 
 		sz = virtqueue_get_vring_size(channels[i].vqueue);
@@ -573,4 +926,5 @@ const struct scmi_desc scmi_virtio_desc = {
 	.max_rx_timeout_ms = VIRTIO_MAX_RX_TIMEOUT_MS,
 	.max_msg = 0, /* overridden by virtio_get_max_msg() */
 	.max_msg_size = VIRTIO_SCMI_MAX_MSG_SIZE,
+	.atomic_enabled = IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE),
 };
-- 
2.17.1

Re: [PATCH v5 3/8] firmware: arm_scmi: Add atomic mode support to virtio transport

[Peter Hilber]

On 17.02.22 14:12, Cristian Marussi wrote:
> Add support for .mark_txdone and .poll_done transport operations to SCMI
> VirtIO transport as pre-requisites to enable atomic operations.
> 
> Add a Kernel configuration option to enable SCMI VirtIO transport polling
> and atomic mode for selected SCMI transactions while leaving it default
> disabled.
> 
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
> Cc: Peter Hilber <peter.hilber@opensynergy.com>
> Cc: virtualization@lists.linux-foundation.org
> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>

Acked-by: Peter Hilber <peter.hilber@opensynergy.com>

> ---
> v4 --> v5
> - introduced vio_msg refcounts and helpers to avoid premature reuse of
>   freed messages when both poling and IRQ path are active on a buffer
> - better handling of timed out polled messages on late replies using new
>   VIO_MSG_POLL_TIMEOUT state
> - fixed comments on locks
> 
> v1 --> v2
> - shrinked spinlocked section within virtio_poll_done to exclude
>   virtqueue_poll
> - removed poll_lock
> - use vio channel refcount acquire/release logic when polling
> - using new free_list accessors
> - added new dedicated pending_lock to access pending_cmds_list
> - fixed a few comments
> 
> v0 --> v1
> - check for deferred_wq existence before queueing work to avoid
>   race at driver removal time
> - changed mark_txdone decision-logic about message release
> - fixed race while checking for msg polled from another thread
> - using dedicated poll_status instead of poll_idx upper bits
> - pick initial poll_idx earlier inside send_message to avoid missing
>   early replies
> - removed F_NOTIFY mention in comment
> - clearing xfer->priv on the IRQ tx path once message has been fetched
> - added some store barriers
> - updated some comments
> ---
>  drivers/firmware/arm_scmi/Kconfig  |  15 ++
>  drivers/firmware/arm_scmi/driver.c |   6 +-
>  drivers/firmware/arm_scmi/virtio.c | 390 +++++++++++++++++++++++++++--
>  3 files changed, 391 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/firmware/arm_scmi/Kconfig b/drivers/firmware/arm_scmi/Kconfig
> index d429326433d1..7794bd41eaa0 100644
> --- a/drivers/firmware/arm_scmi/Kconfig
> +++ b/drivers/firmware/arm_scmi/Kconfig
> @@ -118,6 +118,21 @@ config ARM_SCMI_TRANSPORT_VIRTIO_VERSION1_COMPLIANCE
>  	  the ones implemented by kvmtool) and let the core Kernel VirtIO layer
>  	  take care of the needed conversions, say N.
>  
> +config ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> +	bool "Enable atomic mode for SCMI VirtIO transport"
> +	depends on ARM_SCMI_TRANSPORT_VIRTIO
> +	help
> +	  Enable support of atomic operation for SCMI VirtIO based transport.
> +
> +	  If you want the SCMI VirtIO based transport to operate in atomic
> +	  mode, avoiding any kind of sleeping behaviour for selected
> +	  transactions on the TX path, answer Y.
> +
> +	  Enabling atomic mode operations allows any SCMI driver using this
> +	  transport to optionally ask for atomic SCMI transactions and operate
> +	  in atomic context too, at the price of using a number of busy-waiting
> +	  primitives all over instead. If unsure say N.
> +
>  endif #ARM_SCMI_PROTOCOL
>  
>  config ARM_SCMI_POWER_DOMAIN
> diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
> index c2e7897ff56e..4fd5a35ffa2f 100644
> --- a/drivers/firmware/arm_scmi/driver.c
> +++ b/drivers/firmware/arm_scmi/driver.c
> @@ -648,7 +648,8 @@ static void scmi_handle_notification(struct scmi_chan_info *cinfo,
>  
>  	unpack_scmi_header(msg_hdr, &xfer->hdr);
>  	if (priv)
> -		xfer->priv = priv;
> +		/* Ensure order between xfer->priv store and following ops */
> +		smp_store_mb(xfer->priv, priv);
>  	info->desc->ops->fetch_notification(cinfo, info->desc->max_msg_size,
>  					    xfer);
>  	scmi_notify(cinfo->handle, xfer->hdr.protocol_id,
> @@ -680,7 +681,8 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
>  		xfer->rx.len = info->desc->max_msg_size;
>  
>  	if (priv)
> -		xfer->priv = priv;
> +		/* Ensure order between xfer->priv store and following ops */
> +		smp_store_mb(xfer->priv, priv);
>  	info->desc->ops->fetch_response(cinfo, xfer);
>  
>  	trace_scmi_rx_done(xfer->transfer_id, xfer->hdr.id,
> diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
> index 7ec4085cee7e..f69442e97389 100644
> --- a/drivers/firmware/arm_scmi/virtio.c
> +++ b/drivers/firmware/arm_scmi/virtio.c
> @@ -3,8 +3,8 @@
>   * Virtio Transport driver for Arm System Control and Management Interface
>   * (SCMI).
>   *
> - * Copyright (C) 2020-2021 OpenSynergy.
> - * Copyright (C) 2021 ARM Ltd.
> + * Copyright (C) 2020-2022 OpenSynergy.
> + * Copyright (C) 2021-2022 ARM Ltd.
>   */
>  
>  /**
> @@ -42,9 +42,14 @@
>   * @cinfo: SCMI Tx or Rx channel
>   * @free_lock: Protects access to the @free_list.
>   * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
> + * @deferred_tx_work: Worker for TX deferred replies processing
> + * @deferred_tx_wq: Workqueue for TX deferred replies
> + * @pending_lock: Protects access to the @pending_cmds_list.
> + * @pending_cmds_list: List of pre-fetched commands queueud for later processing
>   * @is_rx: Whether channel is an Rx channel
>   * @max_msg: Maximum number of pending messages for this channel.
> - * @lock: Protects access to all members except users, free_list.
> + * @lock: Protects access to all members except users, free_list and
> + *	  pending_cmds_list.
>   * @shutdown_done: A reference to a completion used when freeing this channel.
>   * @users: A reference count to currently active users of this channel.
>   */
> @@ -54,14 +59,29 @@ struct scmi_vio_channel {
>  	/* lock to protect access to the free list. */
>  	spinlock_t free_lock;
>  	struct list_head free_list;
> +	/* lock to protect access to the pending list. */
> +	spinlock_t pending_lock;
> +	struct list_head pending_cmds_list;
> +	struct work_struct deferred_tx_work;
> +	struct workqueue_struct *deferred_tx_wq;
>  	bool is_rx;
>  	unsigned int max_msg;
> -	/* lock to protect access to all members except users, free_list  */
> +	/*
> +	 * Lock to protect access to all members except users, free_list and
> +	 * pending_cmds_list
> +	 */
>  	spinlock_t lock;
>  	struct completion *shutdown_done;
>  	refcount_t users;
>  };
>  
> +enum poll_states {
> +	VIO_MSG_NOT_POLLED,
> +	VIO_MSG_POLL_TIMEOUT,
> +	VIO_MSG_POLLING,
> +	VIO_MSG_POLL_DONE,
> +};
> +
>  /**
>   * struct scmi_vio_msg - Transport PDU information
>   *
> @@ -69,12 +89,23 @@ struct scmi_vio_channel {
>   * @input: SDU used for (delayed) responses and notifications
>   * @list: List which scmi_vio_msg may be part of
>   * @rx_len: Input SDU size in bytes, once input has been received
> + * @poll_idx: Last used index registered for polling purposes if this message
> + *	      transaction reply was configured for polling.
> + * @poll_status: Polling state for this message.
> + * @poll_lock: A lock to protect @poll_status
> + * @users: A reference count to track this message users and avoid premature
> + *	   freeing (and reuse) when polling and IRQ execution paths interleave.
>   */
>  struct scmi_vio_msg {
>  	struct scmi_msg_payld *request;
>  	struct scmi_msg_payld *input;
>  	struct list_head list;
>  	unsigned int rx_len;
> +	unsigned int poll_idx;
> +	enum poll_states poll_status;
> +	/* Lock to protect access to poll_status */
> +	spinlock_t poll_lock;
> +	refcount_t users;
>  };
>  
>  /* Only one SCMI VirtIO device can possibly exist */
> @@ -117,6 +148,7 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
>  {
>  	unsigned long flags;
>  	DECLARE_COMPLETION_ONSTACK(vioch_shutdown_done);
> +	void *deferred_wq = NULL;
>  
>  	/*
>  	 * Prepare to wait for the last release if not already released
> @@ -127,10 +159,19 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
>  		spin_unlock_irqrestore(&vioch->lock, flags);
>  		return;
>  	}
> +
>  	vioch->shutdown_done = &vioch_shutdown_done;
>  	virtio_break_device(vioch->vqueue->vdev);
> +	if (!vioch->is_rx && vioch->deferred_tx_wq) {
> +		deferred_wq = vioch->deferred_tx_wq;
> +		/* Cannot be kicked anymore after this...*/
> +		vioch->deferred_tx_wq = NULL;
> +	}
>  	spin_unlock_irqrestore(&vioch->lock, flags);
>  
> +	if (deferred_wq)
> +		destroy_workqueue(deferred_wq);
> +
>  	scmi_vio_channel_release(vioch);
>  
>  	/* Let any possibly concurrent RX path release the channel */
> @@ -154,18 +195,34 @@ scmi_virtio_get_free_msg(struct scmi_vio_channel *vioch)
>  	list_del_init(&msg->list);
>  	spin_unlock_irqrestore(&vioch->free_lock, flags);
>  
> +	/* Still no users, no need to acquire poll_lock */
> +	msg->poll_status = VIO_MSG_NOT_POLLED;
> +	refcount_set(&msg->users, 1);
> +
>  	return msg;
>  }
>  
> +static inline bool scmi_vio_msg_acquire(struct scmi_vio_msg *msg)
> +{
> +	return refcount_inc_not_zero(&msg->users);
> +}
> +
>  /* Assumes to be called with vio channel acquired already */
> -static void scmi_virtio_put_free_msg(struct scmi_vio_channel *vioch,
> -				     struct scmi_vio_msg *msg)
> +static inline bool scmi_vio_msg_release(struct scmi_vio_channel *vioch,
> +					struct scmi_vio_msg *msg)
>  {
> -	unsigned long flags;
> +	bool ret;
>  
> -	spin_lock_irqsave(&vioch->free_lock, flags);
> -	list_add_tail(&msg->list, &vioch->free_list);
> -	spin_unlock_irqrestore(&vioch->free_lock, flags);
> +	ret = refcount_dec_and_test(&msg->users);
> +	if (ret) {
> +		unsigned long flags;
> +
> +		spin_lock_irqsave(&vioch->free_lock, flags);
> +		list_add_tail(&msg->list, &vioch->free_list);
> +		spin_unlock_irqrestore(&vioch->free_lock, flags);
> +	}
> +
> +	return ret;
>  }
>  
>  static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
> @@ -206,7 +263,7 @@ static void scmi_finalize_message(struct scmi_vio_channel *vioch,
>  	if (vioch->is_rx)
>  		scmi_vio_feed_vq_rx(vioch, msg);
>  	else
> -		scmi_virtio_put_free_msg(vioch, msg);
> +		scmi_vio_msg_release(vioch, msg);
>  }
>  
>  static void scmi_vio_complete_cb(struct virtqueue *vqueue)
> @@ -230,6 +287,7 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
>  			virtqueue_disable_cb(vqueue);
>  			cb_enabled = false;
>  		}
> +
>  		msg = virtqueue_get_buf(vqueue, &length);
>  		if (!msg) {
>  			if (virtqueue_enable_cb(vqueue)) {
> @@ -260,6 +318,49 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
>  	}
>  }
>  
> +static void scmi_vio_deferred_tx_worker(struct work_struct *work)
> +{
> +	unsigned long flags;
> +	struct scmi_vio_channel *vioch;
> +	struct scmi_vio_msg *msg, *tmp;
> +
> +	vioch = container_of(work, struct scmi_vio_channel, deferred_tx_work);
> +
> +	if (!scmi_vio_channel_acquire(vioch))
> +		return;
> +
> +	/*
> +	 * Process pre-fetched messages: these could be non-polled messages or
> +	 * late timed-out replies to polled messages dequeued by chance while
> +	 * polling for some other messages: this worker is in charge to process
> +	 * the valid non-expired messages and anyway finally free all of them.
> +	 */
> +	spin_lock_irqsave(&vioch->pending_lock, flags);
> +
> +	/* Scan the list of possibly pre-fetched messages during polling. */
> +	list_for_each_entry_safe(msg, tmp, &vioch->pending_cmds_list, list) {
> +		list_del(&msg->list);
> +
> +		/*
> +		 * Channel is acquired here (cannot vanish) and this message
> +		 * is no more processed elsewhere so no poll_lock needed.
> +		 */
> +		if (msg->poll_status == VIO_MSG_NOT_POLLED)
> +			scmi_rx_callback(vioch->cinfo,
> +					 msg_read_header(msg->input), msg);
> +
> +		/* Free the processed message once done */
> +		scmi_vio_msg_release(vioch, msg);
> +	}
> +
> +	spin_unlock_irqrestore(&vioch->pending_lock, flags);
> +
> +	/* Process possibly still pending messages */
> +	scmi_vio_complete_cb(vioch->vqueue);
> +
> +	scmi_vio_channel_release(vioch);
> +}
> +
>  static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
>  
>  static vq_callback_t *scmi_vio_complete_callbacks[] = {
> @@ -327,6 +428,19 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
>  
>  	vioch = &((struct scmi_vio_channel *)scmi_vdev->priv)[index];
>  
> +	/* Setup a deferred worker for polling. */
> +	if (tx && !vioch->deferred_tx_wq) {
> +		vioch->deferred_tx_wq =
> +			alloc_workqueue(dev_name(&scmi_vdev->dev),
> +					WQ_UNBOUND | WQ_FREEZABLE | WQ_SYSFS,
> +					0);
> +		if (!vioch->deferred_tx_wq)
> +			return -ENOMEM;
> +
> +		INIT_WORK(&vioch->deferred_tx_work,
> +			  scmi_vio_deferred_tx_worker);
> +	}
> +
>  	for (i = 0; i < vioch->max_msg; i++) {
>  		struct scmi_vio_msg *msg;
>  
> @@ -340,6 +454,8 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
>  						    GFP_KERNEL);
>  			if (!msg->request)
>  				return -ENOMEM;
> +			spin_lock_init(&msg->poll_lock);
> +			refcount_set(&msg->users, 1);
>  		}
>  
>  		msg->input = devm_kzalloc(cinfo->dev, VIRTIO_SCMI_MAX_PDU_SIZE,
> @@ -394,6 +510,21 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
>  
>  	spin_lock_irqsave(&vioch->lock, flags);
>  
> +	/*
> +	 * If polling was requested for this transaction:
> +	 *  - retrieve last used index (will be used as polling reference)
> +	 *  - bind the polled message to the xfer via .priv
> +	 *  - grab an additional msg refcount for the poll-path
> +	 */
> +	if (xfer->hdr.poll_completion) {
> +		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
> +		/* Still no users, no need to acquire poll_lock */
> +		msg->poll_status = VIO_MSG_POLLING;
> +		scmi_vio_msg_acquire(msg);
> +		/* Ensure initialized msg is visibly bound to xfer */
> +		smp_store_mb(xfer->priv, msg);
> +	}
> +
>  	rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, msg, GFP_ATOMIC);
>  	if (rc)
>  		dev_err(vioch->cinfo->dev,
> @@ -403,8 +534,13 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
>  
>  	spin_unlock_irqrestore(&vioch->lock, flags);
>  
> -	if (rc)
> -		scmi_virtio_put_free_msg(vioch, msg);
> +	if (rc) {
> +		/* Ensure order between xfer->priv clear and vq feeding */
> +		smp_store_mb(xfer->priv, NULL);
> +		if (xfer->hdr.poll_completion)
> +			scmi_vio_msg_release(vioch, msg);
> +		scmi_vio_msg_release(vioch, msg);
> +	}
>  
>  	scmi_vio_channel_release(vioch);
>  
> @@ -416,10 +552,8 @@ static void virtio_fetch_response(struct scmi_chan_info *cinfo,
>  {
>  	struct scmi_vio_msg *msg = xfer->priv;
>  
> -	if (msg) {
> +	if (msg)
>  		msg_fetch_response(msg->input, msg->rx_len, xfer);
> -		xfer->priv = NULL;
> -	}
>  }
>  
>  static void virtio_fetch_notification(struct scmi_chan_info *cinfo,
> @@ -427,10 +561,225 @@ static void virtio_fetch_notification(struct scmi_chan_info *cinfo,
>  {
>  	struct scmi_vio_msg *msg = xfer->priv;
>  
> -	if (msg) {
> +	if (msg)
>  		msg_fetch_notification(msg->input, msg->rx_len, max_len, xfer);
> -		xfer->priv = NULL;
> +}
> +
> +/**
> + * virtio_mark_txdone  - Mark transmission done
> + *
> + * Free only completed polling transfer messages.
> + *
> + * Note that in the SCMI VirtIO transport we never explicitly release still
> + * outstanding but timed-out messages by forcibly re-adding them to the
> + * free-list inside the TX code path; we instead let IRQ/RX callbacks, or the
> + * TX deferred worker, eventually clean up such messages once, finally, a late
> + * reply is received and discarded (if ever).
> + *
> + * This approach was deemed preferable since those pending timed-out buffers are
> + * still effectively owned by the SCMI platform VirtIO device even after timeout
> + * expiration: forcibly freeing and reusing them before they had been returned
> + * explicitly by the SCMI platform could lead to subtle bugs due to message
> + * corruption.
> + * An SCMI platform VirtIO device which never returns message buffers is
> + * anyway broken and it will quickly lead to exhaustion of available messages.
> + *
> + * For this same reason, here, we take care to free only the polled messages
> + * that had been somehow replied (only if not by chance already processed on the
> + * IRQ path - the initial scmi_vio_msg_release() takes care of this) and also
> + * any timed-out polled message if that indeed appears to have been at least
> + * dequeued from the virtqueues (VIO_MSG_POLL_DONE): this is needed since such
> + * messages won't be freed elsewhere. Any other polled message is marked as
> + * VIO_MSG_POLL_TIMEOUT.
> + *
> + * Possible late replies to timed-out polled messages will be eventually freed
> + * by RX callbacks if delivered on the IRQ path or by the deferred TX worker if
> + * dequeued on some other polling path.
> + *
> + * @cinfo: SCMI channel info
> + * @ret: Transmission return code
> + * @xfer: Transfer descriptor
> + */
> +static void virtio_mark_txdone(struct scmi_chan_info *cinfo, int ret,
> +			       struct scmi_xfer *xfer)
> +{
> +	unsigned long flags;
> +	struct scmi_vio_channel *vioch = cinfo->transport_info;
> +	struct scmi_vio_msg *msg = xfer->priv;
> +
> +	if (!msg || !scmi_vio_channel_acquire(vioch))
> +		return;
> +
> +	/* Ensure msg is unbound from xfer anyway at this point */
> +	smp_store_mb(xfer->priv, NULL);
> +
> +	/* Must be a polled xfer and not already freed on the IRQ path */
> +	if (!xfer->hdr.poll_completion || scmi_vio_msg_release(vioch, msg)) {
> +		scmi_vio_channel_release(vioch);
> +		return;
>  	}
> +
> +	spin_lock_irqsave(&msg->poll_lock, flags);
> +	/* Do not free timedout polled messages only if still inflight */
> +	if (ret != -ETIMEDOUT || msg->poll_status == VIO_MSG_POLL_DONE)
> +		scmi_vio_msg_release(vioch, msg);
> +	else if (msg->poll_status == VIO_MSG_POLLING)
> +		msg->poll_status = VIO_MSG_POLL_TIMEOUT;
> +	spin_unlock_irqrestore(&msg->poll_lock, flags);
> +
> +	scmi_vio_channel_release(vioch);
> +}
> +
> +/**
> + * virtio_poll_done  - Provide polling support for VirtIO transport
> + *
> + * @cinfo: SCMI channel info
> + * @xfer: Reference to the transfer being poll for.
> + *
> + * VirtIO core provides a polling mechanism based only on last used indexes:
> + * this means that it is possible to poll the virtqueues waiting for something
> + * new to arrive from the host side, but the only way to check if the freshly
> + * arrived buffer was indeed what we were waiting for is to compare the newly
> + * arrived message descriptor with the one we are polling on.
> + *
> + * As a consequence it can happen to dequeue something different from the buffer
> + * we were poll-waiting for: if that is the case such early fetched buffers are
> + * then added to a the @pending_cmds_list list for later processing by a
> + * dedicated deferred worker.
> + *
> + * So, basically, once something new is spotted we proceed to de-queue all the
> + * freshly received used buffers until we found the one we were polling on, or,
> + * we have 'seemingly' emptied the virtqueue; if some buffers are still pending
> + * in the vqueue at the end of the polling loop (possible due to inherent races
> + * in virtqueues handling mechanisms), we similarly kick the deferred worker
> + * and let it process those, to avoid indefinitely looping in the .poll_done
> + * busy-waiting helper.
> + *
> + * Finally, we delegate to the deferred worker also the final free of any timed
> + * out reply to a polled message that we should dequeue.
> + *
> + * Note that, since we do NOT have per-message suppress notification mechanism,
> + * the message we are polling for could be alternatively delivered via usual
> + * IRQs callbacks on another core which happened to have IRQs enabled while we
> + * are actively polling for it here: in such a case it will be handled as such
> + * by scmi_rx_callback() and the polling loop in the SCMI Core TX path will be
> + * transparently terminated anyway.
> + *
> + * Return: True once polling has successfully completed.
> + */
> +static bool virtio_poll_done(struct scmi_chan_info *cinfo,
> +			     struct scmi_xfer *xfer)
> +{
> +	bool pending, found = false;
> +	unsigned int length, any_prefetched = 0;
> +	unsigned long flags;
> +	struct scmi_vio_msg *next_msg, *msg = xfer->priv;
> +	struct scmi_vio_channel *vioch = cinfo->transport_info;
> +
> +	if (!msg)
> +		return true;
> +
> +	/*
> +	 * Processed already by other polling loop on another CPU ?
> +	 *
> +	 * Note that this message is acquired on the poll path so cannot vanish
> +	 * while inside this loop iteration even if concurrently processed on
> +	 * the IRQ path.
> +	 *
> +	 * Avoid to acquire poll_lock since polled_status can be changed
> +	 * in a relevant manner only later in this same thread of execution:
> +	 * any other possible changes made concurrently by other polling loops
> +	 * or by a reply delivered on the IRQ path have no meaningful impact on
> +	 * this loop iteration: in other words it is harmless to allow this
> +	 * possible race but let has avoid spinlocking with irqs off in this
> +	 * initial part of the polling loop.
> +	 */
> +	if (msg->poll_status == VIO_MSG_POLL_DONE)
> +		return true;
> +
> +	if (!scmi_vio_channel_acquire(vioch))
> +		return true;
> +
> +	/* Has cmdq index moved at all ? */
> +	pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
> +	if (!pending) {
> +		scmi_vio_channel_release(vioch);
> +		return false;
> +	}
> +
> +	spin_lock_irqsave(&vioch->lock, flags);
> +	virtqueue_disable_cb(vioch->vqueue);
> +
> +	/*
> +	 * Process all new messages till the polled-for message is found OR
> +	 * the vqueue is empty.
> +	 */
> +	while ((next_msg = virtqueue_get_buf(vioch->vqueue, &length))) {
> +		bool next_msg_done = false;
> +
> +		/*
> +		 * Mark any dequeued buffer message as VIO_MSG_POLL_DONE so
> +		 * that can be properly freed even on timeout in mark_txdone.
> +		 */
> +		spin_lock(&next_msg->poll_lock);
> +		if (next_msg->poll_status == VIO_MSG_POLLING) {
> +			next_msg->poll_status = VIO_MSG_POLL_DONE;
> +			next_msg_done = true;
> +		}
> +		spin_unlock(&next_msg->poll_lock);
> +
> +		next_msg->rx_len = length;
> +		/* Is the message we were polling for ? */
> +		if (next_msg == msg) {
> +			found = true;
> +			break;
> +		} else if (next_msg_done) {
> +			/* Skip the rest if this was another polled msg */
> +			continue;
> +		}
> +
> +		/*
> +		 * Enqueue for later processing any non-polled message and any
> +		 * timed-out polled one that we happen to have dequeued.
> +		 */
> +		spin_lock(&next_msg->poll_lock);
> +		if (next_msg->poll_status == VIO_MSG_NOT_POLLED ||
> +		    next_msg->poll_status == VIO_MSG_POLL_TIMEOUT) {
> +			spin_unlock(&next_msg->poll_lock);
> +
> +			any_prefetched++;
> +			spin_lock(&vioch->pending_lock);
> +			list_add_tail(&next_msg->list,
> +				      &vioch->pending_cmds_list);
> +			spin_unlock(&vioch->pending_lock);
> +		} else {
> +			spin_unlock(&next_msg->poll_lock);
> +		}
> +	}
> +
> +	/*
> +	 * When the polling loop has successfully terminated if something
> +	 * else was queued in the meantime, it will be served by a deferred
> +	 * worker OR by the normal IRQ/callback OR by other poll loops.
> +	 *
> +	 * If we are still looking for the polled reply, the polling index has
> +	 * to be updated to the current vqueue last used index.
> +	 */
> +	if (found) {
> +		pending = !virtqueue_enable_cb(vioch->vqueue);
> +	} else {
> +		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
> +		pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
> +	}
> +
> +	if (vioch->deferred_tx_wq && (any_prefetched || pending))
> +		queue_work(vioch->deferred_tx_wq, &vioch->deferred_tx_work);
> +
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	scmi_vio_channel_release(vioch);
> +
> +	return found;
>  }
>  
>  static const struct scmi_transport_ops scmi_virtio_ops = {
> @@ -442,6 +791,8 @@ static const struct scmi_transport_ops scmi_virtio_ops = {
>  	.send_message = virtio_send_message,
>  	.fetch_response = virtio_fetch_response,
>  	.fetch_notification = virtio_fetch_notification,
> +	.mark_txdone = virtio_mark_txdone,
> +	.poll_done = virtio_poll_done,
>  };
>  
>  static int scmi_vio_probe(struct virtio_device *vdev)
> @@ -484,6 +835,8 @@ static int scmi_vio_probe(struct virtio_device *vdev)
>  		spin_lock_init(&channels[i].lock);
>  		spin_lock_init(&channels[i].free_lock);
>  		INIT_LIST_HEAD(&channels[i].free_list);
> +		spin_lock_init(&channels[i].pending_lock);
> +		INIT_LIST_HEAD(&channels[i].pending_cmds_list);
>  		channels[i].vqueue = vqs[i];
>  
>  		sz = virtqueue_get_vring_size(channels[i].vqueue);
> @@ -573,4 +926,5 @@ const struct scmi_desc scmi_virtio_desc = {
>  	.max_rx_timeout_ms = VIRTIO_MAX_RX_TIMEOUT_MS,
>  	.max_msg = 0, /* overridden by virtio_get_max_msg() */
>  	.max_msg_size = VIRTIO_SCMI_MAX_MSG_SIZE,
> +	.atomic_enabled = IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE),
>  };

[PATCH v5 4/8] dt-bindings: firmware: arm,scmi: Add atomic-threshold-us optional property

[Cristian Marussi]

SCMI protocols in the platform can optionally signal to the OSPM agent
the expected execution latency for a specific resource/operation pair.

Introduce an SCMI system wide optional property to describe a global time
threshold which can be configured on a per-platform base to determine the
opportunity, or not, for an SCMI command advertised to have a higher
latency than the threshold, to be considered for atomic operations:
high-latency SCMI synchronous commands should be preferably issued in the
usual non-atomic mode.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v4 --> v5:
- fixed example and removed dtschema warnings/errors :
  arm,scmi.yaml: properties:atomic-threshold-us:
  	'$ref' should not be valid under {'const': '$ref'}
- added default: 0 clause
v3 --> v4
- renamed property to atomic-threshold-us
v1 --> v2
- rephrased the property description
---
 .../devicetree/bindings/firmware/arm,scmi.yaml         | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Documentation/devicetree/bindings/firmware/arm,scmi.yaml b/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
index eae15df36eef..590743883802 100644
--- a/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
+++ b/Documentation/devicetree/bindings/firmware/arm,scmi.yaml
@@ -81,6 +81,14 @@ properties:
   '#size-cells':
     const: 0
 
+  atomic-threshold-us:
+    description:
+      An optional time value, expressed in microseconds, representing, on this
+      platform, the threshold above which any SCMI command, advertised to have
+      an higher-than-threshold execution latency, should not be considered for
+      atomic mode of operation, even if requested.
+    default: 0
+
   arm,smc-id:
     $ref: /schemas/types.yaml#/definitions/uint32
     description:
@@ -264,6 +272,8 @@ examples:
             #address-cells = <1>;
             #size-cells = <0>;
 
+            atomic-threshold-us = <10000>;
+
             scmi_devpd: protocol@11 {
                 reg = <0x11>;
                 #power-domain-cells = <1>;
-- 
2.17.1

Re: [PATCH v5 4/8] dt-bindings: firmware: arm,scmi: Add atomic-threshold-us optional property

[Rob Herring]

On Thu, Feb 17, 2022 at 7:13 AM Cristian Marussi
<cristian.marussi@arm.com> wrote:
>
> SCMI protocols in the platform can optionally signal to the OSPM agent
> the expected execution latency for a specific resource/operation pair.
>
> Introduce an SCMI system wide optional property to describe a global time
> threshold which can be configured on a per-platform base to determine the
> opportunity, or not, for an SCMI command advertised to have a higher
> latency than the threshold, to be considered for atomic operations:
> high-latency SCMI synchronous commands should be preferably issued in the
> usual non-atomic mode.
>
> Cc: Rob Herring <robh+dt@kernel.org>
> Cc: devicetree@vger.kernel.org
> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
> ---
> v4 --> v5:
> - fixed example and removed dtschema warnings/errors :
>   arm,scmi.yaml: properties:atomic-threshold-us:
>         '$ref' should not be valid under {'const': '$ref'}

Seems an old version got picked up for linux-next:

/builds/robherring/linux-dt/Documentation/devicetree/bindings/firmware/arm,scmi.yaml:
properties:atomic-threshold-us: '$ref' should not be valid under
{'const': '$ref'}
hint: Standard unit suffix properties don't need a type $ref
from schema $id: http://devicetree.org/meta-schemas/core.yaml#
/builds/robherring/linux-dt/Documentation/devicetree/bindings/firmware/arm,scmi.yaml:
ignoring, error in schema: properties: atomic-threshold-us

> - added default: 0 clause
> v3 --> v4
> - renamed property to atomic-threshold-us
> v1 --> v2
> - rephrased the property description
> ---
>  .../devicetree/bindings/firmware/arm,scmi.yaml         | 10 ++++++++++
>  1 file changed, 10 insertions(+)

For this version:

Reviewed-by: Rob Herring <robh@kernel.org>

[PATCH v5 5/8] firmware: arm_scmi: Support optional system wide atomic-threshold-us

[Cristian Marussi]

An SCMI agent can be configured system-wide with a well-defined atomic
threshold: only SCMI synchronous command whose latency has been advertised
by the SCMI platform to be lower or equal to this configured threshold will
be considered for atomic operations, when requested and if supported by the
underlying transport at all.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v3 --> v4
- renamed DT property to atomic-threshold-us
---
 drivers/firmware/arm_scmi/driver.c | 27 ++++++++++++++++++++++++---
 include/linux/scmi_protocol.h      |  5 ++++-
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index 4fd5a35ffa2f..7436c475e708 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -131,6 +131,12 @@ struct scmi_protocol_instance {
  *	MAX_PROTOCOLS_IMP elements allocated by the base protocol
  * @active_protocols: IDR storing device_nodes for protocols actually defined
  *		      in the DT and confirmed as implemented by fw.
+ * @atomic_threshold: Optional system wide DT-configured threshold, expressed
+ *		      in microseconds, for atomic operations.
+ *		      Only SCMI synchronous commands reported by the platform
+ *		      to have an execution latency lesser-equal to the threshold
+ *		      should be considered for atomic mode operation: such
+ *		      decision is finally left up to the SCMI drivers.
  * @notify_priv: Pointer to private data structure specific to notifications.
  * @node: List head
  * @users: Number of users of this instance
@@ -149,6 +155,7 @@ struct scmi_info {
 	struct mutex protocols_mtx;
 	u8 *protocols_imp;
 	struct idr active_protocols;
+	unsigned int atomic_threshold;
 	void *notify_priv;
 	struct list_head node;
 	int users;
@@ -1406,15 +1413,22 @@ static void scmi_devm_protocol_put(struct scmi_device *sdev, u8 protocol_id)
  * SCMI instance is configured as atomic.
  *
  * @handle: A reference to the SCMI platform instance.
+ * @atomic_threshold: An optional return value for the system wide currently
+ *		      configured threshold for atomic operations.
  *
  * Return: True if transport is configured as atomic
  */
-static bool scmi_is_transport_atomic(const struct scmi_handle *handle)
+static bool scmi_is_transport_atomic(const struct scmi_handle *handle,
+				     unsigned int *atomic_threshold)
 {
+	bool ret;
 	struct scmi_info *info = handle_to_scmi_info(handle);
 
-	return info->desc->atomic_enabled &&
-		is_transport_polling_capable(info);
+	ret = info->desc->atomic_enabled && is_transport_polling_capable(info);
+	if (ret && atomic_threshold)
+		*atomic_threshold = info->atomic_threshold;
+
+	return ret;
 }
 
 static inline
@@ -1954,6 +1968,13 @@ static int scmi_probe(struct platform_device *pdev)
 	handle->version = &info->version;
 	handle->devm_protocol_get = scmi_devm_protocol_get;
 	handle->devm_protocol_put = scmi_devm_protocol_put;
+
+	/* System wide atomic threshold for atomic ops .. if any */
+	if (!of_property_read_u32(np, "atomic-threshold-us",
+				  &info->atomic_threshold))
+		dev_info(dev,
+			 "SCMI System wide atomic threshold set to %d us\n",
+			 info->atomic_threshold);
 	handle->is_transport_atomic = scmi_is_transport_atomic;
 
 	if (desc->ops->link_supplier) {
diff --git a/include/linux/scmi_protocol.h b/include/linux/scmi_protocol.h
index 9f895cb81818..fdf6bd83cc59 100644
--- a/include/linux/scmi_protocol.h
+++ b/include/linux/scmi_protocol.h
@@ -619,6 +619,8 @@ struct scmi_notify_ops {
  *			 be interested to know if they can assume SCMI
  *			 command transactions associated to this handle will
  *			 never sleep and act accordingly.
+ *			 An optional atomic threshold value could be returned
+ *			 where configured.
  * @notify_ops: pointer to set of notifications related operations
  */
 struct scmi_handle {
@@ -629,7 +631,8 @@ struct scmi_handle {
 		(*devm_protocol_get)(struct scmi_device *sdev, u8 proto,
 				     struct scmi_protocol_handle **ph);
 	void (*devm_protocol_put)(struct scmi_device *sdev, u8 proto);
-	bool (*is_transport_atomic)(const struct scmi_handle *handle);
+	bool (*is_transport_atomic)(const struct scmi_handle *handle,
+				    unsigned int *atomic_threshold);
 
 	const struct scmi_notify_ops *notify_ops;
 };
-- 
2.17.1

[PATCH v5 6/8] firmware: arm_scmi: Add atomic support to clock protocol

[Cristian Marussi]

Introduce new _atomic variant for SCMI clock protocol operations related
to enable disable operations: when an atomic operation is required the xfer
poll_completion flag is set for that transaction.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v1 --> v2
- removed enable_latency flag in clock_info since it does not belong to
  this commit really
---
 drivers/firmware/arm_scmi/clock.c | 22 +++++++++++++++++++---
 include/linux/scmi_protocol.h     |  3 +++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/arm_scmi/clock.c b/drivers/firmware/arm_scmi/clock.c
index 35b56c8ba0c0..72f930c0e3e2 100644
--- a/drivers/firmware/arm_scmi/clock.c
+++ b/drivers/firmware/arm_scmi/clock.c
@@ -273,7 +273,7 @@ static int scmi_clock_rate_set(const struct scmi_protocol_handle *ph,
 
 static int
 scmi_clock_config_set(const struct scmi_protocol_handle *ph, u32 clk_id,
-		      u32 config)
+		      u32 config, bool atomic)
 {
 	int ret;
 	struct scmi_xfer *t;
@@ -284,6 +284,8 @@ scmi_clock_config_set(const struct scmi_protocol_handle *ph, u32 clk_id,
 	if (ret)
 		return ret;
 
+	t->hdr.poll_completion = atomic;
+
 	cfg = t->tx.buf;
 	cfg->id = cpu_to_le32(clk_id);
 	cfg->attributes = cpu_to_le32(config);
@@ -296,12 +298,24 @@ scmi_clock_config_set(const struct scmi_protocol_handle *ph, u32 clk_id,
 
 static int scmi_clock_enable(const struct scmi_protocol_handle *ph, u32 clk_id)
 {
-	return scmi_clock_config_set(ph, clk_id, CLOCK_ENABLE);
+	return scmi_clock_config_set(ph, clk_id, CLOCK_ENABLE, false);
 }
 
 static int scmi_clock_disable(const struct scmi_protocol_handle *ph, u32 clk_id)
 {
-	return scmi_clock_config_set(ph, clk_id, 0);
+	return scmi_clock_config_set(ph, clk_id, 0, false);
+}
+
+static int scmi_clock_enable_atomic(const struct scmi_protocol_handle *ph,
+				    u32 clk_id)
+{
+	return scmi_clock_config_set(ph, clk_id, CLOCK_ENABLE, true);
+}
+
+static int scmi_clock_disable_atomic(const struct scmi_protocol_handle *ph,
+				     u32 clk_id)
+{
+	return scmi_clock_config_set(ph, clk_id, 0, true);
 }
 
 static int scmi_clock_count_get(const struct scmi_protocol_handle *ph)
@@ -330,6 +344,8 @@ static const struct scmi_clk_proto_ops clk_proto_ops = {
 	.rate_set = scmi_clock_rate_set,
 	.enable = scmi_clock_enable,
 	.disable = scmi_clock_disable,
+	.enable_atomic = scmi_clock_enable_atomic,
+	.disable_atomic = scmi_clock_disable_atomic,
 };
 
 static int scmi_clock_protocol_init(const struct scmi_protocol_handle *ph)
diff --git a/include/linux/scmi_protocol.h b/include/linux/scmi_protocol.h
index fdf6bd83cc59..306e576835f8 100644
--- a/include/linux/scmi_protocol.h
+++ b/include/linux/scmi_protocol.h
@@ -82,6 +82,9 @@ struct scmi_clk_proto_ops {
 			u64 rate);
 	int (*enable)(const struct scmi_protocol_handle *ph, u32 clk_id);
 	int (*disable)(const struct scmi_protocol_handle *ph, u32 clk_id);
+	int (*enable_atomic)(const struct scmi_protocol_handle *ph, u32 clk_id);
+	int (*disable_atomic)(const struct scmi_protocol_handle *ph,
+			      u32 clk_id);
 };
 
 /**
-- 
2.17.1

[PATCH v5 7/8] firmware: arm_scmi: Add support for clock_enable_latency

[Cristian Marussi]

An SCMI platform can optionally advertise an enable latency typically
associated with a specific clock resource: add support for parsing such
optional message field and export such information in the usual publicly
accessible clock descriptor.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v1 --> v2
- moved enable_latency flag definition from the previous commit

v2 --> v3
- checking for clock_enable_latencty presence, removed RFC tag
---
 drivers/firmware/arm_scmi/clock.c | 12 +++++++++---
 include/linux/scmi_protocol.h     |  1 +
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/arm_scmi/clock.c b/drivers/firmware/arm_scmi/clock.c
index 72f930c0e3e2..cf6fed6dec77 100644
--- a/drivers/firmware/arm_scmi/clock.c
+++ b/drivers/firmware/arm_scmi/clock.c
@@ -27,7 +27,8 @@ struct scmi_msg_resp_clock_protocol_attributes {
 struct scmi_msg_resp_clock_attributes {
 	__le32 attributes;
 #define	CLOCK_ENABLE	BIT(0)
-	    u8 name[SCMI_MAX_STR_SIZE];
+	u8 name[SCMI_MAX_STR_SIZE];
+	__le32 clock_enable_latency;
 };
 
 struct scmi_clock_set_config {
@@ -116,10 +117,15 @@ static int scmi_clock_attributes_get(const struct scmi_protocol_handle *ph,
 	attr = t->rx.buf;
 
 	ret = ph->xops->do_xfer(ph, t);
-	if (!ret)
+	if (!ret) {
 		strlcpy(clk->name, attr->name, SCMI_MAX_STR_SIZE);
-	else
+		/* Is optional field clock_enable_latency provided ? */
+		if (t->rx.len == sizeof(*attr))
+			clk->enable_latency =
+				le32_to_cpu(attr->clock_enable_latency);
+	} else {
 		clk->name[0] = '\0';
+	}
 
 	ph->xops->xfer_put(ph, t);
 	return ret;
diff --git a/include/linux/scmi_protocol.h b/include/linux/scmi_protocol.h
index 306e576835f8..b87551f41f9f 100644
--- a/include/linux/scmi_protocol.h
+++ b/include/linux/scmi_protocol.h
@@ -42,6 +42,7 @@ struct scmi_revision_info {
 
 struct scmi_clock_info {
 	char name[SCMI_MAX_STR_SIZE];
+	unsigned int enable_latency;
 	bool rate_discrete;
 	union {
 		struct {
-- 
2.17.1

[PATCH v5 8/8] clk: scmi: Support atomic clock enable/disable API

[Cristian Marussi]

Support also atomic enable/disable clk_ops beside the bare non-atomic one
(prepare/unprepare) when the underlying SCMI transport is configured to
support atomic transactions for synchronous commands.

Compare the SCMI system-wide configured atomic threshold latency time and
the per-clock advertised enable latency (if any) to choose whether to
provide sleeping prepare/unprepare vs atomic enable/disable.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-clk@vger.kernel.org
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v2 --> v3
- removed RFC tag
---
 drivers/clk/clk-scmi.c | 71 +++++++++++++++++++++++++++++++++++-------
 1 file changed, 60 insertions(+), 11 deletions(-)

diff --git a/drivers/clk/clk-scmi.c b/drivers/clk/clk-scmi.c
index 1e357d364ca2..2c7a830ce308 100644
--- a/drivers/clk/clk-scmi.c
+++ b/drivers/clk/clk-scmi.c
@@ -2,7 +2,7 @@
 /*
  * System Control and Power Interface (SCMI) Protocol based clock driver
  *
- * Copyright (C) 2018-2021 ARM Ltd.
+ * Copyright (C) 2018-2022 ARM Ltd.
  */
 
 #include <linux/clk-provider.h>
@@ -88,21 +88,51 @@ static void scmi_clk_disable(struct clk_hw *hw)
 	scmi_proto_clk_ops->disable(clk->ph, clk->id);
 }
 
+static int scmi_clk_atomic_enable(struct clk_hw *hw)
+{
+	struct scmi_clk *clk = to_scmi_clk(hw);
+
+	return scmi_proto_clk_ops->enable_atomic(clk->ph, clk->id);
+}
+
+static void scmi_clk_atomic_disable(struct clk_hw *hw)
+{
+	struct scmi_clk *clk = to_scmi_clk(hw);
+
+	scmi_proto_clk_ops->disable_atomic(clk->ph, clk->id);
+}
+
+/*
+ * We can provide enable/disable atomic callbacks only if the underlying SCMI
+ * transport for an SCMI instance is configured to handle SCMI commands in an
+ * atomic manner.
+ *
+ * When no SCMI atomic transport support is available we instead provide only
+ * the prepare/unprepare API, as allowed by the clock framework when atomic
+ * calls are not available.
+ *
+ * Two distinct sets of clk_ops are provided since we could have multiple SCMI
+ * instances with different underlying transport quality, so they cannot be
+ * shared.
+ */
 static const struct clk_ops scmi_clk_ops = {
 	.recalc_rate = scmi_clk_recalc_rate,
 	.round_rate = scmi_clk_round_rate,
 	.set_rate = scmi_clk_set_rate,
-	/*
-	 * We can't provide enable/disable callback as we can't perform the same
-	 * in atomic context. Since the clock framework provides standard API
-	 * clk_prepare_enable that helps cases using clk_enable in non-atomic
-	 * context, it should be fine providing prepare/unprepare.
-	 */
 	.prepare = scmi_clk_enable,
 	.unprepare = scmi_clk_disable,
 };
 
-static int scmi_clk_ops_init(struct device *dev, struct scmi_clk *sclk)
+static const struct clk_ops scmi_atomic_clk_ops = {
+	.recalc_rate = scmi_clk_recalc_rate,
+	.round_rate = scmi_clk_round_rate,
+	.set_rate = scmi_clk_set_rate,
+	.enable = scmi_clk_atomic_enable,
+	.disable = scmi_clk_atomic_disable,
+};
+
+static int scmi_clk_ops_init(struct device *dev, struct scmi_clk *sclk,
+			     const struct clk_ops *scmi_ops)
 {
 	int ret;
 	unsigned long min_rate, max_rate;
@@ -110,7 +140,7 @@ static int scmi_clk_ops_init(struct device *dev, struct scmi_clk *sclk)
 	struct clk_init_data init = {
 		.flags = CLK_GET_RATE_NOCACHE,
 		.num_parents = 0,
-		.ops = &scmi_clk_ops,
+		.ops = scmi_ops,
 		.name = sclk->info->name,
 	};
 
@@ -139,6 +169,8 @@ static int scmi_clk_ops_init(struct device *dev, struct scmi_clk *sclk)
 static int scmi_clocks_probe(struct scmi_device *sdev)
 {
 	int idx, count, err;
+	unsigned int atomic_threshold;
+	bool is_atomic;
 	struct clk_hw **hws;
 	struct clk_hw_onecell_data *clk_data;
 	struct device *dev = &sdev->dev;
@@ -168,8 +200,11 @@ static int scmi_clocks_probe(struct scmi_device *sdev)
 	clk_data->num = count;
 	hws = clk_data->hws;
 
+	is_atomic = handle->is_transport_atomic(handle, &atomic_threshold);
+
 	for (idx = 0; idx < count; idx++) {
 		struct scmi_clk *sclk;
+		const struct clk_ops *scmi_ops;
 
 		sclk = devm_kzalloc(dev, sizeof(*sclk), GFP_KERNEL);
 		if (!sclk)
@@ -184,13 +219,27 @@ static int scmi_clocks_probe(struct scmi_device *sdev)
 		sclk->id = idx;
 		sclk->ph = ph;
 
-		err = scmi_clk_ops_init(dev, sclk);
+		/*
+		 * Note that when transport is atomic but SCMI protocol did not
+		 * specify (or support) an enable_latency associated with a
+		 * clock, we default to use atomic operations mode.
+		 */
+		if (is_atomic &&
+		    sclk->info->enable_latency <= atomic_threshold)
+			scmi_ops = &scmi_atomic_clk_ops;
+		else
+			scmi_ops = &scmi_clk_ops;
+
+		err = scmi_clk_ops_init(dev, sclk, scmi_ops);
 		if (err) {
 			dev_err(dev, "failed to register clock %d\n", idx);
 			devm_kfree(dev, sclk);
 			hws[idx] = NULL;
 		} else {
-			dev_dbg(dev, "Registered clock:%s\n", sclk->info->name);
+			dev_dbg(dev, "Registered clock:%s%s\n",
+				sclk->info->name,
+				scmi_ops == &scmi_atomic_clk_ops ?
+				" (atomic ops)" : "");
 			hws[idx] = &sclk->hw;
 		}
 	}
-- 
2.17.1

Re: [PATCH v5 0/8] Add SCMI Virtio & Clock atomic support

[Sudeep Holla]

On Thu, 17 Feb 2022 13:12:26 +0000, Cristian Marussi wrote:
> This small series is the tail-subset of the previous V8 series about atomic
> support in SCMI [1], whose 8-patches head-subset has now been queued on
> [2]; as such, it is based on [2] on top of tag scmi-updates-5.17:
> 
> commit 94d0cd1da14a ("firmware: arm_scmi: Add new parameter to
> 		     mark_txdone")
> 
> [...]


Applied to sudeep.holla/linux (for-next/scmi/updates), thanks!

[1/8] firmware: arm_scmi: Add a virtio channel refcount
      https://git.kernel.org/sudeep.holla/c/42e90eb53b
[2/8] firmware: arm_scmi: Review virtio free_list handling
      https://git.kernel.org/sudeep.holla/c/9a1699bda0
[3/8] firmware: arm_scmi: Add atomic mode support to virtio transport
      https://git.kernel.org/sudeep.holla/c/5a3b7185c4
[4/8] dt-bindings: firmware: arm,scmi: Add atomic-threshold-us optional property
      https://git.kernel.org/sudeep.holla/c/0539884ccc
[5/8] firmware: arm_scmi: Support optional system wide atomic-threshold-us
      https://git.kernel.org/sudeep.holla/c/05976c5f3b
[6/8] firmware: arm_scmi: Add atomic support to clock protocol
      https://git.kernel.org/sudeep.holla/c/b7bd36f2e9
[7/8] firmware: arm_scmi: Add support for clock_enable_latency
      https://git.kernel.org/sudeep.holla/c/18f295b758
[8/8] clk: scmi: Support atomic clock enable/disable API
      https://git.kernel.org/sudeep.holla/c/38a0e5b735

--
Regards,
Sudeep

Re: [PATCH v5 0/8] Add SCMI Virtio & Clock atomic support

[Michael S. Tsirkin]

On Thu, Feb 17, 2022 at 01:12:26PM +0000, Cristian Marussi wrote:
> Hi,
> 
> This small series is the tail-subset of the previous V8 series about atomic
> support in SCMI [1], whose 8-patches head-subset has now been queued on
> [2]; as such, it is based on [2] on top of tag scmi-updates-5.17:
> 
> commit 94d0cd1da14a ("firmware: arm_scmi: Add new parameter to
> 		     mark_txdone")
> 
> Patch [1/8] substitute virtio-scmi ready flag and lock with a reference
> counter to keep track of vio channels lifetime while removing the need of
> a wide spinlocked section (that would cause issues with introduction of
> virtio polling support)
> 
> Patch [2/8] adds a few helpers to handle the TX free_list and a dedicated
> spinlock to reduce the reliance on the main one.
> 
> Patch [3/8] adds polling mode to SCMI VirtIO transport in order to support
> atomic operations on such transport.
> 
> Patches [4,5/8] introduce a new optional SCMI binding, atomic-threshold-us,
> to configure a platform specific time threshold used in the following
> patches to select with a finer grain which SCMI resources should be
> eligible for atomic operations when requested.
> 
> Patch [6/8] exposes new SCMI Clock protocol operations to allow an SCMI
> user to request atomic mode on clock enable commands.
> 
> Patch [7/8] adds support to SCMI Clock protocol for a new clock attributes
> field which advertises typical enable latency for a specific resource.
> 
> Finally patch [8/8] add support for atomic operations to the SCMI clock
> driver; the underlying logic here is that we register with the Clock
> framework atomic-capable clock resources if and only if the backing SCMI
> transport is capable of atomic operations AND the specific clock resource
> has been advertised by the SCMI platform as having:
> 
> 	clock_enable_latency <= atomic-threshold-us
> 
> The idea is to avoid costly atomic busy-waiting for resources that have
> been advertised as 'slow' to operate upon. (i.e. a PLL vs a gating clock)
> 
> To ease testing the whole series can be find at [3].
> 
> Any feedback/testing welcome as usual.
> 
> Thanks,
> Cristian


SCMI specific stuff so I don't have anything to add here.

By the way, it does not look like anything regarding SCMI atomic support
is needed in the virtio spec - is it true the interface with the device
is unaffected?

> [1]: https://lore.kernel.org/linux-arm-kernel/20211220195646.44498-1-cristian.marussi@arm.com/
> [2]: https://git.kernel.org/pub/scm/linux/kernel/git/sudeep.holla/linux.git/tag/?h=scmi-updates-5.17
> [3]: https://gitlab.arm.com/linux-arm/linux-cm/-/commits/scmi_atomic_clk_virtio_V5/
> 
> ---
> v4 --> v5
> - dt_bindings: fixed example and removed dtschema warnings/errors
> - dt_bindings: added 'default: 0' clause
> - introduced vio_msg refcounts and helpers to avoid premature reuse of
>   freed messages when both poling and IRQ path are active on a buffer
> - better handling of timed out polled messages on late replies using
>   new VIO_MSG_POLL_TIMEOUT state
> - fixed comments on locks
> - removed unneeded virtqueue re-enable when fail to acquire channel in
>   complete_cb
> 
> V3 --> V4
> - renamed optional DT property to atomic-threshold-us
> 
> V2 --> V3
>  - split out virtio_ring RFC patch into a distinct series
>  - calling virtqueue_broke_device when cleaning up channel
>  - removed RFC tags from CLK related patches
> 
> V1 --> V2
>  - added vio channel refcount support patch
>  - reviewed free_list support and usage
>  - added virtio_ring RFC patch
>  - shrinked spinlocked section within virtio_poll_done to exclude
>    virtqueue_poll call
>  - removed poll_lock
>  - use vio channel refcount acquire/release logic when polling
>  - using new free_list accessors
>  - added new dedicated pending_lock to access pending_cmds_list
>  - fixed a few comments
> 
> Cristian Marussi (8):
>   firmware: arm_scmi: Add a virtio channel refcount
>   firmware: arm_scmi: Review virtio free_list handling
>   firmware: arm_scmi: Add atomic mode support to virtio transport
>   dt-bindings: firmware: arm,scmi: Add atomic-threshold-us optional
>     property
>   firmware: arm_scmi: Support optional system wide atomic-threshold-us
>   firmware: arm_scmi: Add atomic support to clock protocol
>   firmware: arm_scmi: Add support for clock_enable_latency
>   clk: scmi: Support atomic clock enable/disable API
> 
>  .../bindings/firmware/arm,scmi.yaml           |  10 +
>  drivers/clk/clk-scmi.c                        |  71 ++-
>  drivers/firmware/arm_scmi/Kconfig             |  15 +
>  drivers/firmware/arm_scmi/clock.c             |  34 +-
>  drivers/firmware/arm_scmi/driver.c            |  33 +-
>  drivers/firmware/arm_scmi/virtio.c            | 591 +++++++++++++++---
>  include/linux/scmi_protocol.h                 |   9 +-
>  7 files changed, 655 insertions(+), 108 deletions(-)
> 
> -- 
> 2.17.1
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 

Re: [PATCH v5 0/8] Add SCMI Virtio & Clock atomic support

[Cristian Marussi]

On Fri, Mar 04, 2022 at 11:13:47AM -0500, Michael S. Tsirkin wrote:
> On Thu, Feb 17, 2022 at 01:12:26PM +0000, Cristian Marussi wrote:
> > Hi,
> > 
> > This small series is the tail-subset of the previous V8 series about atomic
> > support in SCMI [1], whose 8-patches head-subset has now been queued on
> > [2]; as such, it is based on [2] on top of tag scmi-updates-5.17:
> > 
> > commit 94d0cd1da14a ("firmware: arm_scmi: Add new parameter to
> > 		     mark_txdone")
> > 
> > Patch [1/8] substitute virtio-scmi ready flag and lock with a reference
> > counter to keep track of vio channels lifetime while removing the need of
> > a wide spinlocked section (that would cause issues with introduction of
> > virtio polling support)
> > 
> > Patch [2/8] adds a few helpers to handle the TX free_list and a dedicated
> > spinlock to reduce the reliance on the main one.
> > 
> > Patch [3/8] adds polling mode to SCMI VirtIO transport in order to support
> > atomic operations on such transport.
> > 
> > Patches [4,5/8] introduce a new optional SCMI binding, atomic-threshold-us,
> > to configure a platform specific time threshold used in the following
> > patches to select with a finer grain which SCMI resources should be
> > eligible for atomic operations when requested.
> > 
> > Patch [6/8] exposes new SCMI Clock protocol operations to allow an SCMI
> > user to request atomic mode on clock enable commands.
> > 
> > Patch [7/8] adds support to SCMI Clock protocol for a new clock attributes
> > field which advertises typical enable latency for a specific resource.
> > 
> > Finally patch [8/8] add support for atomic operations to the SCMI clock
> > driver; the underlying logic here is that we register with the Clock
> > framework atomic-capable clock resources if and only if the backing SCMI
> > transport is capable of atomic operations AND the specific clock resource
> > has been advertised by the SCMI platform as having:
> > 
> > 	clock_enable_latency <= atomic-threshold-us
> > 
> > The idea is to avoid costly atomic busy-waiting for resources that have
> > been advertised as 'slow' to operate upon. (i.e. a PLL vs a gating clock)
> > 
> > To ease testing the whole series can be find at [3].
> > 
> > Any feedback/testing welcome as usual.
> > 
> > Thanks,
> > Cristian
> 
> 
> SCMI specific stuff so I don't have anything to add here.
> 
> By the way, it does not look like anything regarding SCMI atomic support
> is needed in the virtio spec - is it true the interface with the device
> is unaffected?
> 

Yes SCMI atomic uses the existing VirtIO SCMI Device specification and the
existing  VirtIO common API for polling.

The only addition on the implementation side is the polling ABA problem
mitigation that I proposed as an addiional mechanism (wrap counters) in
the Virtio core with:

https://lore.kernel.org/linux-arm-kernel/20220208152520.52983-1-cristian.marussi@arm.com/#r

I'll repost soon this latter series about wrap counters on top of the merged
SCMI atomic series so as to make use of this mitigation from SCMI while in
polling mode.

Thanks for the feedback,
Cristian