[PATCH 0/6] Add SCMI Virtio & Clock atomic support

[PATCH 0/6] 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 1/5 adds polling mode to SCMI VirtIO transport in order to support
atomic operations on such transport and it could have been an independent
patch indeed, it is provided here only for ease of testing.

Patch 2,3 introduce a new optional SCMI binding, atomic_threshold, 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 4 exposes new SCMI Clock protocol operations to allow an SCMI user
to request atomic mode on clock enable commands.

Patch 5 adds support to SCMI Clock protocol for a new clock attributes
field which advertises typical enable latency for a specific resource.
It is marked as RFC since the SCMI spec including such addition is still
to be finalized.

Finally patch 6 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

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)
This last patch is marked as RFC too since it is dependent on the previous
one (that is RFC-tagged too)

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_V1/

Cristian Marussi (6):
  firmware: arm_scmi: Add atomic mode support to virtio transport
  dt-bindings: firmware: arm,scmi: Add atomic_threshold optional
    property
  firmware: arm_scmi: Support optional system wide atomic_threshold
  firmware: arm_scmi: Add atomic support to clock protocol
  [RFC] firmware: arm_scmi: Add support for clock_enable_latency
  [RFC] 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            |  36 +-
 drivers/firmware/arm_scmi/virtio.c            | 315 ++++++++++++++++--
 include/linux/scmi_protocol.h                 |   9 +-
 7 files changed, 440 insertions(+), 50 deletions(-)

-- 
2.17.1

[PATCH 1/6] 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>
---
- 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 insde 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 |   9 +-
 drivers/firmware/arm_scmi/virtio.c | 315 ++++++++++++++++++++++++++---
 3 files changed, 310 insertions(+), 29 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..dc972a54e93e 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,
@@ -680,8 +681,12 @@ 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);
+	if (priv)
+		/* Ensure order between xfer->priv clear and later accesses */
+		smp_store_mb(xfer->priv, NULL);
 
 	trace_scmi_rx_done(xfer->transfer_id, xfer->hdr.id,
 			   xfer->hdr.protocol_id, xfer->hdr.seq,
diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
index fd0f6f91fc0b..39c293f0ffd3 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.
  */
 
 /**
@@ -38,6 +38,9 @@
  * @vqueue: Associated virtqueue
  * @cinfo: SCMI Tx or Rx channel
  * @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_cmds_list: List of pre-fetched commands queueud for later processing
  * @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.
@@ -49,6 +52,9 @@ struct scmi_vio_channel {
 	struct virtqueue *vqueue;
 	struct scmi_chan_info *cinfo;
 	struct list_head free_list;
+	struct list_head pending_cmds_list;
+	struct work_struct deferred_tx_work;
+	struct workqueue_struct *deferred_tx_wq;
 	bool is_rx;
 	bool ready;
 	unsigned int max_msg;
@@ -58,6 +64,12 @@ struct scmi_vio_channel {
 	spinlock_t ready_lock;
 };
 
+enum poll_states {
+	VIO_MSG_NOT_POLLED,
+	VIO_MSG_POLLING,
+	VIO_MSG_POLL_DONE,
+};
+
 /**
  * struct scmi_vio_msg - Transport PDU information
  *
@@ -65,12 +77,22 @@ 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.
+ *	      Note that since virtqueue used index is an unsigned 16-bit we can
+ *	      use some out-of-scale values to signify particular conditions.
+ * @poll_status: Polling state for this message.
+ * @poll_lock: Protect access to @poll_idx and @poll_status.
  */
 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_idx. */
+	spinlock_t poll_lock;
 };
 
 /* Only one SCMI VirtIO device can possibly exist */
@@ -81,40 +103,43 @@ static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
 	return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
 }
 
+/* Expect to be called with vioch->lock acquired by the caller and IRQs off */
 static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
 			       struct scmi_vio_msg *msg,
 			       struct device *dev)
 {
 	struct scatterlist sg_in;
 	int rc;
-	unsigned long flags;
 
 	sg_init_one(&sg_in, msg->input, VIRTIO_SCMI_MAX_PDU_SIZE);
 
-	spin_lock_irqsave(&vioch->lock, flags);
-
 	rc = virtqueue_add_inbuf(vioch->vqueue, &sg_in, 1, msg, GFP_ATOMIC);
 	if (rc)
 		dev_err(dev, "failed to add to RX virtqueue (%d)\n", rc);
 	else
 		virtqueue_kick(vioch->vqueue);
 
-	spin_unlock_irqrestore(&vioch->lock, flags);
-
 	return rc;
 }
 
+/* Expect to be called with vioch->lock acquired by the caller and IRQs off */
+static inline void scmi_vio_feed_vq_tx(struct scmi_vio_channel *vioch,
+				       struct scmi_vio_msg *msg)
+{
+	spin_lock(&msg->poll_lock);
+	msg->poll_status = VIO_MSG_NOT_POLLED;
+	spin_unlock(&msg->poll_lock);
+
+	list_add(&msg->list, &vioch->free_list);
+}
+
 static void scmi_finalize_message(struct scmi_vio_channel *vioch,
 				  struct scmi_vio_msg *msg)
 {
-	if (vioch->is_rx) {
+	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);
-	}
+	else
+		scmi_vio_feed_vq_tx(vioch, msg);
 }
 
 static void scmi_vio_complete_cb(struct virtqueue *vqueue)
@@ -144,6 +169,7 @@ 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))
@@ -157,7 +183,9 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 			scmi_rx_callback(vioch->cinfo,
 					 msg_read_header(msg->input), msg);
 
+			spin_lock(&vioch->lock);
 			scmi_finalize_message(vioch, msg);
+			spin_unlock(&vioch->lock);
 		}
 
 		/*
@@ -176,6 +204,34 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 	spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
 }
 
+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);
+
+	/* Process pre-fetched messages */
+	spin_lock_irqsave(&vioch->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);
+
+		scmi_rx_callback(vioch->cinfo,
+				 msg_read_header(msg->input), msg);
+
+		/* Free the processed message once done */
+		scmi_vio_feed_vq_tx(vioch, msg);
+	}
+
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	/* Process possibly still pending messages */
+	scmi_vio_complete_cb(vioch->vqueue);
+}
+
 static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
 
 static vq_callback_t *scmi_vio_complete_callbacks[] = {
@@ -244,6 +300,19 @@ 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;
 
@@ -257,6 +326,7 @@ 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);
 		}
 
 		msg->input = devm_kzalloc(cinfo->dev, VIRTIO_SCMI_MAX_PDU_SIZE,
@@ -264,13 +334,12 @@ 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 {
+		spin_lock_irqsave(&vioch->lock, flags);
+		if (tx)
+			scmi_vio_feed_vq_tx(vioch, msg);
+		else
 			scmi_vio_feed_vq_rx(vioch, msg, cinfo->dev);
-		}
+		spin_unlock_irqrestore(&vioch->lock, flags);
 	}
 
 	spin_lock_irqsave(&vioch->lock, flags);
@@ -291,11 +360,22 @@ 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;
+	void *deferred_wq = NULL;
 
 	spin_lock_irqsave(&vioch->ready_lock, flags);
 	vioch->ready = false;
 	spin_unlock_irqrestore(&vioch->ready_lock, flags);
 
+	spin_lock_irqsave(&vioch->lock, flags);
+	if (!vioch->is_rx && vioch->deferred_tx_wq) {
+		deferred_wq = vioch->deferred_tx_wq;
+		vioch->deferred_tx_wq = NULL;
+	}
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	if (deferred_wq)
+		destroy_workqueue(deferred_wq);
+
 	scmi_free_channel(cinfo, data, id);
 
 	spin_lock_irqsave(&vioch->lock, flags);
@@ -324,16 +404,33 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
 	}
 
 	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
-	list_del(&msg->list);
+	/* Re-init element so we can discern anytime if it is still in-flight */
+	list_del_init(&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));
 
+	/*
+	 * 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
+	 */
+	if (xfer->hdr.poll_completion) {
+		spin_lock(&msg->poll_lock);
+		msg->poll_status = VIO_MSG_POLLING;
+		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+		spin_unlock(&msg->poll_lock);
+		/* 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) {
-		list_add(&msg->list, &vioch->free_list);
+		/* Ensure order between xfer->priv clear and vq feeding */
+		smp_store_mb(xfer->priv, NULL);
+		scmi_vio_feed_vq_tx(vioch, msg);
 		dev_err(vioch->cinfo->dev,
 			"failed to add to TX virtqueue (%d)\n", rc);
 	} else {
@@ -350,10 +447,8 @@ 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,
@@ -361,12 +456,174 @@ 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 timed-out
+ * messages by forcibly re-adding them to the free-list inside the TX code path;
+ * we instead let IRQ/RX callbacks 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, since they won't be freed elsewhere;
+ * late replies to timed-out polled messages would be anyway freed by RX
+ * callbacks instead.
+ *
+ * @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;
+
+	/* Must be a polled xfer and not already freed on the IRQ path */
+	if (!xfer->hdr.poll_completion || !msg)
+		return;
+
+	/* Ensure msg is unbound from xfer anyway at this point */
+	smp_store_mb(xfer->priv, NULL);
+
+	/* Do not free timedout polled messages */
+	if (ret != -ETIMEDOUT) {
+		spin_lock_irqsave(&vioch->lock, flags);
+		scmi_vio_feed_vq_tx(vioch, msg);
+		spin_unlock_irqrestore(&vioch->lock, flags);
 	}
 }
 
+/**
+ * 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 what we were waiting for is to compare the newly arrived
+ * message descriptors 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
+ * helper.
+ *
+ * Note that, since we do NOT have per-message suppress notification mechanism,
+ * the message we are polling for could be delivered via usual IRQs callbacks
+ * on another core which happened to have IRQs enabled: in such 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, ret = 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;
+
+	spin_lock_irqsave(&vioch->lock, flags);
+	spin_lock(&msg->poll_lock);
+	/* Processed already by other polling loop on another CPU ? */
+	if (msg->poll_status == VIO_MSG_POLL_DONE) {
+		spin_unlock(&msg->poll_lock);
+		spin_unlock_irqrestore(&vioch->lock, flags);
+		return true;
+	}
+
+	/* Has cmdq index moved at all ? */
+	pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+	spin_unlock(&msg->poll_lock);
+	if (!pending) {
+		spin_unlock_irqrestore(&vioch->lock, flags);
+		return false;
+	}
+
+	virtqueue_disable_cb(vioch->vqueue);
+
+	/*
+	 * If something arrived we cannot be sure, without dequeueing, if it
+	 * was the reply to the xfer we are polling for, or, to other, even
+	 * possibly non-polling, pending xfers: 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))) {
+		next_msg->rx_len = length;
+		/* Is the message we were polling for ? */
+		if (next_msg == msg) {
+			ret = true;
+			break;
+		}
+
+		spin_lock(&next_msg->poll_lock);
+		if (next_msg->poll_status == VIO_MSG_NOT_POLLED) {
+			any_prefetched++;
+			list_add_tail(&next_msg->list,
+				      &vioch->pending_cmds_list);
+		} else {
+			/* We picked another currently polled msg */
+			next_msg->poll_status = VIO_MSG_POLL_DONE;
+		}
+		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 (ret) {
+		pending = !virtqueue_enable_cb(vioch->vqueue);
+	} else {
+		spin_lock(&msg->poll_lock);
+		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+		pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+		spin_unlock(&msg->poll_lock);
+	}
+
+	if (vioch->deferred_tx_wq && (any_prefetched || pending))
+		queue_work(vioch->deferred_tx_wq, &vioch->deferred_tx_work);
+
+	spin_unlock_irqrestore(&vioch->lock, flags);
+
+	return ret;
+}
+
 static const struct scmi_transport_ops scmi_virtio_ops = {
 	.link_supplier = virtio_link_supplier,
 	.chan_available = virtio_chan_available,
@@ -376,6 +633,8 @@ 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)
@@ -418,6 +677,7 @@ static int scmi_vio_probe(struct virtio_device *vdev)
 		spin_lock_init(&channels[i].lock);
 		spin_lock_init(&channels[i].ready_lock);
 		INIT_LIST_HEAD(&channels[i].free_list);
+		INIT_LIST_HEAD(&channels[i].pending_cmds_list);
 		channels[i].vqueue = vqs[i];
 
 		sz = virtqueue_get_vring_size(channels[i].vqueue);
@@ -506,4 +766,5 @@ const struct scmi_desc scmi_virtio_desc = {
 	.max_rx_timeout_ms = 60000, /* for non-realtime virtio devices */
 	.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

[PATCH 2/6] dt-bindings: firmware: arm,scmi: Add atomic_threshold 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>
---
 .../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..8edda54dff5b 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:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      An optional time value, expressed in microseconds, representing the
+      threshold above which, on this platform, any SCMI command advertised to
+      have a higher execution latency, should not be used in atomic mode, even
+      if requested. If left unconfigured defaults to zero.
+
   arm,smc-id:
     $ref: /schemas/types.yaml#/definitions/uint32
     description:
@@ -264,6 +272,8 @@ examples:
             #address-cells = <1>;
             #size-cells = <0>;
 
+            atomic_threshold = <10000>;
+
             scmi_devpd: protocol@11 {
                 reg = <0x11>;
                 #power-domain-cells = <1>;
-- 
2.17.1

[PATCH 3/6] firmware: arm_scmi: Support optional system wide atomic_threshold

[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 supported by the
underlying transport at all.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
 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 dc972a54e93e..d2ac6291b84b 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;
@@ -1409,15 +1416,22 @@ 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
@@ -1957,6 +1971,13 @@ 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",
+				  &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 4/6] 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>
---
 drivers/firmware/arm_scmi/clock.c | 22 +++++++++++++++++++---
 include/linux/scmi_protocol.h     |  4 ++++
 2 files changed, 23 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..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 {
@@ -82,6 +83,9 @@ 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 5/6] [RFC] 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 sich information in the usual publicly
accessible clock descriptor.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
RFC tag is due to the fact that this SCMI spec Clock protocol field
addition is still to be published.
---
 drivers/firmware/arm_scmi/clock.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/arm_scmi/clock.c b/drivers/firmware/arm_scmi/clock.c
index 72f930c0e3e2..552610bfb91e 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
+		/* TODO Use CLOCK Proto Versioning once SCMI spec is updated */
+		if (t->rx.len > sizeof(attr->attributes) + SCMI_MAX_STR_SIZE)
+			clk->enable_latency =
+				le32_to_cpu(attr->clock_enable_latency);
+	} else {
 		clk->name[0] = '\0';
+	}
 
 	ph->xops->xfer_put(ph, t);
 	return ret;
-- 
2.17.1

[PATCH 6/6] [RFC] 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>
---
Tagged as RFC since dependent on a previous RFC patch
---
 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 1/6] firmware: arm_scmi: Add atomic mode support to virtio transport

[Peter Hilber]

On 24.01.22 11:03, 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.
> 

Hi Cristian,

please see one remark below.

Best regards,

Peter

> 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>
> ---
> - 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 insde 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 |   9 +-
>  drivers/firmware/arm_scmi/virtio.c | 315 ++++++++++++++++++++++++++---
>  3 files changed, 310 insertions(+), 29 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..dc972a54e93e 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,
> @@ -680,8 +681,12 @@ 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);
> +	if (priv)
> +		/* Ensure order between xfer->priv clear and later accesses */
> +		smp_store_mb(xfer->priv, NULL);
>  
>  	trace_scmi_rx_done(xfer->transfer_id, xfer->hdr.id,
>  			   xfer->hdr.protocol_id, xfer->hdr.seq,
> diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
> index fd0f6f91fc0b..39c293f0ffd3 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.
>   */
>  
>  /**
> @@ -38,6 +38,9 @@
>   * @vqueue: Associated virtqueue
>   * @cinfo: SCMI Tx or Rx channel
>   * @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_cmds_list: List of pre-fetched commands queueud for later processing
>   * @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.
> @@ -49,6 +52,9 @@ struct scmi_vio_channel {
>  	struct virtqueue *vqueue;
>  	struct scmi_chan_info *cinfo;
>  	struct list_head free_list;
> +	struct list_head pending_cmds_list;
> +	struct work_struct deferred_tx_work;
> +	struct workqueue_struct *deferred_tx_wq;
>  	bool is_rx;
>  	bool ready;
>  	unsigned int max_msg;
> @@ -58,6 +64,12 @@ struct scmi_vio_channel {
>  	spinlock_t ready_lock;
>  };
>  
> +enum poll_states {
> +	VIO_MSG_NOT_POLLED,
> +	VIO_MSG_POLLING,
> +	VIO_MSG_POLL_DONE,
> +};
> +
>  /**
>   * struct scmi_vio_msg - Transport PDU information
>   *
> @@ -65,12 +77,22 @@ 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.
> + *	      Note that since virtqueue used index is an unsigned 16-bit we can
> + *	      use some out-of-scale values to signify particular conditions.
> + * @poll_status: Polling state for this message.
> + * @poll_lock: Protect access to @poll_idx and @poll_status.
>   */
>  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_idx. */
> +	spinlock_t poll_lock;
>  };
>  
>  /* Only one SCMI VirtIO device can possibly exist */
> @@ -81,40 +103,43 @@ static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
>  	return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
>  }
>  
> +/* Expect to be called with vioch->lock acquired by the caller and IRQs off */
>  static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
>  			       struct scmi_vio_msg *msg,
>  			       struct device *dev)
>  {
>  	struct scatterlist sg_in;
>  	int rc;
> -	unsigned long flags;
>  
>  	sg_init_one(&sg_in, msg->input, VIRTIO_SCMI_MAX_PDU_SIZE);
>  
> -	spin_lock_irqsave(&vioch->lock, flags);
> -
>  	rc = virtqueue_add_inbuf(vioch->vqueue, &sg_in, 1, msg, GFP_ATOMIC);
>  	if (rc)
>  		dev_err(dev, "failed to add to RX virtqueue (%d)\n", rc);
>  	else
>  		virtqueue_kick(vioch->vqueue);
>  
> -	spin_unlock_irqrestore(&vioch->lock, flags);
> -
>  	return rc;
>  }
>  
> +/* Expect to be called with vioch->lock acquired by the caller and IRQs off */
> +static inline void scmi_vio_feed_vq_tx(struct scmi_vio_channel *vioch,
> +				       struct scmi_vio_msg *msg)
> +{
> +	spin_lock(&msg->poll_lock);
> +	msg->poll_status = VIO_MSG_NOT_POLLED;
> +	spin_unlock(&msg->poll_lock);
> +
> +	list_add(&msg->list, &vioch->free_list);
> +}
> +
>  static void scmi_finalize_message(struct scmi_vio_channel *vioch,
>  				  struct scmi_vio_msg *msg)
>  {
> -	if (vioch->is_rx) {
> +	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);
> -	}
> +	else
> +		scmi_vio_feed_vq_tx(vioch, msg);
>  }
>  
>  static void scmi_vio_complete_cb(struct virtqueue *vqueue)
> @@ -144,6 +169,7 @@ 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))
> @@ -157,7 +183,9 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
>  			scmi_rx_callback(vioch->cinfo,
>  					 msg_read_header(msg->input), msg);
>  
> +			spin_lock(&vioch->lock);
>  			scmi_finalize_message(vioch, msg);
> +			spin_unlock(&vioch->lock);
>  		}
>  
>  		/*
> @@ -176,6 +204,34 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
>  	spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
>  }
>  
> +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);
> +
> +	/* Process pre-fetched messages */
> +	spin_lock_irqsave(&vioch->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);
> +
> +		scmi_rx_callback(vioch->cinfo,
> +				 msg_read_header(msg->input), msg);
> +
> +		/* Free the processed message once done */
> +		scmi_vio_feed_vq_tx(vioch, msg);
> +	}
> +
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	/* Process possibly still pending messages */
> +	scmi_vio_complete_cb(vioch->vqueue);
> +}
> +
>  static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
>  
>  static vq_callback_t *scmi_vio_complete_callbacks[] = {
> @@ -244,6 +300,19 @@ 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;
>  
> @@ -257,6 +326,7 @@ 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);
>  		}
>  
>  		msg->input = devm_kzalloc(cinfo->dev, VIRTIO_SCMI_MAX_PDU_SIZE,
> @@ -264,13 +334,12 @@ 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 {
> +		spin_lock_irqsave(&vioch->lock, flags);
> +		if (tx)
> +			scmi_vio_feed_vq_tx(vioch, msg);
> +		else
>  			scmi_vio_feed_vq_rx(vioch, msg, cinfo->dev);
> -		}
> +		spin_unlock_irqrestore(&vioch->lock, flags);
>  	}
>  
>  	spin_lock_irqsave(&vioch->lock, flags);
> @@ -291,11 +360,22 @@ 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;
> +	void *deferred_wq = NULL;
>  
>  	spin_lock_irqsave(&vioch->ready_lock, flags);
>  	vioch->ready = false;
>  	spin_unlock_irqrestore(&vioch->ready_lock, flags);
>  
> +	spin_lock_irqsave(&vioch->lock, flags);
> +	if (!vioch->is_rx && vioch->deferred_tx_wq) {
> +		deferred_wq = vioch->deferred_tx_wq;
> +		vioch->deferred_tx_wq = NULL;
> +	}
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	if (deferred_wq)
> +		destroy_workqueue(deferred_wq);
> +
>  	scmi_free_channel(cinfo, data, id);
>  
>  	spin_lock_irqsave(&vioch->lock, flags);
> @@ -324,16 +404,33 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
>  	}
>  
>  	msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
> -	list_del(&msg->list);
> +	/* Re-init element so we can discern anytime if it is still in-flight */
> +	list_del_init(&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));
>  
> +	/*
> +	 * 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
> +	 */
> +	if (xfer->hdr.poll_completion) {
> +		spin_lock(&msg->poll_lock);
> +		msg->poll_status = VIO_MSG_POLLING;
> +		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
> +		spin_unlock(&msg->poll_lock);
> +		/* 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) {
> -		list_add(&msg->list, &vioch->free_list);
> +		/* Ensure order between xfer->priv clear and vq feeding */
> +		smp_store_mb(xfer->priv, NULL);
> +		scmi_vio_feed_vq_tx(vioch, msg);
>  		dev_err(vioch->cinfo->dev,
>  			"failed to add to TX virtqueue (%d)\n", rc);
>  	} else {
> @@ -350,10 +447,8 @@ 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,
> @@ -361,12 +456,174 @@ 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 timed-out
> + * messages by forcibly re-adding them to the free-list inside the TX code path;
> + * we instead let IRQ/RX callbacks 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, since they won't be freed elsewhere;
> + * late replies to timed-out polled messages would be anyway freed by RX
> + * callbacks instead.
> + *
> + * @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;
> +
> +	/* Must be a polled xfer and not already freed on the IRQ path */
> +	if (!xfer->hdr.poll_completion || !msg)
> +		return;
> +
> +	/* Ensure msg is unbound from xfer anyway at this point */
> +	smp_store_mb(xfer->priv, NULL);
> +
> +	/* Do not free timedout polled messages */
> +	if (ret != -ETIMEDOUT) {
> +		spin_lock_irqsave(&vioch->lock, flags);
> +		scmi_vio_feed_vq_tx(vioch, msg);
> +		spin_unlock_irqrestore(&vioch->lock, flags);
>  	}
>  }
>  
> +/**
> + * 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 what we were waiting for is to compare the newly arrived
> + * message descriptors 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
> + * helper.
> + *
> + * Note that, since we do NOT have per-message suppress notification mechanism,
> + * the message we are polling for could be delivered via usual IRQs callbacks
> + * on another core which happened to have IRQs enabled: in such 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, ret = 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;
> +
> +	spin_lock_irqsave(&vioch->lock, flags);

If now acquiring vioch->lock here, I see no need to virtqueue_poll() any more.
After checking msg->poll_status, we could just directly try virtqueue_get_buf().

On the other hand, always taking the vioch->lock in a busy loop might better be
avoided (I assumed before that taking it was omitted on purpose), since it
might hamper tx channel progress in other cores (but I'm not sure about the
actual impact).

Also, I don't yet understand why the vioch->lock would need to be taken here.

> +	spin_lock(&msg->poll_lock);
> +	/* Processed already by other polling loop on another CPU ? */
> +	if (msg->poll_status == VIO_MSG_POLL_DONE) {
> +		spin_unlock(&msg->poll_lock);
> +		spin_unlock_irqrestore(&vioch->lock, flags);
> +		return true;
> +	}
> +
> +	/* Has cmdq index moved at all ? */
> +	pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
> +	spin_unlock(&msg->poll_lock);
> +	if (!pending) {
> +		spin_unlock_irqrestore(&vioch->lock, flags);
> +		return false;
> +	}
> +
> +	virtqueue_disable_cb(vioch->vqueue);
> +
> +	/*
> +	 * If something arrived we cannot be sure, without dequeueing, if it
> +	 * was the reply to the xfer we are polling for, or, to other, even
> +	 * possibly non-polling, pending xfers: 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))) {
> +		next_msg->rx_len = length;
> +		/* Is the message we were polling for ? */
> +		if (next_msg == msg) {
> +			ret = true;
> +			break;
> +		}
> +
> +		spin_lock(&next_msg->poll_lock);
> +		if (next_msg->poll_status == VIO_MSG_NOT_POLLED) {
> +			any_prefetched++;
> +			list_add_tail(&next_msg->list,
> +				      &vioch->pending_cmds_list);
> +		} else {
> +			/* We picked another currently polled msg */
> +			next_msg->poll_status = VIO_MSG_POLL_DONE;
> +		}
> +		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 (ret) {
> +		pending = !virtqueue_enable_cb(vioch->vqueue);
> +	} else {
> +		spin_lock(&msg->poll_lock);
> +		msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
> +		pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
> +		spin_unlock(&msg->poll_lock);
> +	}
> +
> +	if (vioch->deferred_tx_wq && (any_prefetched || pending))
> +		queue_work(vioch->deferred_tx_wq, &vioch->deferred_tx_work);
> +
> +	spin_unlock_irqrestore(&vioch->lock, flags);
> +
> +	return ret;
> +}
> +
>  static const struct scmi_transport_ops scmi_virtio_ops = {
>  	.link_supplier = virtio_link_supplier,
>  	.chan_available = virtio_chan_available,
> @@ -376,6 +633,8 @@ 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)
> @@ -418,6 +677,7 @@ static int scmi_vio_probe(struct virtio_device *vdev)
>  		spin_lock_init(&channels[i].lock);
>  		spin_lock_init(&channels[i].ready_lock);
>  		INIT_LIST_HEAD(&channels[i].free_list);
> +		INIT_LIST_HEAD(&channels[i].pending_cmds_list);
>  		channels[i].vqueue = vqs[i];
>  
>  		sz = virtqueue_get_vring_size(channels[i].vqueue);
> @@ -506,4 +766,5 @@ const struct scmi_desc scmi_virtio_desc = {
>  	.max_rx_timeout_ms = 60000, /* for non-realtime virtio devices */
>  	.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),
>  };

Re: [PATCH 1/6] firmware: arm_scmi: Add atomic mode support to virtio transport

[Cristian Marussi]

On Wed, Jan 26, 2022 at 03:28:52PM +0100, Peter Hilber wrote:
> On 24.01.22 11:03, 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.
> > 
> 
> Hi Cristian,
> 

Hi Peter,

> please see one remark below.
> 
> Best regards,
> 
> Peter
> 
> > 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>
> > ---

[snip]

> > +/**
> > + * 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 what we were waiting for is to compare the newly arrived
> > + * message descriptors 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
> > + * helper.
> > + *
> > + * Note that, since we do NOT have per-message suppress notification mechanism,
> > + * the message we are polling for could be delivered via usual IRQs callbacks
> > + * on another core which happened to have IRQs enabled: in such 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, ret = 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;
> > +
> > +	spin_lock_irqsave(&vioch->lock, flags);
> 
> If now acquiring vioch->lock here, I see no need to virtqueue_poll() any more.
> After checking msg->poll_status, we could just directly try virtqueue_get_buf().
> 
> On the other hand, always taking the vioch->lock in a busy loop might better be
> avoided (I assumed before that taking it was omitted on purpose), since it
> might hamper tx channel progress in other cores (but I'm not sure about the
> actual impact).
> 
> Also, I don't yet understand why the vioch->lock would need to be taken here.

There was a race I could reproduce between the below check against
VIO_MSG_POLL_DONE and the poll_idx later update near the end of the poll loop
where another thread could have set VIO_MSG_POLL_DONE after this thread
had checked it and then this same thread would have cleared it rewriting
the new poll_idx; so at first I needlessly enlanrged the spinlocked section
(even though I knew was subtopimal given virtqueue_poll does not need
serialization) and then forget to properly review this thing.

BUT now that, following your suggestion, I introduced a dedicated
poll_status that race is gone, so I shrinked back the spinlocked section
as before and works fine (even poll_idx itself does not need to be
protected really given it can be accessed only here)

I'll post the fix in -rc2 together with the core change in the
virtio-core I proposed last week to Michael (if not too costly as perfs)

Thanks,
Cristian

Re: [PATCH 1/6] firmware: arm_scmi: Add atomic mode support to virtio transport

[Cristian Marussi]

On Thu, Jan 27, 2022 at 09:07:59AM +0000, Cristian Marussi wrote:
> On Wed, Jan 26, 2022 at 03:28:52PM +0100, Peter Hilber wrote:
> > On 24.01.22 11:03, 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.
> > > 
> > 

Hi Peter,

> > Hi Cristian,
> > 
> 
> Hi Peter,
> 
> > please see one remark below.
> > 
> > Best regards,
> > 
> > Peter
> > 
> > > 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>
> > > ---
> 
> [snip]
> 
> > > +/**
> > > + * 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 what we were waiting for is to compare the newly arrived
> > > + * message descriptors 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
> > > + * helper.
> > > + *
> > > + * Note that, since we do NOT have per-message suppress notification mechanism,
> > > + * the message we are polling for could be delivered via usual IRQs callbacks
> > > + * on another core which happened to have IRQs enabled: in such 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, ret = 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;
> > > +
> > > +	spin_lock_irqsave(&vioch->lock, flags);
> > 
> > If now acquiring vioch->lock here, I see no need to virtqueue_poll() any more.
> > After checking msg->poll_status, we could just directly try virtqueue_get_buf().
> > 
> > On the other hand, always taking the vioch->lock in a busy loop might better be
> > avoided (I assumed before that taking it was omitted on purpose), since it
> > might hamper tx channel progress in other cores (but I'm not sure about the
> > actual impact).
> > 
> > Also, I don't yet understand why the vioch->lock would need to be taken here.
> 
> There was a race I could reproduce between the below check against
> VIO_MSG_POLL_DONE and the poll_idx later update near the end of the poll loop
> where another thread could have set VIO_MSG_POLL_DONE after this thread
> had checked it and then this same thread would have cleared it rewriting
> the new poll_idx; so at first I needlessly enlanrged the spinlocked section
> (even though I knew was subtopimal given virtqueue_poll does not need
> serialization) and then forget to properly review this thing.
> 
> BUT now that, following your suggestion, I introduced a dedicated
> poll_status that race is gone, so I shrinked back the spinlocked section
> as before and works fine (even poll_idx itself does not need to be
> protected really given it can be accessed only here)
> 
> I'll post the fix in -rc2 together with the core change in the
> virtio-core I proposed last week to Michael (if not too costly as perfs)
> 

Looking again at the polling support, beside the above fixes, I realized that
also the handling of the virtqueue ready status was not addressed at all
in polling mode, so I got rid of it and introduced a dedicated refcount
mechanism that should serve the same purpose; moreover I shrinked a bit the
main spinlocked areas introducing dedicated spinlocks for free_list and
pending_lists. (and removed some redundant locks on msg...hopefully :P)

..and everuything still works apparently in my test scenario ...
( but ....  you know... :P)

So just a heads up that the next series will include a couple of
preparatory patches, and the core virtio_ring fix for the ABA problem,
before the virtio polling one.

Thanks,
Cristian