[PATCH v7 14/16] firmware: arm_scmi: Add atomic mode support to virtio transport

[Cristian Marussi <cristian.marussi@arm.com>]

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>
---
V6 --> V7
- added a few comments about virtio polling internals
- fixed missing list_del on pending_cmds_list processing
- shrinked spinlocked areas in virtio_poll_done
- added proper spinlocking to scmi_vio_complete_cb while scanning list
  of pending cmds
---
 drivers/firmware/arm_scmi/Kconfig  |  15 ++
 drivers/firmware/arm_scmi/virtio.c | 241 +++++++++++++++++++++++++++--
 2 files changed, 243 insertions(+), 13 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/virtio.c b/drivers/firmware/arm_scmi/virtio.c
index fd0f6f91fc0b..0598e185a786 100644
--- a/drivers/firmware/arm_scmi/virtio.c
+++ b/drivers/firmware/arm_scmi/virtio.c
@@ -38,6 +38,7 @@
  * @vqueue: Associated virtqueue
  * @cinfo: SCMI Tx or Rx channel
  * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
+ * @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 +50,9 @@ struct scmi_vio_channel {
 	struct virtqueue *vqueue;
 	struct scmi_chan_info *cinfo;
 	struct list_head free_list;
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+	struct list_head pending_cmds_list;
+#endif
 	bool is_rx;
 	bool ready;
 	unsigned int max_msg;
@@ -65,12 +69,22 @@ 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 virtqueue used index is an unsigned 16-bit.
+ * @poll_lock: Protect access to @poll_idx.
  */
 struct scmi_vio_msg {
 	struct scmi_msg_payld *request;
 	struct scmi_msg_payld *input;
 	struct list_head list;
 	unsigned int rx_len;
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+#define VIO_MSG_POLL_DONE	0xffffffffUL
+	unsigned int poll_idx;
+	/* lock to protect access to poll_idx. */
+	spinlock_t poll_lock;
+#endif
 };
 
 /* Only one SCMI VirtIO device can possibly exist */
@@ -104,17 +118,22 @@ static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
 	return rc;
 }
 
+static inline void scmi_vio_feed_vq_tx(struct scmi_vio_channel *vioch,
+				       struct scmi_vio_msg *msg)
+{
+	/* 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);
+}
+
 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)
@@ -140,6 +159,26 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 
 		/* IRQs already disabled here no need to irqsave */
 		spin_lock(&vioch->lock);
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+		/* At first scan the list of possibly pre-fetched messages */
+		if (!vioch->is_rx) {
+			struct scmi_vio_msg *tmp;
+
+			list_for_each_entry_safe(msg, tmp,
+						 &vioch->pending_cmds_list,
+						 list) {
+				list_del(&msg->list);
+				spin_unlock(&vioch->lock);
+
+				scmi_rx_callback(vioch->cinfo,
+						 msg_read_header(msg->input),
+						 msg);
+				/* Free the processed message once done */
+				spin_lock(&vioch->lock);
+				list_add(&msg->list, &vioch->free_list);
+			}
+		}
+#endif
 		if (cb_enabled) {
 			virtqueue_disable_cb(vqueue);
 			cb_enabled = false;
@@ -257,6 +296,9 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
 						    GFP_KERNEL);
 			if (!msg->request)
 				return -ENOMEM;
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+			spin_lock_init(&msg->poll_lock);
+#endif
 		}
 
 		msg->input = devm_kzalloc(cinfo->dev, VIRTIO_SCMI_MAX_PDU_SIZE,
@@ -324,7 +366,8 @@ 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);
 
@@ -337,6 +380,20 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
 		dev_err(vioch->cinfo->dev,
 			"failed to add to TX virtqueue (%d)\n", rc);
 	} else {
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+		/*
+		 * 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_idx =
+				virtqueue_enable_cb_prepare(vioch->vqueue);
+			spin_unlock(&msg->poll_lock);
+			xfer->priv = msg;
+		}
+#endif
 		virtqueue_kick(vioch->vqueue);
 	}
 
@@ -350,10 +407,8 @@ 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,
@@ -361,11 +416,163 @@ 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;
+}
+
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+/**
+ * virtio_mark_txdone  - Mark transmission done
+ *
+ * Free only successfully 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 on timeout 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 beeen returned
+ * 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 message exhaustion.
+ *
+ * For this same reason, here, we take care to free only the successfully
+ * completed polled messages, 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;
+
+	if (!msg)
+		return;
+
+	/* Is a successfully completed polled message still to be finalized ? */
+	spin_lock_irqsave(&msg->poll_lock, flags);
+	if (!ret && xfer->hdr.poll_completion && list_empty(&msg->list))
+		scmi_vio_feed_vq_tx(vioch, msg);
+	spin_unlock_irqrestore(&msg->poll_lock, flags);
+
+	xfer->priv = NULL;
+}
+
+/**
+ * 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 within the
+ * usual VirtIO callbacks; 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 empty the virtqueue.
+ *
+ * Note that we do NOT suppress notification with VIRTQ_USED_F_NO_NOTIFY even
+ * when polling since such flag is per-virtqueues and we do not want to
+ * suppress notifications as a whole: so, if the message we are polling for is
+ * delivered via usual IRQs callbacks, it will be handled as such 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 ret;
+	unsigned int poll_idx;
+	unsigned long flags;
+	struct scmi_vio_msg *msg = xfer->priv;
+	struct scmi_vio_channel *vioch = cinfo->transport_info;
+
+	if (!msg)
+		return true;
+
+	/*
+	 * Keep the spinlocked region as small as possible: don't care if
+	 * missing something this time it will be polled again next.
+	 */
+	spin_lock_irqsave(&msg->poll_lock, flags);
+	poll_idx = msg->poll_idx;
+	spin_unlock_irqrestore(&msg->poll_lock, flags);
+
+	/* Processed already by other polling loop on another CPU ? */
+	if (poll_idx == VIO_MSG_POLL_DONE)
+		return true;
+
+	/* Has cmdq index moved at all ? */
+	ret = virtqueue_poll(vioch->vqueue, poll_idx);
+	if (ret) {
+		struct scmi_vio_msg *next_msg;
+
+		spin_lock_irqsave(&vioch->lock, flags);
+		virtqueue_disable_cb(vioch->vqueue);
+		/*
+		 * If something arrived we cannot be sure if it was the reply to
+		 * the xfer we are polling for, or some replies to other, even
+		 * possibly non-polling, pending xfers: process all new messages
+		 * till the polled-for message is found OR the vqueue is empty.
+		 */
+		do {
+			unsigned int length;
+
+			next_msg = virtqueue_get_buf(vioch->vqueue, &length);
+			if (next_msg) {
+				next_msg->rx_len = length;
+				if (next_msg == msg) {
+					ret = true;
+					break;
+				}
+
+				list_add_tail(&next_msg->list,
+					      &vioch->pending_cmds_list);
+				spin_lock(&next_msg->poll_lock);
+				next_msg->poll_idx = VIO_MSG_POLL_DONE;
+				spin_unlock(&next_msg->poll_lock);
+				ret = false;
+			}
+		} while (next_msg);
+
+		/*
+		 * When the polling loop has successfully terminated simply
+		 * restart the vqueue, no matter if something else was queued
+		 * in the meantime, it will be served by normal IRQ/callback
+		 * or by the next poll loop.
+		 *
+		 * Update the polling index to the current vqueue last used
+		 * index, if still looking for a reply.
+		 */
+		if (ret) {
+			virtqueue_enable_cb(vioch->vqueue);
+		} else {
+			spin_lock(&msg->poll_lock);
+			msg->poll_idx =
+				virtqueue_enable_cb_prepare(vioch->vqueue);
+			spin_unlock(&msg->poll_lock);
+		}
+		spin_unlock_irqrestore(&vioch->lock, flags);
 	}
+
+	return ret;
 }
+#endif
 
 static const struct scmi_transport_ops scmi_virtio_ops = {
 	.link_supplier = virtio_link_supplier,
@@ -376,6 +583,10 @@ static const struct scmi_transport_ops scmi_virtio_ops = {
 	.send_message = virtio_send_message,
 	.fetch_response = virtio_fetch_response,
 	.fetch_notification = virtio_fetch_notification,
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+	.mark_txdone = virtio_mark_txdone,
+	.poll_done = virtio_poll_done,
+#endif
 };
 
 static int scmi_vio_probe(struct virtio_device *vdev)
@@ -418,6 +629,9 @@ 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);
+#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+		INIT_LIST_HEAD(&channels[i].pending_cmds_list);
+#endif
 		channels[i].vqueue = vqs[i];
 
 		sz = virtqueue_get_vring_size(channels[i].vqueue);
@@ -506,4 +720,5 @@ 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