From:   Cristian Marussi <cristian.marussi@arm.com>
To:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Cc:     sudeep.holla@arm.com, james.quinlan@broadcom.com,
        Jonathan.Cameron@Huawei.com, f.fainelli@gmail.com,
        etienne.carriere@linaro.org, vincent.guittot@linaro.org,
        souvik.chakravarty@arm.com, peter.hilber@opensynergy.com,
        igor.skalkin@opensynergy.com, cristian.marussi@arm.com,
        "Michael S. Tsirkin" <mst@redhat.com>,
        virtualization@lists.linux-foundation.org
Subject: [PATCH v9 09/11] firmware: arm_scmi: Add atomic mode support to virtio transport
Date:   Tue, 21 Dec 2021 14:00:27 +0000
Message-Id: <20211221140027.41524-1-cristian.marussi@arm.com>
In-Reply-To: <20211220195646.44498-10-cristian.marussi@arm.com>
References: <20211220195646.44498-10-cristian.marussi@arm.com>
Precedence: bulk

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>
---
v8 --> v9
- check for deferred_wq existence before queueing work to avoid
  race at driver removal time
---
 drivers/firmware/arm_scmi/Kconfig  |  15 ++
 drivers/firmware/arm_scmi/virtio.c | 298 ++++++++++++++++++++++++++---
 2 files changed, 287 insertions(+), 26 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..e54d14971c07 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;
@@ -65,12 +71,22 @@ 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_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;
+#define VIO_MSG_NOT_POLLED	0xeeeeeeeeUL
+#define VIO_MSG_POLL_DONE	0xffffffffUL
+	unsigned int poll_idx;
+	/* lock to protect access to poll_idx. */
+	spinlock_t poll_lock;
 };
 
 /* Only one SCMI VirtIO device can possibly exist */
@@ -81,40 +97,43 @@ 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_idx = 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 +163,7 @@ 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 +177,9 @@ 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 +198,34 @@ 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 +294,19 @@ 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 +320,7 @@ 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 +328,12 @@ 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 +354,22 @@ 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);
@@ -324,7 +398,8 @@ 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 +412,19 @@ 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 {
+		/*
+		 * 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);
+			/* Ensure initialized msg is visibly bound to xfer */
+			smp_store_mb(xfer->priv, msg);
+		}
 		virtqueue_kick(vioch->vqueue);
 	}
 
@@ -350,10 +438,8 @@ 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,
@@ -361,10 +447,166 @@ 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 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, even 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 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 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;
+
+	/* Ensure msg is unbound from xfer before pushing onto the free list  */
+	smp_store_mb(xfer->priv, NULL);
+
+	/* Is a successfully completed polled message still to be finalized ? */
+	spin_lock_irqsave(&vioch->lock, flags);
+	if (!ret && xfer->hdr.poll_completion && list_empty(&msg->list))
+		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 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, on another core which are IRQs-on, 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(&msg->poll_lock, flags);
+	/* Processed already by other polling loop on another CPU ? */
+	if (msg->poll_idx == VIO_MSG_POLL_DONE) {
+		spin_unlock_irqrestore(&msg->poll_lock, flags);
+		return true;
+	}
+
+	/* Has cmdq index moved at all ? */
+	pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+	spin_unlock_irqrestore(&msg->poll_lock, flags);
+	if (!pending)
+		return false;
+
+	spin_lock_irqsave(&vioch->lock, flags);
+	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_idx == VIO_MSG_NOT_POLLED) {
+			any_prefetched++;
+			list_add_tail(&next_msg->list,
+				      &vioch->pending_cmds_list);
+		} else {
+			next_msg->poll_idx = 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 = {
@@ -376,6 +618,8 @@ 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 +662,7 @@ 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 +751,5 @@ 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