[RFC PATCH v2 2/8] firmware: arm_scmi: Add support for atomic transports

[Cristian Marussi]

An SCMI transport can declare itself as .atomic_capable in order to signal
to the SCMI core that all its transmit path can be executed in atomic
context: the core as a consequence will take care not to sleep to in the
corresponding rx path while waiting for a response or a delayed response.

Signed-off-by: Cristian Marussi <cristian.marussi@xxxxxxx>
---
 drivers/firmware/arm_scmi/common.h |   3 +
 drivers/firmware/arm_scmi/driver.c | 168 ++++++++++++++++++++++-------
 2 files changed, 132 insertions(+), 39 deletions(-)

diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index eb45f2dced03..9be04e4bb2f5 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -395,6 +395,8 @@ struct scmi_device *scmi_child_dev_find(struct device *parent,
  *			    proper related assumptions.
  * @force_polling: Flag to force this whole transport to use SCMI core polling
  *		   mechanism instead of completion interrupts even if available.
+ * @atomic_capable: Flag to indicate that this transport is assured not to sleep
+ *		    on the TX path.
  */
 struct scmi_desc {
 	int (*init)(void);
@@ -405,6 +407,7 @@ struct scmi_desc {
 	int max_msg_size;
 	bool using_xfers_delegation;
 	bool force_polling;
+	bool atomic_capable;
 };
 
 #ifdef CONFIG_ARM_SCMI_TRANSPORT_MAILBOX
diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index d1814c58f4ba..2e920df73e46 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -839,6 +839,10 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo, u32 msg_hdr)
 		info->desc->ops->clear_channel(cinfo);
 		complete(xfer->async_done);
 	} else {
+		/*
+		 * This same xfer->done completion is used in atomic mode as a
+		 * flag for polling.
+		 */
 		complete(&xfer->done);
 	}
 
@@ -890,8 +894,6 @@ static void xfer_put(const struct scmi_protocol_handle *ph,
 	__scmi_xfer_put(&info->tx_minfo, xfer);
 }
 
-#define SCMI_MAX_POLL_TO_NS	(100 * NSEC_PER_USEC)
-
 static bool scmi_xfer_done_no_timeout(struct scmi_chan_info *cinfo,
 				      struct scmi_xfer *xfer, ktime_t stop)
 {
@@ -906,6 +908,90 @@ static bool scmi_xfer_done_no_timeout(struct scmi_chan_info *cinfo,
 	       ktime_after(ktime_get(), stop);
 }
 
+static bool xfer_complete_or_timeout(struct completion *done, ktime_t stop)
+{
+	return try_wait_for_completion(done) || ktime_after(ktime_get(), stop);
+}
+
+static int spin_for_completion_timeout(struct completion *done, int timeout_ms)
+{
+	ktime_t stop = ktime_add_ms(ktime_get(), timeout_ms);
+
+	spin_until_cond(xfer_complete_or_timeout(done, stop));
+	if (ktime_after(ktime_get(), stop))
+		return -ETIMEDOUT;
+
+	return 0;
+}
+
+/**
+ * scmi_wait_for_message_response  - An helper to group all the possible ways of
+ * waiting for a synchronous message response.
+ *
+ * @cinfo: SCMI channel info
+ * @xfer: Reference to the transfer being waited for.
+ *
+ * Chooses waiting strategy (sleep-waiting vs busy-waiting) depending on flags
+ * configuration like xfer->hdr.poll_completion and scmi_desc.atomic.capable.
+ *
+ * Return: 0 on Success, error otherwise.
+ */
+static int scmi_wait_for_message_response(struct scmi_chan_info *cinfo,
+					  struct scmi_xfer *xfer)
+{
+	struct scmi_info *info = handle_to_scmi_info(cinfo->handle);
+	struct device *dev = info->dev;
+	int ret = 0, timeout_ms = info->desc->max_rx_timeout_ms;
+
+	if (!xfer->hdr.poll_completion) {
+		if (!info->desc->atomic_capable) {
+			if (!wait_for_completion_timeout(&xfer->done,
+							 msecs_to_jiffies(timeout_ms))) {
+				dev_err(dev, "timed out in resp(caller: %pS)\n",
+					(void *)_RET_IP_);
+				ret = -ETIMEDOUT;
+			}
+		} else {
+			/* Poll on xfer->done waiting for completion by interrupt */
+			ret = spin_for_completion_timeout(&xfer->done,
+							  timeout_ms);
+			if (ret)
+				dev_err(dev,
+					"timed out in resp(caller: %pS) - atomic\n",
+					(void *)_RET_IP_);
+		}
+	} else {
+		/*
+		 * Poll on xfer using transport provided .poll_done();
+		 * assumes no completion interrupt was available.
+		 */
+		ktime_t stop = ktime_add_ms(ktime_get(), timeout_ms);
+
+		spin_until_cond(scmi_xfer_done_no_timeout(cinfo, xfer, stop));
+		if (ktime_before(ktime_get(), stop)) {
+			unsigned long flags;
+
+			/*
+			 * Do not fetch_response if an out-of-order delayed
+			 * response is being processed.
+			 */
+			spin_lock_irqsave(&xfer->lock, flags);
+			if (xfer->state == SCMI_XFER_SENT_OK) {
+				info->desc->ops->fetch_response(cinfo, xfer);
+				xfer->state = SCMI_XFER_RESP_OK;
+			}
+			spin_unlock_irqrestore(&xfer->lock, flags);
+		} else {
+			dev_err(dev,
+				"timed out in resp(caller: %pS) - polling\n",
+				(void *)_RET_IP_);
+			ret = -ETIMEDOUT;
+		}
+	}
+
+	return ret;
+}
+
 /**
  * do_xfer() - Do one transfer
  *
@@ -920,7 +1006,6 @@ static int do_xfer(const struct scmi_protocol_handle *ph,
 		   struct scmi_xfer *xfer)
 {
 	int ret;
-	int timeout;
 	const struct scmi_protocol_instance *pi = ph_to_pi(ph);
 	struct scmi_info *info = handle_to_scmi_info(pi->handle);
 	struct device *dev = info->dev;
@@ -952,37 +1037,7 @@ static int do_xfer(const struct scmi_protocol_handle *ph,
 		return ret;
 	}
 
-	if (xfer->hdr.poll_completion) {
-		ktime_t stop = ktime_add_ns(ktime_get(), SCMI_MAX_POLL_TO_NS);
-
-		spin_until_cond(scmi_xfer_done_no_timeout(cinfo, xfer, stop));
-
-		if (ktime_before(ktime_get(), stop)) {
-			unsigned long flags;
-
-			/*
-			 * Do not fetch_response if an out-of-order delayed
-			 * response is being processed.
-			 */
-			spin_lock_irqsave(&xfer->lock, flags);
-			if (xfer->state == SCMI_XFER_SENT_OK) {
-				info->desc->ops->fetch_response(cinfo, xfer);
-				xfer->state = SCMI_XFER_RESP_OK;
-			}
-			spin_unlock_irqrestore(&xfer->lock, flags);
-		} else {
-			ret = -ETIMEDOUT;
-		}
-	} else {
-		/* And we wait for the response. */
-		timeout = msecs_to_jiffies(info->desc->max_rx_timeout_ms);
-		if (!wait_for_completion_timeout(&xfer->done, timeout)) {
-			dev_err(dev, "timed out in resp(caller: %pS)\n",
-				(void *)_RET_IP_);
-			ret = -ETIMEDOUT;
-		}
-	}
-
+	ret = scmi_wait_for_message_response(cinfo, xfer);
 	if (!ret && xfer->hdr.status)
 		ret = scmi_to_linux_errno(xfer->hdr.status);
 
@@ -1004,7 +1059,7 @@ static void reset_rx_to_maxsz(const struct scmi_protocol_handle *ph,
 	xfer->rx.len = info->desc->max_msg_size;
 }
 
-#define SCMI_MAX_RESPONSE_TIMEOUT	(2 * MSEC_PER_SEC)
+#define SCMI_DRESP_TIMEOUT	(2 * MSEC_PER_SEC)
 
 /**
  * do_xfer_with_response() - Do one transfer and wait until the delayed
@@ -1013,22 +1068,57 @@ static void reset_rx_to_maxsz(const struct scmi_protocol_handle *ph,
  * @ph: Pointer to SCMI protocol handle
  * @xfer: Transfer to initiate and wait for response
  *
+ * Avois sleeping in favour of busy-waiting if the underlying transport was
+ * declared as .atomic_capable.
+ *
+ * Note that using asynchronous commands when running on top of atomic
+ * transports should be avoided since it could cause long busy-waiting here,
+ * but, once a transport is declared atomic, upper layers using the SCMI stack
+ * can freely make assumptions about the 'non-sleeping' nature of the stack
+ * (e.g. Clock framework) and it cannot be excluded that asynchronous commands
+ * could be exposed by the platform and so used.
+ *
+ * The only other option would have been to refrain from using any asynchronous
+ * command even if made available, when an atomic transport is detected, and
+ * instead forcibly use the synchronous version (thing that can be easily
+ * attained at the protocol layer), but this would also have led to longer
+ * stalls of the channel for synchronous commands and possibly timeouts.
+ * (in other words there is usually a good reason if a platform provides an
+ *  asynchronous version of a command and we should prefer to use it)
+ *
  * Return: -ETIMEDOUT in case of no delayed response, if transmit error,
  *	return corresponding error, else if all goes well, return 0.
  */
 static int do_xfer_with_response(const struct scmi_protocol_handle *ph,
 				 struct scmi_xfer *xfer)
 {
-	int ret, timeout = msecs_to_jiffies(SCMI_MAX_RESPONSE_TIMEOUT);
+	int ret, timeout = msecs_to_jiffies(SCMI_DRESP_TIMEOUT);
+	const struct scmi_protocol_instance *pi = ph_to_pi(ph);
+	struct scmi_info *info = handle_to_scmi_info(pi->handle);
 	DECLARE_COMPLETION_ONSTACK(async_response);
 
 	xfer->async_done = &async_response;
 
 	ret = do_xfer(ph, xfer);
 	if (!ret) {
-		if (!wait_for_completion_timeout(xfer->async_done, timeout))
-			ret = -ETIMEDOUT;
-		else if (xfer->hdr.status)
+		if (!info->desc->atomic_capable) {
+			if (!wait_for_completion_timeout(xfer->async_done,
+							 timeout)) {
+				dev_err(ph->dev,
+					"timed out in delayed resp(caller: %pS)\n",
+					(void *)_RET_IP_);
+				ret = -ETIMEDOUT;
+			}
+		} else {
+			ret = spin_for_completion_timeout(xfer->async_done,
+							  SCMI_DRESP_TIMEOUT);
+			if (ret)
+				dev_err(ph->dev,
+					"timed out in delayed resp(caller: %pS) - atomic\n",
+					(void *)_RET_IP_);
+		}
+
+		if (!ret && xfer->hdr.status)
 			ret = scmi_to_linux_errno(xfer->hdr.status);
 	}
 
-- 
2.17.1