Date:   Mon, 12 Jul 2021 17:26:19 +0100
From:   Cristian Marussi <cristian.marussi@....com>
To:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     sudeep.holla@....com, james.quinlan@...adcom.com,
        Jonathan.Cameron@...wei.com, f.fainelli@...il.com,
        etienne.carriere@...aro.org, vincent.guittot@...aro.org,
        souvik.chakravarty@....com, cristian.marussi@....com
Subject: [PATCH v3 2/8] firmware: arm_scmi: Add support for atomic transports

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@....com>
---
 drivers/firmware/arm_scmi/common.h |   3 +
 drivers/firmware/arm_scmi/driver.c | 185 +++++++++++++++++++++--------
 2 files changed, 140 insertions(+), 48 deletions(-)

diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index 2b5fa351d89f..d203ed408dde 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h
@@ -392,6 +392,8 @@ struct scmi_device *scmi_child_dev_find(struct device *parent,
  * @max_msg_size: Maximum size of data per message that can be handled.
  * @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);
@@ -401,6 +403,7 @@ struct scmi_desc {
 	int max_msg;
 	int max_msg_size;
 	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 36cb7a12d72f..a2a62910e805 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -676,6 +676,10 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
 		scmi_clear_channel(info, cinfo);
 		complete(xfer->async_done);
 	} else {
+		/*
+		 * This same xfer->done completion is used in atomic mode as a
+		 * flag for polling.
+		 */
 		complete(&xfer->done);
 	}
 
@@ -728,8 +732,6 @@ 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)
 {
@@ -744,6 +746,98 @@ 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 {
+		if (info->desc->ops->poll_done) {
+			/*
+			 * 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;
+			}
+		} else {
+			dev_warn_once(dev,
+				      "Polling mode is not supported by transport.\n");
+			ret = EINVAL;
+		}
+	}
+
+	return ret;
+}
+
 /**
  * do_xfer() - Do one transfer
  *
@@ -758,7 +852,6 @@ 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;
@@ -790,46 +883,7 @@ static int do_xfer(const struct scmi_protocol_handle *ph,
 		return ret;
 	}
 
-	if (xfer->hdr.poll_completion) {
-		if (info->desc->ops->poll_done) {
-			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 {
-			dev_warn_once(dev,
-				      "Polling mode is not supported by transport.\n");
-			ret = EINVAL;
-		}
-	} 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);
 
@@ -851,7 +905,7 @@ 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
@@ -860,22 +914,57 @@ 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

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