[PATCH v2 0/4] drm/dp_mst: drm_dp_check_act_status() fixes

[PATCH v2 0/4] drm/dp_mst: drm_dp_check_act_status() fixes

[Lyude Paul]

Noticed this while fixing some unrelated issues with NAKs being dropped
- we don't wait nearly long enough to receive ACTs from MST hubs in some
situations. Also, we take the time to refactor this function a bit.

This fixes some ACT timeouts I observed on an EVGA MST hub with i915.

Lyude Paul (4):
  drm/dp_mst: Improve kdocs for drm_dp_check_act_status()
  drm/dp_mst: Reformat drm_dp_check_act_status() a bit
  drm/dp_mst: Increase ACT retry timeout to 3s
  drm/dp_mst: Print errors on ACT timeouts

 drivers/gpu/drm/drm_dp_mst_topology.c | 71 +++++++++++++++++----------
 1 file changed, 44 insertions(+), 27 deletions(-)

-- 
2.25.1

[PATCH v2 1/4] drm/dp_mst: Improve kdocs for drm_dp_check_act_status()

[Lyude Paul]

No functional changes.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Sean Paul <sean@poorly.run>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 5b205aea58d4..828ca63cc576 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -4462,10 +4462,14 @@ static int drm_dp_dpcd_write_payload(struct drm_dp_mst_topology_mgr *mgr,
 
 
 /**
- * drm_dp_check_act_status() - Check ACT handled status.
+ * drm_dp_check_act_status() - Polls for ACT handled status.
  * @mgr: manager to use
  *
- * Check the payload status bits in the DPCD for ACT handled completion.
+ * Tries waiting for the MST hub to finish updating it's payload table by
+ * polling for the ACT handled bit.
+ *
+ * Returns:
+ * 0 if the ACT was handled in time, negative error code on failure.
  */
 int drm_dp_check_act_status(struct drm_dp_mst_topology_mgr *mgr)
 {
-- 
2.25.1

[PATCH v2 2/4] drm/dp_mst: Reformat drm_dp_check_act_status() a bit

[Lyude Paul]

Just add a bit more line wrapping, get rid of some extraneous
whitespace, remove an unneeded goto label, and move around some variable
declarations. No functional changes here.

Signed-off-by: Lyude Paul <lyude@redhat.com>
[this isn't a fix, but it's needed for the fix that comes after this]
Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
Cc: Sean Paul <sean@poorly.run>
Cc: <stable@vger.kernel.org> # v3.17+
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 828ca63cc576..c83adbdfc1cd 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -4473,33 +4473,31 @@ static int drm_dp_dpcd_write_payload(struct drm_dp_mst_topology_mgr *mgr,
  */
 int drm_dp_check_act_status(struct drm_dp_mst_topology_mgr *mgr)
 {
+	int count = 0, ret;
 	u8 status;
-	int ret;
-	int count = 0;
 
 	do {
-		ret = drm_dp_dpcd_readb(mgr->aux, DP_PAYLOAD_TABLE_UPDATE_STATUS, &status);
-
+		ret = drm_dp_dpcd_readb(mgr->aux,
+					DP_PAYLOAD_TABLE_UPDATE_STATUS,
+					&status);
 		if (ret < 0) {
-			DRM_DEBUG_KMS("failed to read payload table status %d\n", ret);
-			goto fail;
+			DRM_DEBUG_KMS("failed to read payload table status %d\n",
+				      ret);
+			return ret;
 		}
 
 		if (status & DP_PAYLOAD_ACT_HANDLED)
 			break;
 		count++;
 		udelay(100);
-
 	} while (count < 30);
 
 	if (!(status & DP_PAYLOAD_ACT_HANDLED)) {
-		DRM_DEBUG_KMS("failed to get ACT bit %d after %d retries\n", status, count);
-		ret = -EINVAL;
-		goto fail;
+		DRM_DEBUG_KMS("failed to get ACT bit %d after %d retries\n",
+			      status, count);
+		return -EINVAL;
 	}
 	return 0;
-fail:
-	return ret;
 }
 EXPORT_SYMBOL(drm_dp_check_act_status);
 
-- 
2.25.1

[PATCH v2 3/4] drm/dp_mst: Increase ACT retry timeout to 3s

[Lyude Paul]

Currently we only poll for an ACT up to 30 times, with a busy-wait delay
of 100µs between each attempt - giving us a timeout of 2900µs. While
this might seem sensible, it would appear that in certain scenarios it
can take dramatically longer then that for us to receive an ACT. On one
of the EVGA MST hubs that I have available, I observed said hub
sometimes taking longer then a second before signalling the ACT. These
delays mostly seem to occur when previous sideband messages we've sent
are NAKd by the hub, however it wouldn't be particularly surprising if
it's possible to reproduce times like this simply by introducing branch
devices with large LCTs since payload allocations have to take effect on
every downstream device up to the payload's target.

So, instead of just retrying 30 times we poll for the ACT for up to 3ms,
and additionally use usleep_range() to avoid a very long and rude
busy-wait. Note that the previous retry count of 30 appears to have been
arbitrarily chosen, as I can't find any mention of a recommended timeout
or retry count for ACTs in the DisplayPort 2.0 specification. This also
goes for the range we were previously using for udelay(), although I
suspect that was just copied from the recommended delay for link
training on SST devices.

Changes since v1:
* Use readx_poll_timeout() instead of open-coding timeout loop - Sean
  Paul

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
Cc: Sean Paul <sean@poorly.run>
Cc: <stable@vger.kernel.org> # v3.17+
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 57 ++++++++++++++++-----------
 1 file changed, 34 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index c83adbdfc1cd..ce61964baa7c 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -27,6 +27,7 @@
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/seq_file.h>
+#include <linux/iopoll.h>
 
 #if IS_ENABLED(CONFIG_DRM_DEBUG_DP_MST_TOPOLOGY_REFS)
 #include <linux/stacktrace.h>
@@ -4460,43 +4461,53 @@ static int drm_dp_dpcd_write_payload(struct drm_dp_mst_topology_mgr *mgr,
 	return ret;
 }
 
+static int do_get_act_status(struct drm_dp_aux *aux)
+{
+	int ret;
+	u8 status;
+
+	ret = drm_dp_dpcd_readb(aux, DP_PAYLOAD_TABLE_UPDATE_STATUS, &status);
+	if (ret < 0)
+		return ret;
+
+	return status;
+}
 
 /**
  * drm_dp_check_act_status() - Polls for ACT handled status.
  * @mgr: manager to use
  *
  * Tries waiting for the MST hub to finish updating it's payload table by
- * polling for the ACT handled bit.
+ * polling for the ACT handled bit for up to 3 seconds (yes-some hubs really
+ * take that long).
  *
  * Returns:
  * 0 if the ACT was handled in time, negative error code on failure.
  */
 int drm_dp_check_act_status(struct drm_dp_mst_topology_mgr *mgr)
 {
-	int count = 0, ret;
-	u8 status;
-
-	do {
-		ret = drm_dp_dpcd_readb(mgr->aux,
-					DP_PAYLOAD_TABLE_UPDATE_STATUS,
-					&status);
-		if (ret < 0) {
-			DRM_DEBUG_KMS("failed to read payload table status %d\n",
-				      ret);
-			return ret;
-		}
-
-		if (status & DP_PAYLOAD_ACT_HANDLED)
-			break;
-		count++;
-		udelay(100);
-	} while (count < 30);
-
-	if (!(status & DP_PAYLOAD_ACT_HANDLED)) {
-		DRM_DEBUG_KMS("failed to get ACT bit %d after %d retries\n",
-			      status, count);
+	/*
+	 * There doesn't seem to be any recommended retry count or timeout in
+	 * the MST specification. Since some hubs have been observed to take
+	 * over 1 second to update their payload allocations under certain
+	 * conditions, we use a rather large timeout value.
+	 */
+	const int timeout_ms = 3000;
+	int ret, status;
+
+	ret = readx_poll_timeout(do_get_act_status, mgr->aux, status,
+				 status & DP_PAYLOAD_ACT_HANDLED || status < 0,
+				 100, timeout_ms * USEC_PER_MSEC);
+	if (ret < 0 && status >= 0) {
+		DRM_DEBUG_KMS("Failed to get ACT bit %d after %dms\n",
+			      status, timeout_ms);
 		return -EINVAL;
+	} else if (status < 0) {
+		DRM_DEBUG_KMS("Failed to read payload table status: %d\n",
+			      status);
+		return status;
 	}
+
 	return 0;
 }
 EXPORT_SYMBOL(drm_dp_check_act_status);
-- 
2.25.1

[PATCH v2 4/4] drm/dp_mst: Print errors on ACT timeouts

[Lyude Paul]

Although it's not unexpected for drm_dp_check_act_status() to fail due
to DPCD read failures (as the hub may have just been unplugged
suddenly), timeouts are a bit more worrying as they either mean we need
a longer timeout value, or we aren't setting up payload allocations
properly. So, let's start printing errors on timeouts.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Sean Paul <sean@poorly.run>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index ce61964baa7c..0cbeb0f5c834 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -4499,10 +4499,14 @@ int drm_dp_check_act_status(struct drm_dp_mst_topology_mgr *mgr)
 				 status & DP_PAYLOAD_ACT_HANDLED || status < 0,
 				 100, timeout_ms * USEC_PER_MSEC);
 	if (ret < 0 && status >= 0) {
-		DRM_DEBUG_KMS("Failed to get ACT bit %d after %dms\n",
-			      status, timeout_ms);
+		DRM_ERROR("Failed to get ACT after %dms, last status: %02x\n",
+			  timeout_ms, status);
 		return -EINVAL;
 	} else if (status < 0) {
+		/*
+		 * Failure here isn't unexpected - the hub may have
+		 * just been unplugged
+		 */
 		DRM_DEBUG_KMS("Failed to read payload table status: %d\n",
 			      status);
 		return status;
-- 
2.25.1

Re: [PATCH v2 3/4] drm/dp_mst: Increase ACT retry timeout to 3s

[Sean Paul]

On Mon, Apr 6, 2020 at 6:13 PM Lyude Paul <lyude@redhat.com> wrote:
>
> Currently we only poll for an ACT up to 30 times, with a busy-wait delay
> of 100µs between each attempt - giving us a timeout of 2900µs. While
> this might seem sensible, it would appear that in certain scenarios it
> can take dramatically longer then that for us to receive an ACT. On one
> of the EVGA MST hubs that I have available, I observed said hub
> sometimes taking longer then a second before signalling the ACT. These
> delays mostly seem to occur when previous sideband messages we've sent
> are NAKd by the hub, however it wouldn't be particularly surprising if
> it's possible to reproduce times like this simply by introducing branch
> devices with large LCTs since payload allocations have to take effect on
> every downstream device up to the payload's target.
>
> So, instead of just retrying 30 times we poll for the ACT for up to 3ms,
> and additionally use usleep_range() to avoid a very long and rude
> busy-wait. Note that the previous retry count of 30 appears to have been
> arbitrarily chosen, as I can't find any mention of a recommended timeout
> or retry count for ACTs in the DisplayPort 2.0 specification. This also
> goes for the range we were previously using for udelay(), although I
> suspect that was just copied from the recommended delay for link
> training on SST devices.
>
> Changes since v1:
> * Use readx_poll_timeout() instead of open-coding timeout loop - Sean
>   Paul
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
> Cc: Sean Paul <sean@poorly.run>
> Cc: <stable@vger.kernel.org> # v3.17+
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 57 ++++++++++++++++-----------
>  1 file changed, 34 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index c83adbdfc1cd..ce61964baa7c 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -27,6 +27,7 @@
>  #include <linux/kernel.h>
>  #include <linux/sched.h>
>  #include <linux/seq_file.h>
> +#include <linux/iopoll.h>
>
>  #if IS_ENABLED(CONFIG_DRM_DEBUG_DP_MST_TOPOLOGY_REFS)
>  #include <linux/stacktrace.h>
> @@ -4460,43 +4461,53 @@ static int drm_dp_dpcd_write_payload(struct drm_dp_mst_topology_mgr *mgr,
>         return ret;
>  }
>
> +static int do_get_act_status(struct drm_dp_aux *aux)
> +{
> +       int ret;
> +       u8 status;
> +
> +       ret = drm_dp_dpcd_readb(aux, DP_PAYLOAD_TABLE_UPDATE_STATUS, &status);
> +       if (ret < 0)
> +               return ret;
> +
> +       return status;
> +}
>
>  /**
>   * drm_dp_check_act_status() - Polls for ACT handled status.
>   * @mgr: manager to use
>   *
>   * Tries waiting for the MST hub to finish updating it's payload table by
> - * polling for the ACT handled bit.
> + * polling for the ACT handled bit for up to 3 seconds (yes-some hubs really
> + * take that long).
>   *
>   * Returns:
>   * 0 if the ACT was handled in time, negative error code on failure.
>   */
>  int drm_dp_check_act_status(struct drm_dp_mst_topology_mgr *mgr)
>  {
> -       int count = 0, ret;
> -       u8 status;
> -
> -       do {
> -               ret = drm_dp_dpcd_readb(mgr->aux,
> -                                       DP_PAYLOAD_TABLE_UPDATE_STATUS,
> -                                       &status);
> -               if (ret < 0) {
> -                       DRM_DEBUG_KMS("failed to read payload table status %d\n",
> -                                     ret);
> -                       return ret;
> -               }
> -
> -               if (status & DP_PAYLOAD_ACT_HANDLED)
> -                       break;
> -               count++;
> -               udelay(100);
> -       } while (count < 30);
> -
> -       if (!(status & DP_PAYLOAD_ACT_HANDLED)) {
> -               DRM_DEBUG_KMS("failed to get ACT bit %d after %d retries\n",
> -                             status, count);
> +       /*
> +        * There doesn't seem to be any recommended retry count or timeout in
> +        * the MST specification. Since some hubs have been observed to take
> +        * over 1 second to update their payload allocations under certain
> +        * conditions, we use a rather large timeout value.
> +        */
> +       const int timeout_ms = 3000;
> +       int ret, status;
> +
> +       ret = readx_poll_timeout(do_get_act_status, mgr->aux, status,
> +                                status & DP_PAYLOAD_ACT_HANDLED || status < 0,
> +                                100, timeout_ms * USEC_PER_MSEC);

In v1 the usleep range was 100 -> 1000, in v2 it's going to be 51 ->
100. Perhaps bump this up to 200?

> +       if (ret < 0 && status >= 0) {
> +               DRM_DEBUG_KMS("Failed to get ACT bit %d after %dms\n",
> +                             status, timeout_ms);

I still think status should be base 16 when printed

With those nits addressed,

Reviewed-by: Sean Paul <sean@poorly.run>

>                 return -EINVAL;
> +       } else if (status < 0) {
> +               DRM_DEBUG_KMS("Failed to read payload table status: %d\n",
> +                             status);
> +               return status;
>         }
> +
>         return 0;
>  }
>  EXPORT_SYMBOL(drm_dp_check_act_status);
> --
> 2.25.1
>

Re: [PATCH v2 4/4] drm/dp_mst: Print errors on ACT timeouts

[Sean Paul]

On Mon, Apr 6, 2020 at 6:13 PM Lyude Paul <lyude@redhat.com> wrote:
>
> Although it's not unexpected for drm_dp_check_act_status() to fail due
> to DPCD read failures (as the hub may have just been unplugged
> suddenly), timeouts are a bit more worrying as they either mean we need
> a longer timeout value, or we aren't setting up payload allocations
> properly. So, let's start printing errors on timeouts.
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Cc: Sean Paul <sean@poorly.run>

Patches 1,2,4 are still

Reviewed-by: Sean Paul <sean@poorly.run>

> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index ce61964baa7c..0cbeb0f5c834 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -4499,10 +4499,14 @@ int drm_dp_check_act_status(struct drm_dp_mst_topology_mgr *mgr)
>                                  status & DP_PAYLOAD_ACT_HANDLED || status < 0,
>                                  100, timeout_ms * USEC_PER_MSEC);
>         if (ret < 0 && status >= 0) {
> -               DRM_DEBUG_KMS("Failed to get ACT bit %d after %dms\n",
> -                             status, timeout_ms);
> +               DRM_ERROR("Failed to get ACT after %dms, last status: %02x\n",
> +                         timeout_ms, status);
>                 return -EINVAL;
>         } else if (status < 0) {
> +               /*
> +                * Failure here isn't unexpected - the hub may have
> +                * just been unplugged
> +                */
>                 DRM_DEBUG_KMS("Failed to read payload table status: %d\n",
>                               status);
>                 return status;
> --
> 2.25.1
>