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

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Message-ID: <CAMavQK+yVxFYNUR1wdfwB_UhRS2ziy0N5k+WTwAqUwRovX3GMA@mail.gmail.com>
Date:   Mon, 6 Apr 2020 15:41:05 -0400
From:   Sean Paul <sean@...rly.run>
To:     Lyude Paul <lyude@...hat.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        stable <stable@...r.kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Todd Previte <tprevite@...il.com>,
        Dave Airlie <airlied@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] drm/dp_mst: Increase ACT retry timeout to 3s

On Fri, Apr 3, 2020 at 4:08 PM Lyude Paul <lyude@...hat.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.
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
> Cc: Sean Paul <sean@...rly.run>
> Cc: <stable@...r.kernel.org> # v3.17+
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 7aaf184a2e5f..f313407374ed 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -4466,17 +4466,30 @@ static int drm_dp_dpcd_write_payload(struct drm_dp_mst_topology_mgr *mgr,
>   * @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;
> +       /*
> +        * 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;
> +      unsigned long timeout = jiffies + msecs_to_jiffies(timeout_ms);
> +       int ret;
> +       bool retrying = false;
>         u8 status;
>
>         do {
> +               if (retrying)
> +                       usleep_range(100, 1000);
> +
>                 ret = drm_dp_dpcd_readb(mgr->aux,
>                                         DP_PAYLOAD_TABLE_UPDATE_STATUS,
>                                         &status);
> @@ -4488,13 +4501,12 @@ int drm_dp_check_act_status(struct drm_dp_mst_topology_mgr *mgr)
>
>                 if (status & DP_PAYLOAD_ACT_HANDLED)
>                         break;
> -               count++;
> -               udelay(100);
> -       } while (count < 30);
> +               retrying = true;
> +       } while (jiffies < timeout);

Somewhat academic, but I think there's an overflow possibility here if
timeout is near ulong_max and jiffies overflows during the usleep. In
that case we'll be retrying for a very loong time.

I wish we had i915's wait_for() macro available to all drm...

Sean

>
>         if (!(status & DP_PAYLOAD_ACT_HANDLED)) {
> -               DRM_DEBUG_KMS("failed to get ACT bit %d after %d retries\n",
> -                             status, count);
> +               DRM_DEBUG_KMS("failed to get ACT bit %d after %dms\n",
> +                             status, timeout_ms);
>                 return -EINVAL;
>         }
>         return 0;
> --
> 2.25.1
>