On Fri, Apr 3, 2020 at 4:08 PM Lyude Paul <lyude@xxxxxxxxxx> 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@xxxxxxxxxx>
> Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
> Cc: Sean Paul <sean@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 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
>
