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

[Lyude Paul]

On Mon, 2020-04-06 at 15:41 -0400, Sean Paul wrote:
> 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 100Âs between each attempt - giving us a timeout of 2900Âs. 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...

Maybe we could add it to the kernel library somewhere? I don't see why we'd
need to stop at 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
> > 
-- 
Cheers,
	Lyude Paul (she/her)
	Associate Software Engineer at Red Hat