Re: [PATCH] drm: make drm_dp_add_payload_part2 gracefully handle NULL state pointer

[Jani Nikula]

On Fri, 14 Apr 2023, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Fri, 2023-04-14 at 04:40 +0000, Lin, Wayne wrote:
>> [Public]
>> 
>> Hi Jeff,
>> 
>> Thanks. I might need more information to understand why we can't retrieve
>> the drm atomic state. Also , "Failed to create MST payload for port" indicates
>> error while configuring DPCD payload ID table. Could you help to provide log
>> with KMS + ATOMIC + DP debug on please? Thanks in advance!
>> 
>> Regards,
>> Wayne
>> 
>
> Possibly. I'm not that familiar with display driver debugging. Can you
> send me some directions on how to crank up that sort of debug logging?
>
> Note that this problem is _very_ intermittent too: I went about 2 weeks
> between crashes, and then I got 3 in one day. I'd rather not run with a
> lot of debug logging for a long time if that's what this is going to
> require, as this is my main workstation.
>
> The last time I got this log message, my proposed patch did prevent the
> box from oopsing, so I'd really like to see it go in unless it's just
> categorically wrong for the caller to pass down a NULL state pointer to
> drm_dp_add_payload_part2.

Cc: Lyude.

Looks like the state parameter was added in commit 4d07b0bc4034
("drm/display/dp_mst: Move all payload info into the atomic state") and
its only use is to get at state->dev for debug logging.

What's the plan for the parameter? Surely something more than that! :)

Instead of "state ? state->dev : NULL" I guess we could use mgr->dev
like the other logging calls do. It's papering over the NULL parameter
too, but perhaps in a slightly cleaner way...


BR,
Jani.


>
>> > -----Original Message-----
>> > From: Alex Deucher <alexdeucher@xxxxxxxxx>
>> > Sent: Thursday, April 13, 2023 8:59 PM
>> > To: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>; Lin, Wayne
>> > <Wayne.Lin@xxxxxxx>
>> > Cc: Jeff Layton <jlayton@xxxxxxxxxx>; David Airlie <airlied@xxxxxxxxx>;
>> > Daniel Vetter <daniel@xxxxxxxx>; Deucher, Alexander
>> > <Alexander.Deucher@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; dri-
>> > devel@xxxxxxxxxxxxxxxxxxxxx
>> > Subject: Re: [PATCH] drm: make drm_dp_add_payload_part2 gracefully
>> > handle NULL state pointer
>> > 
>> > + Wayne
>> > 
>> > On Thu, Apr 13, 2023 at 8:31 AM Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
>> > wrote:
>> > > 
>> > > On Thu, 13 Apr 2023, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>> > > > I've been experiencing some intermittent crashes down in the display
>> > > > driver code. The symptoms are ususally a line like this in dmesg:
>> > > > 
>> > > >     amdgpu 0000:30:00.0: [drm] Failed to create MST payload for port
>> > > > 000000006d3a3885: -5
>> > > > 
>> > > > ...followed by an Oops due to a NULL pointer dereference.
>> > > > 
>> > > > The real bug is probably in the caller of this function, which is
>> > > > passing it a NULL state pointer, but this patch at least keeps my
>> > > > machine from oopsing when this occurs.
>> > > 
>> > > My fear is that papering over this makes the root cause harder to find.
>> > > 
>> > > Cc: Harry, Alex
>> > > 
>> > > 
>> > > BR,
>> > > Jani.
>> > > 
>> > > 
>> > > > 
>> > > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2184855
>> > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
>> > > > ---
>> > > >  drivers/gpu/drm/display/drm_dp_mst_topology.c | 3 ++-
>> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
>> > > > 
>> > > > diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
>> > > > b/drivers/gpu/drm/display/drm_dp_mst_topology.c
>> > > > index 38dab76ae69e..87ad406c50f9 100644
>> > > > --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
>> > > > +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
>> > > > @@ -3404,7 +3404,8 @@ int drm_dp_add_payload_part2(struct
>> > > > drm_dp_mst_topology_mgr *mgr,
>> > > > 
>> > > >       /* Skip failed payloads */
>> > > >       if (payload->vc_start_slot == -1) {
>> > > > -             drm_dbg_kms(state->dev, "Part 1 of payload creation for %s
>> > failed, skipping part 2\n",
>> > > > +             drm_dbg_kms(state ? state->dev : NULL,
>> > > > +                         "Part 1 of payload creation for %s failed,
>> > > > + skipping part 2\n",
>> > > >                           payload->port->connector->name);
>> > > >               return -EIO;
>> > > >       }
>> > > 
>> > > --
>> > > Jani Nikula, Intel Open Source Graphics Center

-- 
Jani Nikula, Intel Open Source Graphics Center