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

[Jani Nikula]

On Fri, 14 Apr 2023, Lyude Paul <lyude at redhat.com> wrote:
> On Fri, 2023-04-14 at 13:35 +0300, Jani Nikula wrote:
>> On Fri, 14 Apr 2023, Jeff Layton <jlayton at kernel.org> 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! :)
>
> I don't think there was any plan for that, or at least I certainly don't even
> remember adding that D:. It must totally have been by mistake and snuck by
> review, if that's the only thing that we're using it for I'd say it's
> definitely fine to just drop it entirely

I guess we could use two patches then, first replace state->dev with
mgr->dev as something that can be backported as needed, and second drop
the state parameter altogether.

Jeff, up for it? At least the first one?


BR,
Jani.


>
>> 
>> 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 at gmail.com>
>> > > > Sent: Thursday, April 13, 2023 8:59 PM
>> > > > To: Jani Nikula <jani.nikula at linux.intel.com>; Lin, Wayne
>> > > > <Wayne.Lin at amd.com>
>> > > > Cc: Jeff Layton <jlayton at kernel.org>; David Airlie <airlied at gmail.com>;
>> > > > Daniel Vetter <daniel at ffwll.ch>; Deucher, Alexander
>> > > > <Alexander.Deucher at amd.com>; linux-kernel at vger.kernel.org; dri-
>> > > > devel at lists.freedesktop.org
>> > > > 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 at linux.intel.com>
>> > > > wrote:
>> > > > > 
>> > > > > On Thu, 13 Apr 2023, Jeff Layton <jlayton at kernel.org> 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 at kernel.org>
>> > > > > > ---
>> > > > > >  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