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

[Lyude Paul]

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

> 
> 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
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat