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

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2023-04-13 at 15:31 +0300, Jani Nikula wrote:
> On Thu, 13 Apr 2023, Jeff Layton <jlayton@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.
> 
> 

I'm happy to help track down the root cause. Display drivers are
somewhat outside my wheelhouse though.

Maybe we can throw a WARNING when this happens? I'd just like it to not
crash my machine.


> > 
> > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2184855
> > Signed-off-by: Jeff Layton <jlayton@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;
> >  	}
> 

-- 
Jeff Layton <jlayton@kernel.org>