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

[Jeff Layton]

On Mon, 2023-04-17 at 10:58 +0000, Lin, Wayne wrote:
> [AMD Official Use Only - General]
> 
> 
> 
> > -----Original Message-----
> > From: Jani Nikula <jani.nikula at linux.intel.com>
> > Sent: Monday, April 17, 2023 6:30 PM
> > To: Jeff Layton <jlayton at kernel.org>; Lyude Paul <lyude at redhat.com>; Lin,
> > Wayne <Wayne.Lin at amd.com>; Alex Deucher <alexdeucher at gmail.com>
> > Cc: 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
> > 
> > On Mon, 17 Apr 2023, Jeff Layton <jlayton at kernel.org> wrote:
> > > On Mon, 2023-04-17 at 11:44 +0300, Jani Nikula wrote:
> > > > 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
> > > > mgr->drop
> > > > the state parameter altogether.
> > > > 
> > > > Jeff, up for it? At least the first one?
> > > > 
> > > > 
> > > > BR,
> > > > Jani.
> > > > 
> > > 
> > > Sure. I'm happy to test patches if you send them along.
> > 
> > I was hoping to lure you into sending patches. ;)
> > 
> > Anyway, I'm not working on this.
> > 
> > 
> Hi Jeff,
> 
> I probably know the root cause. 
> But it doesn't need to use the state in the end, I will just rely on the patch 
> that Jani suggested to fix it. I can help to provide the patch later : )
> 
> 

Sounds good. If you want to send me a patch to solve the root cause,
I'll put it in the kernel with the other one I'm testing.

Thanks,
-- 
Jeff Layton <jlayton at kernel.org>