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    SubjectRe: [PATCH] drm: make drm_dp_add_payload_part2 gracefully handle NULL state pointer
    Date
    On Fri, 14 Apr 2023, Jeff Layton <jlayton@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! :)

    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@gmail.com>
    >> > Sent: Thursday, April 13, 2023 8:59 PM
    >> > To: Jani Nikula <jani.nikula@linux.intel.com>; Lin, Wayne
    >> > <Wayne.Lin@amd.com>
    >> > Cc: Jeff Layton <jlayton@kernel.org>; David Airlie <airlied@gmail.com>;
    >> > Daniel Vetter <daniel@ffwll.ch>; Deucher, Alexander
    >> > <Alexander.Deucher@amd.com>; linux-kernel@vger.kernel.org; dri-
    >> > devel@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@linux.intel.com>
    >> > 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.
    >> > >
    >> > >
    >> > > >
    >> > > > 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;
    >> > > > }
    >> > >
    >> > > --
    >> > > Jani Nikula, Intel Open Source Graphics Center

    --
    Jani Nikula, Intel Open Source Graphics Center

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