[PATCH] drm/dp_mst: Tune down the WARN modesetting a port with full_pbn=0

[Imre Deak]

On Wed, Feb 17, 2021 at 01:07:11PM -0500, Lyude Paul wrote:
> On Tue, 2021-02-16 at 14:34 +0200, Imre Deak wrote:
> > It's possible to modeset a connector/mst port that has a 0 full_pbn
> > value: if the sink on the port deasserts its HPD and a branch device
> > reports this via a CSN with the port's ddps=0 and pdt!=NONE the driver
> > clears full_pbn, but the corresponding connector can be still
> > modesetted.
> > 
> > This happened on a DELL U2719D monitor as the branch device and an LG
> > 27UL650-W daisy-chained to it, the LG monitor generating a long HPD
> > pulse (doing this for some reason always when waking up from some power
> > saving state).
> > 
> > Tune down the WARN about this scenario to a debug message.
> > 
> > References: https://gitlab.freedesktop.org/drm/intel/-/issues/1917
> > Cc: Lyude Paul <lyude at redhat.com>
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > ---
> >  drivers/gpu/drm/drm_dp_mst_topology.c | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > index 0a54506c27738..dcaf4bf9b62f6 100644
> > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > @@ -5118,11 +5118,15 @@ drm_dp_mst_atomic_check_port_bw_limit(struct
> > drm_dp_mst_port *port,
> >                 if (!found)
> >                         return 0;
> >  
> > -               /* This should never happen, as it means we tried to
> > -                * set a mode before querying the full_pbn
> > +               /*
> > +                * This could happen if the sink deasserted its HPD line, but
> > +                * the branch device still reports it as attached (PDT != NONE).
> >                  */
> > -               if (WARN_ON(!port->full_pbn))
> > +               if (!port->full_pbn) {
> > +                       drm_dbg_kms(port->mgr->dev, "[MSTB:%p] [MST PORT:%p] no BW available for the port\n",
> > +                                   port->parent, port);
> 
> Shouldn't this use the atomic debugging level? With that fixed, this is:
> 
> Reviewed-by: Lyude Paul <lyude at redhat.com>

Thanks, yes mixed up the debug level, will fix that.

> >                         return -EINVAL;
> > +               }
> >  
> >                 pbn_used = vcpi->pbn;
> >         } else {
> 
> -- 
> Sincerely,
>    Lyude Paul (she/her)
>    Software Engineer at Red Hat
>    
> Note: I deal with a lot of emails and have a lot of bugs on my plate. If you've
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