Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad

[Doug Anderson]

Hi,

On Tue, Apr 26, 2022 at 12:16 PM Abhinav Kumar
<quic_abhinavk@xxxxxxxxxxx> wrote:
>
> Hi Doug
>
> One minor comment below.
>
> But otherwise, looking at this change this should work for us acc to me.
>
> We will test this out with our equipment and then provide R-b.
>
> Thanks
>
> Abhinav
> On 4/26/2022 11:46 AM, Douglas Anderson wrote:
> > As per Displayport spec section 5.2.1.2 ("Video Timing Format") says
> > that all detachable sinks shall support 640x480 @60Hz as a fail safe
> > mode.
> >
> > A DP compliance test expected us to utilize the above fact when all
> > modes it presented to the DP source were not achievable. It presented
> > only modes that would be achievable with more lanes and/or higher
> > speeds than we had available and expected that when we couldn't do
> > that then we'd fall back to 640x480 even though it didn't advertise
> > this size.
> >
> > In order to pass the compliance test (and also support any users who
> > might fall into a similar situation with their display), we need to
> > add 640x480 into the list of modes. However, we don't want to add
> > 640x480 all the time. Despite the fact that the DP spec says all sinks
> > _shall support_ 640x480, they're not guaranteed to support it
> > _well_. Continuing to read the spec you can see that the display is
> > not required to really treat 640x480 equal to all the other modes. It
> > doesn't need to scale or anything--just display the pixels somehow for
> > failsafe purposes. It should also be noted that it's not hard to find
> > a display hooked up via DisplayPort that _doesn't_ support 640x480 at
> > all. The HP ZR30w screen I'm sitting in front of has a native DP port
> > and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI
> > display via a DP to HDMI adapter and that screen definitely doesn't
> > support 640x480.
> >
> > As a compromise solution, let's only add the 640x480 mode if:
> > * We're on DP.
> > * All other modes have been pruned.
> >
> > This acknowledges that 640x480 might not be the best mode to use but,
> > since sinks are _supposed_ to support it, we will at least fall back
> > to it if there's nothing else.
> >
> > Note that we _don't_ add higher resolution modes like 1024x768 in this
> > case. We only add those modes for a failed EDID read where we have no
> > idea what's going on. In the case where we've pruned all modes then
> > instead we only want 640x480 which is the only defined "Fail Safe"
> > resolution.
> >
> > This patch originated in response to Kuogee Hsieh's patch [1].
> >
> > [1] https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@xxxxxxxxxxx
> >
> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > ---
> >
> >   drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++-----
> >   1 file changed, 21 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
> > index 819225629010..90cd46cbfec1 100644
> > --- a/drivers/gpu/drm/drm_probe_helper.c
> > +++ b/drivers/gpu/drm/drm_probe_helper.c
> > @@ -476,7 +476,6 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
> >       const struct drm_connector_helper_funcs *connector_funcs =
> >               connector->helper_private;
> >       int count = 0, ret;
> > -     bool verbose_prune = true;
> >       enum drm_connector_status old_status;
> >       struct drm_modeset_acquire_ctx ctx;
> >
> > @@ -556,8 +555,8 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
> >               DRM_DEBUG_KMS("[CONNECTOR:%d:%s] disconnected\n",
> >                       connector->base.id, connector->name);
> >               drm_connector_update_edid_property(connector, NULL);
> > -             verbose_prune = false;
> > -             goto prune;
> > +             drm_mode_prune_invalid(dev, &connector->modes, false);
> > +             goto exit;
> >       }
> >
> >       count = (*connector_funcs->get_modes)(connector);
> > @@ -580,9 +579,26 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
> >               }
> >       }
> >
> > -prune:
> > -     drm_mode_prune_invalid(dev, &connector->modes, verbose_prune);
> > +     drm_mode_prune_invalid(dev, &connector->modes, true);
> >
> > +     /*
> > +      * Displayport spec section 5.2.1.2 ("Video Timing Format") says that
> > +      * all detachable sinks shall support 640x480 @60Hz as a fail safe
> > +      * mode. If all modes were pruned, perhaps because they need more
> > +      * lanes or a higher pixel clock than available, at least try to add
> > +      * in 640x480.
> > +      */
> > +     if (list_empty(&connector->modes) &&
> > +         connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) {
> > +             count = drm_add_modes_noedid(connector, 640, 480);
> > +             if (_drm_helper_update_and_validate(connector, maxX, maxY, &ctx)) {
> > +                     drm_modeset_backoff(&ctx);
> > +                     goto retry;
>
> Do we need another retry here? This will again repeat everything from
> get_modes().
> The fact that we are hitting this code is because we have already tried
> that and this is already a second-pass. So I think another retry isnt
> needed?

The retry is still needed. This gets into the whole wait-wound mutexes
that DRM uses, right? Any time we detect deadlock we release all of
our locks and start from scratch. That's still possible here.

-Doug