On Tue, 26 Apr 2022 at 21:47, Douglas Anderson <dianders@xxxxxxxxxxxx> 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> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx> > --- > > 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; > + } > + drm_mode_prune_invalid(dev, &connector->modes, true); > + } > + > +exit: > drm_modeset_drop_locks(&ctx); > drm_modeset_acquire_fini(&ctx); > > -- > 2.36.0.rc2.479.g8af0fa9b8e-goog > -- With best wishes Dmitry