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
