Date:   Thu, 5 May 2022 23:13:01 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     dri-devel@...ts.freedesktop.org, swboyd@...omium.org,
        quic_abhinavk@...cinc.com, quic_aravindh@...cinc.com,
        robdclark@...il.com, quic_khsieh@...cinc.com,
        linux-arm-msm@...r.kernel.org, quic_sbillaka@...cinc.com,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other
 modes are bad

On Tue, 26 Apr 2022 at 21:47, Douglas Anderson <dianders@...omium.org> 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@quicinc.com
>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

> ---
>
>  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

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