Re: [Freedreno] [PATCH v4] drm/probe-helper: Default to 640x480 if no EDID on DP

[Sean Paul <seanpaul@chromium.org>]

On Wed, Jun 1, 2022 at 2:23 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> If we're unable to read the EDID for a display because it's corrupt /
> bogus / invalid then we'll add a set of standard modes for the
> display. Since we have no true information about the connected
> display, these modes are essentially guesses but better than nothing.
> At the moment, none of the modes returned is marked as preferred, but
> the modes are sorted such that the higher resolution modes are listed
> first.
>
> When userspace sees these modes presented by the kernel it needs to
> figure out which one to pick. At least one userspace, ChromeOS [1]
> seems to use the rules (which seem pretty reasonable):
> 1. Try to pick the first mode marked as preferred.
> 2. Try to pick the mode which matches the first detailed timing
>    descriptor in the EDID.
> 3. If no modes were marked as preferred then pick the first mode.
>
> Unfortunately, userspace's rules combined with what the kernel is
> doing causes us to fail section 4.2.2.6 (EDID Corruption Detection) of
> the DP 1.4a Link CTS. That test case says that, while it's OK to allow
> some implementation-specific fall-back modes if the EDID is bad that
> userspace should _default_ to 640x480.
>
> Let's fix this by marking 640x480 as default for DP in the no-EDID
> case.
>
> NOTES:
> - In the discussion around v3 of this patch [2] there was talk about
>   solving this in userspace and I even implemented a patch that would
>   have solved this for ChromeOS, but then the discussion turned back
>   to solving this in the kernel.
> - Also in the discussion of v3 [2] it was requested to limit this
> 83;40900;0c  change to just DP since folks were worried that it would break some
>   subtle corner case on VGA or HDMI.
>
> [1] https://source.chromium.org/chromium/chromium/src/+/a051f741d0a15caff2251301efe081c30e0f4a96:ui/ozone/platform/drm/common/drm_util.cc;l=488
> [2] https://lore.kernel.org/r/20220513130533.v3.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>


Reviewed-by: Sean Paul <seanpaul@chromium.org>

>
> ---
> I put Abhinav's Reviewed-by tag from v2 here since this is nearly the
> same as v2. Hope this is OK.
>
> Changes in v4:
> - Code is back to v2, but limit to just DP.
> - Beefed up the commit message.
>
> Changes in v3:
> - Don't set preferred, just disable the sort.
>
> Changes in v2:
> - Don't modify drm_add_modes_noedid() 'cause that'll break others
> - Set 640x480 as preferred in drm_helper_probe_single_connector_modes()
>
>  drivers/gpu/drm/drm_probe_helper.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
> index 425f56280d51..75a71649b64d 100644
> --- a/drivers/gpu/drm/drm_probe_helper.c
> +++ b/drivers/gpu/drm/drm_probe_helper.c
> @@ -569,8 +569,17 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
>                 count = drm_add_override_edid_modes(connector);
>
>         if (count == 0 && (connector->status == connector_status_connected ||
> -                          connector->status == connector_status_unknown))
> +                          connector->status == connector_status_unknown)) {
>                 count = drm_add_modes_noedid(connector, 1024, 768);
> +
> +               /*
> +                * Section 4.2.2.6 (EDID Corruption Detection) of the DP 1.4a
> +                * Link CTS specifies that 640x480 (the official "failsafe"
> +                * mode) needs to be the default if there's no EDID.
> +                */
> +               if (connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort)
> +                       drm_set_preferred_mode(connector, 640, 480);
> +       }
>         count += drm_helper_probe_add_cmdline_mode(connector);
>         if (count != 0) {
>                 ret = __drm_helper_update_and_validate(connector, maxX, maxY, &ctx);
> --
> 2.36.1.255.ge46751e96f-goog
>