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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 01 Jun 2022, 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>
> ---
> 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)

If we're doing this primarily to appease the CTS, this is fine.

If we think this is a functional improvement for regular use, I suppose
we should consider doing this also for DRM_MODE_CONNECTOR_eDP. Which is
irrelevant for the CTS.

Either way,

Acked-by: Jani Nikula <jani.nikula@intel.com>

> +			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);

-- 
Jani Nikula, Intel Open Source Graphics Center