Re: [PATCH v2] drm/probe-helper: Default to 640x480 if no EDID

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 10.05.22 um 22:51 schrieb Douglas Anderson:
> 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. When userspace looks at these modes it doesn't really have a
> good concept for which mode to pick and it'll likely pick the highest
> resolution one by default. That's probably not ideal because the modes
> were purely guesses on the part of the Linux kernel.

I'm skeptical. Why does the kernel do a better job than userspace here? 
Only the graphics driver could possibly make such a decision.

Not setting any preferred mode at least gives a clear message to userspace.

Best regards
Thomas

> 
> Let's instead set 640x480 as the "preferred" mode when we have no EDID.
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> Note that this is the second of two related and similar-sounding but
> different patches. See also ("drm/probe-helper: For DP, add 640x480 if
> all other modes are bad") [1]. I'm hoping to land _both_ of the
> patches since they address different issues. This patch addresses the
> case of a corrupt EDID and having 640x480 be the default in the
> "guessed" modes. The other patch handles the case where the EDID
> _isn't_ corrupt but all the modes listed can't be made with the
> existing situations. The two patches can land in either order.
> 
> Also note that I didn't carry any Tested-by / Reviewed-by tags since
> the patch is now quite different.
> 
> [1] https://lore.kernel.org/r/20220510131309.v2.2.I4ac7f55aa446699f8c200a23c10463256f6f439f@changeid
> 
> 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 682359512996..1fbb9a8c315c 100644
> --- a/drivers/gpu/drm/drm_probe_helper.c
> +++ b/drivers/gpu/drm/drm_probe_helper.c
> @@ -516,8 +516,17 @@ 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);
> +
> +		/*
> +		 * Give userspace a hint that we don't have a lot of confidence
> +		 * in these modes (we totally guessed) by marking 640x480 as
> +		 * preferred. This is low clock rate and incredibly common as
> +		 * a failsafe mode.
> +		 */
> +		drm_set_preferred_mode(connector, 640, 480);
> +	}
>   	count += drm_helper_probe_add_cmdline_mode(connector);
>   	if (count == 0)
>   		goto prune;

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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