linux-kernel - Re: [RFC PATCH] drm/edid: drm_add_modes_noedid() should set lowest resolution as preferred

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Date:   Tue, 26 Apr 2022 13:45:59 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Douglas Anderson <dianders@...omium.org>,
        <dri-devel@...ts.freedesktop.org>
CC:     <robdclark@...il.com>, <linux-arm-msm@...r.kernel.org>,
        <quic_khsieh@...cinc.com>, <quic_aravindh@...cinc.com>,
        <swboyd@...omium.org>, <dmitry.baryshkov@...aro.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: [RFC PATCH] drm/edid: drm_add_modes_noedid() should set lowest
 resolution as preferred



On 4/26/2022 1:21 PM, Douglas Anderson 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. 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.
> 
> Let's instead set 640x480 as the "preferred" mode when we have no EDID.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

drm_dmt_modes array is sorted but you are also relying on this check to 
eliminate the non-60fps modes

5611 		if (drm_mode_vrefresh(ptr) > 61)
5612 			continue;

I am not sure why we filter out the modes > 61 vrefresh.

If that check will remain this is okay.

If its not, its not reliable that the first mode will be 640x480@60

> ---
> 
>   drivers/gpu/drm/drm_edid.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 7a8482b75071..64ccfff4167e 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -5839,6 +5839,15 @@ int drm_add_modes_noedid(struct drm_connector *connector,
>   			continue;
>   		mode = drm_mode_duplicate(dev, ptr);
>   		if (mode) {
> +			/*
> +			 * The drm_dmt_modes array is sorted so that lower
> +			 * resolutions come first. We'll set the lowest
> +			 * resolution mode as preferred. We have no EDID so
> +			 * we should prefer the lowest resolution mode as
> +			 * the safest one.
> +			 */
> +			if (num_modes == 0)
> +				mode->type |= DRM_MODE_TYPE_PREFERRED;
>   			drm_mode_probed_add(connector, mode);
>   			num_modes++;
>   		}