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Message-ID: <228732a3-47d4-c97c-a016-a14013dc13d8@quicinc.com>
Date:   Tue, 10 May 2022 17:00:50 -0700
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Subject: Re: [PATCH v2] drm/probe-helper: Default to 640x480 if no EDID
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To:     Douglas Anderson <dianders@chromium.org>,
        <dri-devel@lists.freedesktop.org>
CC:     <robdclark@gmail.com>, <swboyd@chromium.org>,
        <quic_aravindh@quicinc.com>, <ville.syrjala@linux.intel.com>,
        <tzimmermann@suse.de>, <linux-arm-msm@vger.kernel.org>,
        <jani.nikula@linux.intel.com>, <quic_sbillaka@quicinc.com>,
        <dmitry.baryshkov@linaro.org>, <freedreno@lists.freedesktop.org>,
        <quic_khsieh@quicinc.com>, Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        <linux-kernel@vger.kernel.org>
References: <20220510135101.v2.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid>
From:   Abhinav Kumar <quic_abhinavk@quicinc.com>
In-Reply-To: <20220510135101.v2.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid>
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On 5/10/2022 1:51 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@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
> ---
> 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;