Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other modes are bad

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

Hi Doug

One minor comment below.

But otherwise, looking at this change this should work for us acc to me.

We will test this out with our equipment and then provide R-b.

Thanks

Abhinav
On 4/26/2022 11:46 AM, Douglas Anderson wrote:
> As per Displayport spec section 5.2.1.2 ("Video Timing Format") says
> that all detachable sinks shall support 640x480 @60Hz as a fail safe
> mode.
> 
> A DP compliance test expected us to utilize the above fact when all
> modes it presented to the DP source were not achievable. It presented
> only modes that would be achievable with more lanes and/or higher
> speeds than we had available and expected that when we couldn't do
> that then we'd fall back to 640x480 even though it didn't advertise
> this size.
> 
> In order to pass the compliance test (and also support any users who
> might fall into a similar situation with their display), we need to
> add 640x480 into the list of modes. However, we don't want to add
> 640x480 all the time. Despite the fact that the DP spec says all sinks
> _shall support_ 640x480, they're not guaranteed to support it
> _well_. Continuing to read the spec you can see that the display is
> not required to really treat 640x480 equal to all the other modes. It
> doesn't need to scale or anything--just display the pixels somehow for
> failsafe purposes. It should also be noted that it's not hard to find
> a display hooked up via DisplayPort that _doesn't_ support 640x480 at
> all. The HP ZR30w screen I'm sitting in front of has a native DP port
> and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI
> display via a DP to HDMI adapter and that screen definitely doesn't
> support 640x480.
> 
> As a compromise solution, let's only add the 640x480 mode if:
> * We're on DP.
> * All other modes have been pruned.
> 
> This acknowledges that 640x480 might not be the best mode to use but,
> since sinks are _supposed_ to support it, we will at least fall back
> to it if there's nothing else.
> 
> Note that we _don't_ add higher resolution modes like 1024x768 in this
> case. We only add those modes for a failed EDID read where we have no
> idea what's going on. In the case where we've pruned all modes then
> instead we only want 640x480 which is the only defined "Fail Safe"
> resolution.
> 
> This patch originated in response to Kuogee Hsieh's patch [1].
> 
> [1] https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quicinc.com
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> 
>   drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++-----
>   1 file changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
> index 819225629010..90cd46cbfec1 100644
> --- a/drivers/gpu/drm/drm_probe_helper.c
> +++ b/drivers/gpu/drm/drm_probe_helper.c
> @@ -476,7 +476,6 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
>   	const struct drm_connector_helper_funcs *connector_funcs =
>   		connector->helper_private;
>   	int count = 0, ret;
> -	bool verbose_prune = true;
>   	enum drm_connector_status old_status;
>   	struct drm_modeset_acquire_ctx ctx;
>   
> @@ -556,8 +555,8 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
>   		DRM_DEBUG_KMS("[CONNECTOR:%d:%s] disconnected\n",
>   			connector->base.id, connector->name);
>   		drm_connector_update_edid_property(connector, NULL);
> -		verbose_prune = false;
> -		goto prune;
> +		drm_mode_prune_invalid(dev, &connector->modes, false);
> +		goto exit;
>   	}
>   
>   	count = (*connector_funcs->get_modes)(connector);
> @@ -580,9 +579,26 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
>   		}
>   	}
>   
> -prune:
> -	drm_mode_prune_invalid(dev, &connector->modes, verbose_prune);
> +	drm_mode_prune_invalid(dev, &connector->modes, true);
>   
> +	/*
> +	 * Displayport spec section 5.2.1.2 ("Video Timing Format") says that
> +	 * all detachable sinks shall support 640x480 @60Hz as a fail safe
> +	 * mode. If all modes were pruned, perhaps because they need more
> +	 * lanes or a higher pixel clock than available, at least try to add
> +	 * in 640x480.
> +	 */
> +	if (list_empty(&connector->modes) &&
> +	    connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) {
> +		count = drm_add_modes_noedid(connector, 640, 480);
> +		if (_drm_helper_update_and_validate(connector, maxX, maxY, &ctx)) {
> +			drm_modeset_backoff(&ctx);
> +			goto retry;

Do we need another retry here? This will again repeat everything from
get_modes().
The fact that we are hitting this code is because we have already tried 
that and this is already a second-pass. So I think another retry isnt 
needed?

> +		}
> +		drm_mode_prune_invalid(dev, &connector->modes, true);
> +	}
> +
> +exit:
>   	drm_modeset_drop_locks(&ctx);
>   	drm_modeset_acquire_fini(&ctx);
>