[PATCH v3] drm: Only create a cmdline mode if no probed modes match

[Ville Syrjälä]

On Wed, Jun 01, 2016 at 10:34:36AM +0100, Chris Wilson wrote:
> The intention of using video=<connector>:<mode> is primarily to select
> the user's preferred resolution at startup. Currently we always create a
> new mode irrespective of whether the monitor has a native mode at the
> desired resolution. This has the issue that we may then select the fake
> mode rather the native mode during fb_helper->inital_config() and so
> if the fake mode is invalid we then end up with a loss of signal. Oops.
> This invalid fake mode would also be exported to userspace, who
> potentially may make the same mistake.
> 
> To avoid this issue, we filter out the added command line mode if we
> detect the desired resolution (and clock if specified) amongst the
> probed modes. This fixes the immediate problem of adding a duplicate
> mode, but perhaps more generically we should avoid adding a GTF mode if
> the monitor has an EDID that is not GTF-compatible, or similarly for
> CVT.
> 
> Fixes regression from
> 
> commit eaf99c749d43ae74ac7ffece5512f3c73f01dfd2
> Author: Chris Wilson <chris at chris-wilson.co.uk>
> Date:   Wed Aug 6 10:08:32 2014 +0200
> 
>     drm: Perform cmdline mode parsing during connector initialisation
> 
> that breaks HDMI output on BeagleBone Black with LG TV (model 19LS4R-ZA).
> 
> v2: Explicitly delete our earlier cmdline mode
> v3: Mode pruning should now be sufficient to delete stale cmdline modes
> 
> Reported-by: Radek Dostál <rd at radekdostal.com>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Radek Dostál <rd at radekdostal.com>
> Cc: Jesse Barnes <jbarnes at virtuousgeek.org>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: dri-devel at lists.freedesktop.org
> Cc: Julia Lemire <jlemire at matrox.com>
> Cc: Dave Airlie <airlied at redhat.com>
> Cc: stable at vger.kernel.org
> ---
>  drivers/gpu/drm/drm_probe_helper.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
> index 0329080d7f7c..a705ed12c062 100644
> --- a/drivers/gpu/drm/drm_probe_helper.c
> +++ b/drivers/gpu/drm/drm_probe_helper.c
> @@ -82,13 +82,28 @@ drm_mode_validate_flag(const struct drm_display_mode *mode,
>  
>  static int drm_helper_probe_add_cmdline_mode(struct drm_connector *connector)
>  {
> +	struct drm_cmdline_mode *cmdline_mode;
>  	struct drm_display_mode *mode;
>  
> -	if (!connector->cmdline_mode.specified)
> +	cmdline_mode = &connector->cmdline_mode;
> +	if (!cmdline_mode->specified)
>  		return 0;
>  
> +	/* Only add a GTF mode if we find no matching probed modes */
> +	list_for_each_entry(mode, &connector->probed_modes, head) {
> +		if (mode->hdisplay != cmdline_mode->xres ||
> +		    mode->vdisplay != cmdline_mode->yres)
> +			continue;
> +
> +		if (cmdline_mode->refresh_specified &&
> +		    mode->vrefresh != cmdline_mode->refresh)

I think we might not have .vrefresh populated for the probed modes.
We update .vrefresh only for the modes left on the real mode list in the
end.

> +			continue;
> +
> +		return 0;
> +	}
> +
>  	mode = drm_mode_create_from_cmdline_mode(connector->dev,
> -						 &connector->cmdline_mode);
> +						 cmdline_mode);
>  	if (mode == NULL)
>  		return 0;
>  
> -- 
> 2.8.1

-- 
Ville Syrjälä
Intel OTC