[PATCH] drm: Fix broken VT switch with video=1366x768 option

[Ville Syrjälä]

On Mon, Jan 09, 2017 at 03:56:14PM +0100, Takashi Iwai wrote:
> I noticed that the VT switch doesn't work any longer with a Dell
> laptop with 1366x768 eDP when the machine is connected with a DP
> monitor.  It behaves as if VT were switched, but the graphics remain
> frozen.  Actually the keyboard works, so I could switch back to VT7
> again.
> 
> I tried to track down the problem, and encountered a long story until
> we reach to this error:
> 
> - The machine is booted with video=1366x768 option (the distro
>   installer seems to add it as default).
> - Recently, drm_helper_probe_single_connector_modes() deals with
>   cmdline modes, and it tries to create a new mode when no
>   matching mode is found.
> - The drm_mode_create_from_cmdline_mode() creates a mode based on
>   either CVT of GFT according to the given cmdline mode; in our case,
>   it's 1366x768.
> - Since both CVT and GFT can't express the width 1366 due to
>   alignment, the resultant mode becomes 1368x768, slightly larger than
>   the given size.
> - Later on, the atomic commit is performed, and in
>   drm_atomic_check_only(), the size of each plane is checked.
> - The size check of 1366x768 fails due to the above, and eventually
>   the whole VT switch fails.
> 
> Back in the history, we've had a manual fix-up of 1368x768 in various
> places via c09dedb7a50e ("drm/edid: Add a workaround for 1366x768 HD
> panel"), but they have been all in drm_edid.c at probing the modes
> from EDID.  For addressing the problem above, we need a similar hack
> to the mode newly created from cmdline, manually adjusting the width
> when the expected size is 1366 while we get 1368 instead.
> 
> Fixes: eaf99c749d43 ("drm: Perform cmdline mode parsing during...")
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai at suse.de>
> ---
>  drivers/gpu/drm/drm_modes.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> index ac6a35212501..e6b19bc9021a 100644
> --- a/drivers/gpu/drm/drm_modes.c
> +++ b/drivers/gpu/drm/drm_modes.c
> @@ -1460,6 +1460,13 @@ drm_mode_create_from_cmdline_mode(struct drm_device *dev,
>  		return NULL;
>  
>  	mode->type |= DRM_MODE_TYPE_USERDEF;
> +	/* fix up 1368x768: GFT/CVT can't express 1366 width due to alignment */
> +	if (cmd->xres == 1366 && mode->hdisplay == 1368) {
> +		mode->hdisplay = 1366;
> +		mode->hsync_start--;
> +		mode->hsync_end--;
> +		drm_mode_set_name(mode);
> +	}

Maybe give fixup_mode_1366x768() a drm_ prefix and make in non-static to
avoid duplicating the code? And I guess move it to drm_modes.c?

Otherwise lgtm:
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

>  	drm_mode_set_crtcinfo(mode, CRTC_INTERLACE_HALVE_V);
>  	return mode;
>  }
> -- 
> 2.11.0

-- 
Ville Syrjälä
Intel OTC