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

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 c09dedb7 ("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: eaf99c74 ("drm: Perform cmdline mode parsing during...")
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109145614.29454-1-tiwai@suse.deReviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

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);
+	}
 	drm_mode_set_crtcinfo(mode, CRTC_INTERLACE_HALVE_V);
 	return mode;
 }