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 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: <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Ville Syrjälä <[e-mail]>
Signed-off-by: Ville Syrjälä <[e-mail]>

drivers/gpu/drm/drm_modes.c

···
 		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;
 }