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

[Takashi Iwai]

On Tue, 10 Jan 2017 12:28:36 +0100,
Ville Syrjälä wrote:
> 
> 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?

Yes, that'll be a good cleanup.  I was afraid of symbol export, but
both are in the same module, so far.  I can post a follow-up once when
this one gets accepted.

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

Thanks!


Takashi

> >  	drm_mode_set_crtcinfo(mode, CRTC_INTERLACE_HALVE_V);
> >  	return mode;
> >  }
> > -- 
> > 2.11.0
> 
> -- 
> Ville Syrjälä
> Intel OTC
>