VT switch broken with docking station DP

[Ville Syrjälä]

On Thu, Jan 05, 2017 at 05:19:58PM +0100, Takashi Iwai wrote:
> On Thu, 05 Jan 2017 16:59:31 +0100,
> Ville Syrjälä wrote:
> > 
> > On Thu, Jan 05, 2017 at 04:37:27PM +0100, Takashi Iwai wrote:
> > > Hi,
> > > 
> > > recently I noticed that VT console doesn't work any longer when I dock
> > > a Dell E7270 laptop with a DP monitor.  The bug detail is like this:
> > > 
> > > At first, I boot the laptop without dock.  I can switch between X and
> > > VT via ctrl-alt-F1, so far.  Then I dock it to a docking station
> > > connected with a DP monitor.  Now, when I switch to VT, it behaves as
> > > if frozen, the X graphics screen remains.  But actually it's only
> > > graphics and the keyboard input is processed in VT.  I can go back to
> > > X via alt-F7 again.  The situation remains until I undock and I kill X
> > > once.
> > > 
> > > After looking more deeply at drm debug log, I found out that it's
> > > caused by the drm atomic check.  Essentially, it's because eDP has the
> > > lower resolution (1366x768) than DP (1920x1080).  Since booting with
> > > eDP, the frame buffer size is 1366x768.  Then it hits the following
> > > check in drm_atomic_plane_check():
> > > 
> > > 	fb_width = state->fb->width << 16;
> > > 	fb_height = state->fb->height << 16;
> > > 
> > > 	/* Make sure source coordinates are inside the fb. */
> > > 	if (state->src_w > fb_width ||
> > > 	    state->src_x > fb_width - state->src_w ||
> > > 	    state->src_h > fb_height ||
> > > 	    state->src_y > fb_height - state->src_h) {
> > > 		DRM_DEBUG_ATOMIC("Invalid source coordinates "
> > > 				 "%u.%06ux%u.%06u+%u.%06u+%u.%06u\n",
> > > 				 state->src_w >> 16, ((state->src_w & 0xffff) * 15625) >> 10,
> > > 				 state->src_h >> 16, ((state->src_h & 0xffff) * 15625) >> 10,
> > > 				 state->src_x >> 16, ((state->src_x & 0xffff) * 15625) >> 10,
> > > 				 state->src_y >> 16, ((state->src_y & 0xffff) * 15625) >> 10);
> > > 		return -ENOSPC;
> > > 	}
> > > 
> > > Actually after commenting out "return -ENOSPC", VT switch works fine.
> > > 
> > > But the code above made me wonder what's the requirement here.  IIRC,
> > > the VT always worked on a display with a higher resolution even if the
> > > frame buffer is smaller.  Only a part of display was used, but it was
> > > OK, far better than the frozen graphics :)
> > > 
> > > Can we simply drop this check, or may we add a flag to skip it for VT
> > > switching?  Or any better idea?
> > 
> > Find out 2why it didn't allocate a big enough framebuffer to begin with,
> > or alternatively why it tried to specify source coordinates exceeding
> > the fb dimensions.
> > 
> > There is clearly a bug somewhere, just not here.
> 
> Hrm, so that's my misunderstanding.  The problem is that it tries to
> assign to 1368x768 resolution for DP while the fb is 1366x768...
> Sounds familiar?

Not really. Sounds like there's a bug in the fb helper somewhere that it
tries to add new connectors to the mix without checking that the fb is
big enough for whatever modes are supported on said connectors.

As far as the 1366 vs. 1368 thing goes, we have quirks in the EDID
parser to convert 1368 to 1366 with the assumption that 1366 is what
they really meant. The reason for the quirk being that EDID can't
actually say 1366 since it's not a multiple of 8. It might be
interesting to know where that 1368 mode came from and why the quirk
didn't convert it to 1366. But even fixing that part (assuming there's
something to fix) doesn't really excuse the fact that the fb helper
picked a resolution for the connector that doesn't fit in the fb.

-- 
Ville Syrjälä
Intel OTC