[PATCH 2/2] drm/i915: Deal with upside-down mounted LCD panels

[Chris Wilson]

On Sun, May 07, 2017 at 11:10:56AM +0200, Hans de Goede wrote:
> On some (Bay Trail) devices the LCD panel is mounted upside-down.
> 
> This commit uses the code to read back the initial rotation of the
> primary plane in get_initial_plane_config from Ville Syrjala's
> "drm/fb-helper: Inherit rotation wip" patch and when re-using the
> initial fb it stores that in intel_crtc.initial_rotation.
> 
> It adds an intel_plane_get_rotation helper which combines this
> initial_rotation with any rotation requested by userspace and
> uses this in all places which look at a planes rotation, thus
> transparently dealing with upside-down LCD panels without requiring
> any user-space or fbcon changes.
> 
> This fixes the kernel boot messages switching from being shown the right
> way up in efifb to being shown upside down as soon as a native kms driver
> loads, as well as any graphics displayed by userspace being upside-down.
> 
> Note this only deals with upside-down LCD panels / 180 degrees
> rotation as the hardware in question only supports 180 degrees
> rotation in hardware. The one model known which has a panel mounted
> with 90/270 degrees rotation will need to be fully dealt with in
> userspace, even the firmware boot screen / menus are rotated 90 degrees
> on this one and there simply is nothing the kernel can do about this.

I shall just mention a concern with hiding the transformation on the
connection, we do expose the subpixel layout to userspace and that
should be adjusted based on this lie. There are probably other places
where the orientation of the output leaks through the current interface.

The commit message fails to explain how userspace, which should already
have the tools available to it, is unable to reapply the existing
rotation - i.e. why this is a kernel problem, and whether by hiding this
from userspace we are not trapping ourselves with warts in the ABI.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre