drm/nouveau: prefer XBGR2101010 for addfb ioctl

Nouveau only exposes support for XBGR2101010. Prior to the atomic
conversion, drm would pass in the wrong format in the framebuffer, but
it was always ignored -- both userspace (xf86-video-nouveau) and the
kernel driver agreed on the layout, so the fact that the format was
wrong didn't matter.

With the atomic conversion, nouveau all of a sudden started caring about
the exact format, and so the previously-working code in
xf86-video-nouveau no longer functioned since the (internally-assigned)
format from the addfb ioctl was wrong.

This change adds infrastructure to allow a drm driver to specify that it
prefers the XBGR format variant for the addfb ioctl, and makes nouveau's
nv50 display driver set it. (Prior gens had no support for 30bpp at all.)

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: stable@vger.kernel.org # v4.10+
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180203191123.31507-1-imirkin@alum.mit.edu

drivers/gpu/drm/drm_framebuffer.c

@@ -121,6 +121,10 @@ int drm_mode_addfb(struct drm_device *dev,
 	r.pixel_format = drm_mode_legacy_fb_format(or->bpp, or->depth);
 	r.handles[0] = or->handle;
 
+	if (r.pixel_format == DRM_FORMAT_XRGB2101010 &&
+	    dev->driver->driver_features & DRIVER_PREFER_XBGR_30BPP)
+		r.pixel_format = DRM_FORMAT_XBGR2101010;
+
 	ret = drm_mode_addfb2(dev, &r, file_priv);
 	if (ret)
 		return ret;

drivers/gpu/drm/nouveau/nv50_display.c

@@ -4477,6 +4477,7 @@ nv50_display_create(struct drm_device *dev)
 	nouveau_display(dev)->fini = nv50_display_fini;
 	disp->disp = &nouveau_display(dev)->disp;
 	dev->mode_config.funcs = &nv50_disp_func;
+	dev->driver->driver_features |= DRIVER_PREFER_XBGR_30BPP;
 	if (nouveau_atomic)
 		dev->driver->driver_features |= DRIVER_ATOMIC;
 

include/drm/drm_drv.h

@@ -56,6 +56,7 @@ struct drm_printer;
 #define DRIVER_ATOMIC			0x10000
 #define DRIVER_KMS_LEGACY_CONTEXT	0x20000
 #define DRIVER_SYNCOBJ                  0x40000
+#define DRIVER_PREFER_XBGR_30BPP        0x80000
 
 /**
  * struct drm_driver - DRM driver structure