[PATCH] drm/fourcc: add LINEAR modifiers with an exact pitch alignment

[Marek Olšák <maraeo@xxxxxxxxx>]

The comment explains the problem with DRM_FORMAT_MOD_LINEAR.
   
Signed-off-by: Marek Olšák <marek.olsak@xxxxxxx>

diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index 78abd819fd62e..8ec4163429014 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -484,9 +484,27 @@ extern "C" {
  * modifier (e.g. not setting DRM_MODE_FB_MODIFIERS in the DRM_ADDFB2 ioctl),
  * which tells the driver to also take driver-internal information into account
  * and so might actually result in a tiled framebuffer.
+ *
+ * WARNING:
+ * There are drivers out there that expose DRM_FORMAT_MOD_LINEAR, but only
+ * support a certain pitch alignment and can't import images with this modifier
+ * if the pitch alignment isn't exactly the one supported. They can however
+ * allocate images with this modifier and other drivers can import them only
+ * if they support the same pitch alignment. Thus, DRM_FORMAT_MOD_LINEAR is
+ * fundamentically incompatible across devices and is the only modifier that
+ * has a chance of not working. The PITCH_ALIGN modifiers should be used
+ * instead.
  */
 #define DRM_FORMAT_MOD_LINEAR  fourcc_mod_code(NONE, 0)
 
+/* Linear layout modifiers with an explicit pitch alignment in bytes.
+ * Exposing this modifier requires that the pitch alignment is exactly
+ * the number in the definition.
+ */
+#define DRM_FORMAT_MOD_LINEAR_PITCH_ALIGN_64B fourcc_mod_code(NONE, 1)