Hi
Am 15.12.24 um 21:53 schrieb Marek Olšák:
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)
I recently went through drivers and tried to harmonize some
pitch-related code. There are essentially all cases from 2 to 256. See
my series at
https://patchwork.freedesktop.org/series/141168/
Sometimes the driver computes the pitch requirements from the
bit-per-pixels. [1]
Hardware also has alignment requirements for framebuffer width (usually
8 pixels).
Best regards
Thomas
[1]
https://elixir.bootlin.com/linux/v6.12.6/source/drivers/gpu/drm/renesas/rcar-du/rcar_du_kms.c#L410
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
