[v4] drm: add modifiers for Apple GPU layouts

Commit Message

Alyssa Rosenzweig March 10, 2025, 7:28 p.m. UTC

Apple GPUs support non-linear "GPU-tiled" image layouts. Add modifiers
for these layouts. Mesa requires these modifiers to share non-linear
buffers across processes, but no other userspace or kernel support is
required/expected.

These layouts are notably not used for interchange across hardware
blocks (e.g. with the display controller). There are other layouts for
that but we don't support them either in userspace or kernelspace yet
(even downstream), so we don't add modifiers here.

Acked-by: Faith Ekstrand <[e-mail]>
Reviewed-by: Sven Peter <[e-mail]>
Signed-off-by: Alyssa Rosenzweig <[e-mail]>
---
Changes in v4:
- Remove twiddled modifiers as it is only used for sparse and therefore
  not shareable and therefore doesn't need a modifier.
- Reflow comments accordingly.
- Increase alignment to 128B out of abundance of caution.
- Collect acks.
- Link to v3: https://lore.kernel.org/r/[e-mail]

Changes in v3:
- Condense comments for clarity and concision.
- Add text explaining strides and planes with justification.
- Add table giving tile sizes for GPU tiled images.
- Tighten up wording.
- Link to v2: https://lore.kernel.org/r/[e-mail]

Changes in v2:
- Rename "Twiddled" to "GPU-tiled" to match what I now believe is the canonical name.
- Add modifiers for the actual "Twiddled" layouts.
- Clarify that the body of compressed images are laid out like their
  uncompressed counterparts.
- Link to v1: https://lore.kernel.org/r/[e-mail]
---
 include/uapi/drm/drm_fourcc.h | 45 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)


---
base-commit: 0ed1356af8f629ae807963b7db4e501e3b580bc2
change-id: 20250218-apple-twiddled-modifiers-fde1a6f4300c

Best regards,

Patch

diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index e41a3cec6a9ed18760f3b0c88ba437c9aba3dd4f..81202a50dc9e2e4363abba91ca164b30d5b2f71d 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -422,6 +422,7 @@  extern "C" {
 #define DRM_FORMAT_MOD_VENDOR_ALLWINNER 0x09
 #define DRM_FORMAT_MOD_VENDOR_AMLOGIC 0x0a
 #define DRM_FORMAT_MOD_VENDOR_MTK     0x0b
+#define DRM_FORMAT_MOD_VENDOR_APPLE   0x0c
 
 /* add more to the end as needed */
 
@@ -1494,6 +1495,50 @@  drm_fourcc_canonicalize_nvidia_format_mod(__u64 modifier)
 /* alias for the most common tiling format */
 #define DRM_FORMAT_MOD_MTK_16L_32S_TILE  DRM_FORMAT_MOD_MTK(MTK_FMT_MOD_TILE_16L32S)
 
+/*
+ * Apple GPU-tiled layouts.
+ *
+ * Apple GPUs support nonlinear tilings with optional lossless compression.
+ *
+ * GPU-tiled images are divided into 16KiB tiles:
+ *
+ *     Bytes per pixel  Tile size
+ *     ---------------  ---------
+ *                   1  128x128
+ *                   2  128x64
+ *                   4  64x64
+ *                   8  64x32
+ *                  16  32x32
+ *
+ * Tiles are raster-order. Pixels within a tile are interleaved (Morton order).
+ *
+ * Compressed images pad the body to 128-bytes and are immediately followed by a
+ * metadata section. The metadata section rounds the image dimensions to
+ * powers-of-two and contains 8 bytes for each 16x16 compression subtile.
+ * Subtiles are interleaved (Morton order).
+ *
+ * All images are 128-byte aligned.
+ *
+ * These layouts fundamentally do not have meaningful strides. No matter how we
+ * specify strides for these layouts, userspace unaware of Apple image layouts
+ * will be unable to use correctly the specified stride for any purpose.
+ * Userspace aware of the image layouts do not use strides. The most "correct"
+ * convention would be setting the image stride to 0. Unfortunately, some
+ * software assumes the stride is at least (width * bytes per pixel). We
+ * therefore require that stride equals (width * bytes per pixel). Since the
+ * stride is arbitrary here, we pick the simplest convention.
+ *
+ * Although containing two sections, compressed image layouts are treated in
+ * software as a single plane. This is modelled after AFBC, a similar
+ * scheme. Attempting to separate the sections to be "explicit" in DRM would
+ * only generate more confusion, as software does not treat the image this way.
+ *
+ * For detailed information on the hardware image layouts, see
+ * https://docs.mesa3d.org/drivers/asahi.html#image-layouts
+ */
+#define DRM_FORMAT_MOD_APPLE_GPU_TILED fourcc_mod_code(APPLE, 1)
+#define DRM_FORMAT_MOD_APPLE_GPU_TILED_COMPRESSED fourcc_mod_code(APPLE, 2)
+
 /*
  * AMD modifiers
  *