[PATCH 0/7] drm/msm, media: define the UBWC buffer layout

[Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>]

The layout of UBWC buffers (DRM_FORMAT_MOD_QCOM_COMPRESSED, V4L2 QC08C
and QC10C) has never been written down. The V4L2 formats are documented
as opaque with four or eight planes, the DRM modifier as "macrotiles".
In practice the video decoders, the GPU and the display controller
exchange these buffers, so each side has to reverse-engineer the layout
from the other drivers. Nicolas Dufresne asked for a contract to know
what the V4L2 plugins have to compute when passing decoded frames to
DRM.

Writing it down showed that the display driver never validated it:
msm_framebuffer_init() checks UBWC framebuffers as linear ones, so short
buffers are accepted and the DPU reads past their end. Patch 1 fixes a
misleading comment in that code, patch 2 moves the layout calculation
out of DPU, patch 3 checks framebuffers against it.

Patches 4-5 document the layout for DRM and V4L2: metadata followed by
data in each plane, strides, line counts, tile sizes and alignment,
with the chroma size rounded up as DPU and the vendor headers do. For
NV12 the CbCr plane follows the Y plane in the first buffer: DPU has
always fetched it from there, ignoring offsets[1] and pitches[1], and
turnip reports offset 0 for both planes, so this is what existing
userspace relies on.

Patches 6-7 bring Venus and Iris in line with the documented QC10C
layout: Venus reported a wrong bytesperline, Iris called the format P010.

Mesa 26.3-rc rounds the NV12 chroma size down for odd sizes; that is
being fixed on the Mesa side, before the layout reaches a release.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
---
Dmitry Baryshkov (7):
      drm/msm/dpu: fix the comment on the RGB UBWC plane count
      drm/msm: move the UBWC plane size calculation out of DPU
      drm/msm: check UBWC framebuffers against their layout
      drm/fourcc: describe the Qualcomm UBWC layout
      media: docs: pixfmt-reserved: describe the QC08C and QC10C layout
      media: venus: report the QC10C data stride
      media: qcom: iris: describe QC10C as TP10

 .../userspace-api/media/v4l/pixfmt-reserved.rst    |  63 +++++++++++--
 drivers/gpu/drm/msm/disp/dpu1/dpu_formats.c        |  84 +----------------
 drivers/gpu/drm/msm/disp/mdp_format.c              | 100 +++++++++++++++++++++
 drivers/gpu/drm/msm/msm_drv.h                      |   3 +
 drivers/gpu/drm/msm/msm_fb.c                       |  39 ++++++--
 drivers/media/platform/qcom/iris/iris_buffer.c     |   2 +-
 drivers/media/platform/qcom/venus/vdec.c           |   5 +-
 include/uapi/drm/drm_fourcc.h                      |  54 +++++++++--
 8 files changed, 247 insertions(+), 103 deletions(-)
---
base-commit: f1ac44c32cba88ad65dcecd4a279661d14e93651
change-id: 20261027-ubwc-format-docs-7942079e1472

Best regards,
--  
With best wishes
Dmitry