claude-noether
ca4dd88007
V4L2 CAPTURE buffers are V4L2_MEMORY_MMAP and mapped cached. Kernel DMA writes don't propagate to CPU cache observer; reading destination_data[] without DMA_BUF_IOCTL_SYNC(START|READ) returns stale data on RK3399 — observed as Bug 4 (H.264 partial-fill) and Bug 5 (HEVC all-zero) when libva goes through cached-mmap readback while kdirect ffmpeg-v4l2request + DRM_PRIME-mmap reads cleanly via implicit sync. Per Tomasz Figa's 2024 linaro-mm-sig discussion + feedback_rfc_v2_ vb2_dma_resv_scope.md: userspace responsibility for cache sync on cached-mmap'd V4L2 buffers. RFC v2 fence work doesn't engage this path; this ioctl pair does. Just-in-time EXPBUF + SYNC + close per copy. Per-call cost is one ioctl pair + one fd lifecycle per plane. Could cache the EXPBUF fd on cap_pool slot but doing it transient keeps lifecycle simple. Closing the EXPBUF fd is a no-op on V4L2 buffer memory. If EXPBUF or SYNC fails, fall through to existing memcpy path — preserves pre-iter13 behavior on the error branch. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
v4l2-request libVA Backend
About
This libVA backend is designed to work with the Linux Video4Linux2 Request API that is used by a number of video codecs drivers, including the Video Engine found in most Allwinner SoCs.
Status
The v4l2-request libVA backend currently supports the following formats:
- MPEG2 (Simple and Main profiles)
- H264 (Baseline, Main and High profiles)
- H265 (Main profile)
Instructions
In order to use this libVA backend, the v4l2_request driver has to
be specified through the LIBVA_DRIVER_NAME environment variable, as
such:
export LIBVA_DRIVER_NAME=v4l2_request
A media player that supports VAAPI (such as VLC) can then be used to decode a video in a supported format:
vlc path/to/video.mpg
Sample media files can be obtained from:
http://samplemedia.linaro.org/MPEG2/
http://samplemedia.linaro.org/MPEG4/SVT/
Technical Notes
Surface
A Surface is an internal data structure never handled by the VA's user containing the output of a rendering. Usualy, a bunch of surfaces are created at the begining of decoding and they are then used alternatively. When created, a surface is assigned a corresponding v4l capture buffer and it is kept until the end of decoding. Syncing a surface waits for the v4l buffer to be available and then dequeue it.
Note: since a Surface is kept private from the VA's user, it can ask to directly render a Surface on screen in an X Drawable. Some kind of implementation is available in PutSurface but this is only for development purpose.
Context
A Context is a global data structure used for rendering a video of a certain format. When a context is created, input buffers are created and v4l's output (which is the compressed data input queue, since capture is the real output) format is set.
Picture
A Picture is an encoded input frame made of several buffers. A single input can contain slice data, headers and IQ matrix. Each Picture is assigned a request ID when created and each corresponding buffer might be turned into a v4l buffers or extended control when rendered. Finally they are submitted to kernel space when reaching EndPicture.
The real rendering is done in EndPicture instead of RenderPicture because the v4l2 driver expects to have the full corresponding extended control when a buffer is queued and we don't know in which order the different RenderPicture will be called.
Image
An Image is a standard data structure containing rendered frames in a usable pixel format. Here we only use NV12 buffers which are converted from sunxi's proprietary tiled pixel format with tiled_yuv when deriving an Image from a Surface.