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The dmabuf import already checks that the backing buffer is contiguous and rejects it if it isn't. vc4 also requires that the buffer is in the bottom 1GB of RAM, and this is all correctly defined via dma-ranges. However the kernel silently uses swiotlb to bounce dma buffers around if they are in the wrong region. This relies on dma sync functions to be called in order to copy the data to/from the bounce buffer. DRM is based on all memory allocations being coherent with the GPU so that any updates to a framebuffer will be acted on without the need for any additional update. This is fairly fundamentally incompatible with needing to call dma_sync_ to handle the bounce buffer copies, and therefore we have to detect and reject mappings that use bounce buffers. Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Linux kernel
There are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. Please read Documentation/admin-guide/README.rst first.
In order to build the documentation, use make htmldocs or
make pdfdocs. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.
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