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SHMEM-buffer backing storage is allocated from system memory; which is typically cachable. The default mode for SHMEM objects is writecombine though. Unify SHMEM semantics by defaulting to cached mappings. The exception is pages imported via dma-buf. DMA memory is usually not cached. DRM drivers that require write-combined mappings set the map_wc flag in struct drm_gem_shmem_object to true. This currently affects lima, panfrost and v3d. The drivers mgag200, udl, virtio and vkms continue to use default shmem mappings. The drivers cirrus and gm12u320 change caching flags. Both used writecombine and now switch over to shmem defaults. Both drivers use SHMEM objects as shadow buffers for internal video memory, so cached mappings will not affect them negatively. v3: * set value of shmem pointer before dereferencing it in __drm_gem_shmem_create() (Dan, kernel test robot) v2: * recreate patch on top of latest SHMEM helpers * update lima, panfrost, v3d to select writecombine (Daniel, Rob) Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de> Acked-by: Maxime Ripard <mripard@kernel.org> Link: https://patchwork.freedesktop.org/patch/msgid/20201117133156.26822-2-tzimmermann@suse.de
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.