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Thomas Zimmermann 0d087de947 drm/fbdev-dma: Add shadow buffering for deferred I/O 
[ Upstream commit 3603996432 ]

DMA areas are not necessarily backed by struct page, so we cannot
rely on it for deferred I/O. Allocate a shadow buffer for drivers
that require deferred I/O and use it as framebuffer memory.

Fixes driver errors about being "Unable to handle kernel NULL pointer
dereference at virtual address" or "Unable to handle kernel paging
request at virtual address".

The patch splits drm_fbdev_dma_driver_fbdev_probe() in an initial
allocation, which creates the DMA-backed buffer object, and a tail
that sets up the fbdev data structures. There is a tail function for
direct memory mappings and a tail function for deferred I/O with
the shadow buffer.

It is no longer possible to use deferred I/O without shadow buffer.
It can be re-added if there exists a reliably test for usable struct
page in the allocated DMA-backed buffer object.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Nuno Gonçalves <nunojpg@gmail.com>
CLoses: https://lore.kernel.org/dri-devel/CAEXMXLR55DziAMbv_+2hmLeH-jP96pmit6nhs6siB22cpQFr9w@mail.gmail.com/
Tested-by: Nuno Gonçalves <nunojpg@gmail.com>
Fixes: 5ab91447aa ("drm/tiny/ili9225: Use fbdev-dma")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: <stable@vger.kernel.org> # v6.11+
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241211090643.74250-1-tzimmermann@suse.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-07-06 11:01:49 +02:00
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README

README 

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 reStructuredText 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.