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Ian Abbott fd1575e28a comedi: allocate DMA coherent buffer as individual pages 
Depending on the driver, the acquisition buffer is allocated either from
normal memory, or from DMA coherent memory.  For normal memory, the
buffer is allocated as individual pages, but for DMA coherent memory, it
is allocated as a single block.  Prior to commit e36472145a ("staging:
comedi: use dma_mmap_coherent for DMA-able buffer mmap"), the buffer was
allocated as individual pages for DMA coherent memory too, but that was
changed to allocate it as a single block to allow `dma_mmap_coherent()`
to be used to mmap it, because that requires the pages being mmap'ed to
be contiguous.

This patch allocates the buffer from DMA coherent memory a page at a
time again, and works around the limitation of `dma_mmap_coherent()` by
calling it in a loop for each page, with temporarily modified `vm_start`
and `vm_end` values in the VMA.  (The `vm_pgoff` value is 0.)

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20250415114008.5977-5-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2025-04-25 15:53:19 +02:00
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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.