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On some platforms (eg armv7 due to the CONFIG_ARM_DMA_MEM_BUFFERABLE) MMIO R/W operations always add memory barriers which can increase load, decrease battery life or in general reduce performance unnecessarily on devices which access a lot of configuration registers and where ordering does not matter (eg. media accelerators like the Verisilicon / Hantro video decoders). Drivers used to call the relaxed MMIO variants directly but since they are now accessing the MMIO registers via regmaps (to compensate for different VPU HW reg layouts via regmap fields), there is a need for a relaxed API / config to preserve existing behaviour. Cc: Mark Brown <broonie@kernel.org> Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com> Link: https://lore.kernel.org/r/20201014203024.954369-1-adrian.ratiu@collabora.com Signed-off-by: Mark Brown <broonie@kernel.org>
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.