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65688d2a05
Back in9730348075("arm64: Increase the max granular size"), ARCH_DMA_MINALIGN was effectively increased to 128 bytes thanks to an increase in L1_CACHE_BYTES due to an unsubstantiated performance claim on the now obsolete ThunderX-1. Although this was reverted ind93277b983, ARCH_DMA_MINALIGN was kept at 128 bytes byebc7e21e0f("arm64: Increase ARCH_DMA_MINALIGN to 128"). During discussion of the original patch, it was reported that the change also prevented a warning during boot on (again, now obsolete) Qualcomm server hardware where the cache writeback granule was larger than 64 bytes. The reason for this warning was because non-coherent DMA could lead to data corruption due to unexpected writeback from the CPU where a cacheline is shared with other allocations. Since then, systems have appeared with larger cachelines still, and so commit8f5c9037a5("arm64/mm: Correct the cache line size warning with non coherent device") reworked the warning so that it only appears on systems where non-coherent DMA is actually required and taints the kernel with TAINT_CPU_OUT_OF_SPEC. We are not aware of any systems, even including the aforementioned obsolete machines, which have a CWG larger than 64 bytes and require non-coherent DMA. More recently, it has been reported that a ARCH_DMA_MINALIGN of 128 bytes wastes considerable memory (~6% immediately after boot on one system). Reduce ARCH_DMA_MINALIGN to 64 bytes and allow the warning/taint to indicate if there are machines that unknowingly rely on this. Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Ard Biesheuvel <ardb@kernel.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Vincent Whitchurch <vincent.whitchurch@axis.com> Link: https://lore.kernel.org/linux-arm-kernel/1442944788-17254-1-git-send-email-rric@kernel.org/ Link: https://lore.kernel.org/linux-arm-kernel/CAOZdJXUiRMAguDV+HEJqPg57MyBNqEcTyaH+ya=U93NHb-pdJA@mail.gmail.com/ Link: https://lore.kernel.org/linux-arm-kernel/20190614131141.4428-1-msys.mizuma@gmail.com/ Link: https://lore.kernel.org/r/20210517074332.28280-1-vincent.whitchurch@axis.com Acked-by: Catalin Marinas <catalin.marinas@arm.com> Acked-by: Mark Rutland <mark.rutland@arm.com> Acked-by: Arnd Bergmann <arnd@arndb.de> Acked-by: Mark Rutland <mark.rutland@arm.com> Acked-by: Ard Biesheuvel <ardb@kernel.org> Link: https://lore.kernel.org/r/20210527124356.22367-1-will@kernel.org Signed-off-by: Will Deacon <will@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.