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The MMC core sets BLK_BOUNCE_HIGH for devices where dma_mask
is unassigned.
For the majority of MMC hosts this path is never taken: the
OF core will unconditionally assign a 32-bit mask to any
OF device, and most MMC hosts are probed from device tree,
see drivers/of/platform.c:
of_platform_device_create_pdata()
dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
if (!dev->dev.dma_mask)
dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
of_amba_device_create()
dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
MMC devices that are probed from ACPI or PCI will likewise
have a proper dma_mask assigned.
The only remaining devices that could have a blank dma_mask
are platform devices instantiated from board files.
These are mostly used on systems without CONFIG_HIGHMEM
enabled which means the block layer will not bounce, and in
the few cases where it is enabled it is not used anyway:
for example some OMAP2 systems such as Nokia n800/n810 will
create a platform_device and not assign a dma_mask, however
they do not have any highmem, so no bouncing will happen
anyway: the block core checks if max_low_pfn >= max_pfn
and this will always be false.
Should it turn out there is a platform_device with blank
DMA mask actually using CONFIG_HIGHMEM somewhere out there
we should set dma_mask for it, not do this trickery.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240125-mmc-no-blk-bounce-high-v1-1-d0f92a30e085@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.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.