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There'a limit to how big a kmalloc buffer can be, and as memory gets fragmented it becomes more difficult to get big buffers. The downside of smaller buffers is that the driver has to split the transfer up which hampers performance. Compression might also take a hit because of the splitting. Solve this by allocating the transfer buffer using vmalloc and create a SG table to be passed on to the USB subsystem. vmalloc_32() is used to avoid DMA bounce buffers on USB controllers that can only access 32-bit addresses. This also solves the problem that split transfers can give host side tearing since flushing is decoupled from rendering. usb_sg_wait() doesn't have timeout handling builtin, so it is wrapped in a timer like 4 out of 6 users in the kernel have done. v2: - Use DIV_ROUND_UP (Linus) - Add timeout note to the commit log (Linus) - Expand note about upper buffer limit (Linus) - Change var name s/timer/ctx/ in gud_usb_bulk_timeout() Reviewed-by: Linus Walleij <linus.walleij@linaro.org> Signed-off-by: Noralf Trønnes <noralf@tronnes.org> Link: https://patchwork.freedesktop.org/patch/msgid/20210701170748.58009-2-noralf@tronnes.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.