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The 6.9 kernel introduced a large patchset [1] designed to make gpiochip usage safe in the presence of potential hotplugging events. The changes included protecting every gpiochip access with a claim of an interlock. Running on a Pi 5 these changes reduce GPIO performance from userspace by around 10%. The penalty would be proportionally higher from kernel, as seen by SPI speed reductions. Patch the gpiolib implementation to remove the protection of gpiochip accesses. By providing alternative implementations of the relevant macros, the changes are localised and therefore easier to verify. See: https://github.com/raspberrypi/linux/issues/6854 [1] https://lwn.net/Articles/960024/ Signed-off-by: Phil Elwell <phil@raspberrypi.com>
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.
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