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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: raspberrypi#6854
[1] https://lwn.net/Articles/960024/
Signed-off-by: Phil Elwell <[email protected]>
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