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In commit37a186225a("platform/chrome: cros_ec_spi: Transfer messages at high priority") we moved transfers to a high priority workqueue. This helped make them much more reliable. ...but, we still saw failures. We were actually finding ourselves competing for time with dm-crypt which also scheduled work on HIGHPRI workqueues. While we can consider reverting the change that made dm-crypt run its work at HIGHPRI, the argument in commita1b89132dc("dm crypt: use WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling. It does make sense for IO to be scheduled at a priority that's higher than the default user priority. It also turns out that dm-crypt isn't alone in using high priority like this. loop_prepare_queue() does something similar for loopback devices. Looking in more detail, it can be seen that the high priority workqueue isn't actually that high of a priority. It runs at MIN_NICE which is _fairly_ high priority but still below all real time priority. Should we move cros_ec_spi to real time priority to fix our problems, or is this just escalating a priority war? I'll argue here that cros_ec_spi _does_ belong at real time priority. Specifically cros_ec_spi actually needs to run quickly for correctness. As I understand this is exactly what real time priority is for. There currently doesn't appear to be any way to use the standard workqueue APIs with a real time priority, so we'll switch over to using using a kthread worker. We'll match the priority that the SPI core uses when it wants to do things on a realtime thread and just use "MAX_RT_PRIO - 1". This commit plus the patch ("platform/chrome: cros_ec_spi: Request the SPI thread be realtime") are enough to get communications very close to 100% reliable (the only known problem left is when serial console is turned on, which isn't something that happens in shipping devices). Specifically this test case now passes (tested on rk3288-veyron-jerry): dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512& while true; do ectool version > /dev/null; done It should be noted that "/var/log" is encrypted (and goes through dm-crypt) and also passes through a loopback device. Signed-off-by: Douglas Anderson <dianders@chromium.org> Reviewed-by: Guenter Roeck <groeck@chromium.org> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.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.