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Date: Wed, 15 May 2019 10:02:12 -0700 From: Guenter Roeck <groeck@...gle.com> To: Douglas Anderson <dianders@...omium.org> Cc: Mark Brown <broonie@...nel.org>, Benson Leung <bleung@...omium.org>, Enric Balletbo i Serra <enric.balletbo@...labora.com>, "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>, Nicolas Boichat <drinkcat@...omium.org>, Guenter Roeck <groeck@...omium.org>, Brian Norris <briannorris@...omium.org>, Matthias Kaehlcke <mka@...omium.org>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v4 1/3] platform/chrome: cros_ec_spi: Move to real time priority for transfers On Wed, May 15, 2019 at 9:48 AM Douglas Anderson <dianders@...omium.org> wrote: > > In commit 37a186225a0c ("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 commit a1b89132dc4f ("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@...omium.org> Reviewed-by: Guenter Roeck <groeck@...omium.org>
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