Re: [PATCH] pinctrl: bcm2835: Use raw spinlock for RT compatibility

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Sat, Oct 27, 2018 at 10:15 AM Lukas Wunner <lukas@xxxxxxxxx> wrote:

> The BCM2835 pinctrl driver acquires a spinlock in its ->irq_enable,
> ->irq_disable and ->irq_set_type callbacks.  Spinlocks become sleeping
> locks with CONFIG_PREEMPT_RT_FULL=y, therefore invocation of one of the
> callbacks in atomic context may cause a hard lockup if at least two GPIO
> pins in the same bank are used as interrupts.  The issue doesn't occur
> with just a single interrupt pin per bank because the lock is never
> contended.  I'm experiencing such lockups with GPIO 8 and 28 used as
> level-triggered interrupts, i.e. with ->irq_disable being invoked on
> reception of every IRQ.
>
> The critical section protected by the spinlock is very small (one bitop
> and one RMW of an MMIO register), hence converting to a raw spinlock
> seems a better trade-off than converting the driver to threaded IRQ
> handling (which would increase latency to handle an interrupt).
>
> Cc: Mathias Duckeck <m.duckeck@xxxxxxxxx>
> Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>

Looks good to me but adding Julia Cartwright and Grygorii
Strashko to this thread because they usually know this
stuff way better than me.

Yours,
Linus Walleij