linux-kernel - Re: [SPLAT 3/3] gpio: dwapb: Sleeping spinlocks down IRQ mapping

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Date:   Tue, 10 Aug 2021 22:33:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Linus Walleij <linusw@...nel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-gpio@...r.kernel.org
Subject: Re: [SPLAT 3/3] gpio: dwapb: Sleeping spinlocks down IRQ mapping

On Tue, Aug 10 2021 at 14:41, Valentin Schneider wrote:
> [   11.549824] rt_spin_lock (kernel/locking/rtmutex.c:1641 (discriminator 4) kernel/locking/spinlock_rt.c:30 (discriminator 4) kernel/locking/spinlock_rt.c:36 (discriminator 4) kernel/locking/spinlock_rt.c:44 (discriminator 4)) 
> [   11.549827] dwapb_irq_ack (drivers/gpio/gpio-dwapb.c:151 drivers/gpio/gpio-dwapb.c:233) gpio_dwapb
> [   11.549831] __irq_do_set_handler (kernel/irq/chip.c:414 kernel/irq/chip.c:406 kernel/irq/chip.c:1009) 
> [   11.549833] __irq_set_handler (kernel/irq/internals.h:178 kernel/irq/chip.c:1053) 

This is gpio_chip->bgpio_lock which is a regular spinlock. AFAICT this
lock should merely serializing access to MMIO registers, so it should
not be a problem to make this lock raw.

Except for the obligatory exception:

 grgpio_irq_handler() holds that lock from the demultiplexing
 handler and invokes all handlers for the individual GPIOs which have
 interrupts enabled without ever consulting a pending register.

 That drivers usage of that lock is interesting in general, see
 grgpio_map_irq() for illustration. Quality stuff for mission critical
 systems...

 But nevertheless it should just work with a raw lock on RT AFACIT.

Thanks,

        tglx