Re: [PATCH 2/2] gpio: mmio: get the direction from pinctrl when there are no direction registers

[Mehmet Fide <mehmet.fide@gmail.com>]

From: Mehmet Fide <mehmet.fide@screeningeagle.com>

> [Severity: High]
> Can assigning this callback cause a scheduling-while-atomic bug?
>
> Generic MMIO GPIO chips are initialized with gc->can_sleep set to false.
>
> When an interrupt is activated, gpiochip_lock_as_irq() executes with the
> IRQ descriptor's raw spinlock held. Since it sees the chip is non-sleeping,
> it invokes this new get_direction callback directly in atomic context:

This one is real and the most important of the three: irq_activate()
runs inside the desc->lock region of __setup_irq(), reaches
gpiochip_lock_as_irq() through gpiochip_irq_domain_activate(), and for a
!can_sleep chip that calls get_direction(), so the pinctrl mutex would be
taken with interrupts off. My test kernel had DEBUG_ATOMIC_SLEEP off, the
uncontended mutex fastpath hid it.

So the callback cannot ask pinctrl. v2 changes the approach: the chip
keeps the direction in its existing shadow (sdir, under the chip's raw
spinlock), the direction setters update it, and get_direction() only
reads the shadow. The pad is asked once, in process context, from the
request() callback: gpiolib calls request() right before it calls
get_direction() for a newly requested line, so the shadow is seeded from
PIN_CONFIG_OUTPUT_ENABLE there and the line reports what the pad says.

> [Severity: High]
> Does returning these pinctrl errors break interrupt setup for valid GPIOs?

With the shadow, get_direction() never fails: a line pinctrl cannot
answer for keeps the input default, which is exactly what gpiolib
assumed before this patch, so gpiochip_lock_as_irq() behaves as it did.

> [Severity: Medium]
> Will this early exit prevent the patch from determining the initial line state?

Yes, the initial scan in gpiochip_add_data_with_key() runs before the
pin ranges exist, so nothing can be read from pinctrl at that point.
The v2 commit message will drop that claim; what the series fixes is the
WARN and the direction reported for requested lines.

Thanks,
Mehmet