linux-kernel - Re: [PATCH] gpio: siox: indicate exclusive support of threaded IRQs

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Date:   Thu, 6 Aug 2020 15:32:41 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Thorsten Scherer <t.scherer@...elmann.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpio: siox: indicate exclusive support of threaded IRQs

On Thu, Aug 6, 2020 at 12:20 PM Thomas Gleixner <tglx@...utronix.de> wrote:

> So the solution for this driver is either to make the dispatch handler
> threaded or use the hard interrupt variant of dispatching the
> demultiplexed GPIO interrupts.

The struct gpio_irq_chip .threaded bool that the patch
sets just instructs the gpio core to issue
irq_set_nested_thread(irq, 1) on the child IRQ.

This is a driver of type "struct siox_driver" handling the
IRQ through the special .get_data callback supplied in the
driver struct and it calls handle_nested_irq(irq) so with
this fix it percolated up to the parent as intended.

So far so good. So I think the patch should be applied.

But what is behind this .get_data() callback for siox drivers?

The siox driver framework in drivers/siox dispatches calls
to .get_data() from a polling thread which is just some ordinary
kthread. It looks like this because the SIOX (I think) needs
to do polled I/O. (drivers/siox/siox-core.c)

So this is a thread but it is not an irq thread from the irq core,
however it is treated like such by the driver, and in a way what
happens is events, just polled by a thread.

So when we call handle_nested_irq() ... we are not really
calling that from an irq handler.

I am just very confused :D

But Uwe must have designed this thread to mimic IRQs
specifically? (Uwe?)

I don't know if the IRQ core even sees a difference between which
thread it gets interfaced with. I suppose it does? :/

Yours,
Linus Walleij