linux-kernel - Re: GPIO: Performance sensitive applications, gpiochip-level locking

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Date:	Wed, 16 Oct 2013 00:02:10 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Alexandre Courbot <acourbot@...dia.com>
Subject: Re: GPIO: Performance sensitive applications, gpiochip-level locking

On Mon, Oct 14, 2013 at 7:29 PM, Darren Hart <dvhart@...ux.intel.com> wrote:

> In the case of the gpio-sch driver, each operation for direction and
> value require a lock/unlock. There is no API in gpiolib to lock the chip
> as a whole and then make lockless calls.

I don't see why the gpiolib should handle a lock? The lock in this
driver seems to be there for this type of read/modify/write sequence:

spin_lock
val = inb()
val &= ~mask;
val |= set;
outb(val)
spin_unlock

It's quite far away from the gpiochip as such ... In the case of ARM
we are now looking at implementing atomic read/modify/write calls
so we don't have to use any locks like this, so it's something that
is not going to be useful for everyone it seems.

> We could do this for this
> specific driver, but it seems to me it would better to do so at the
> gpiolib layer. For some chips these operations might be no-ops, for
> others, like the gpio-sch chip, they could avoid the lock/unlock for
> every call and allow for some performance improvement.

Yeah, we just need to figure out how to do that properly.

> Full disclosure here, I don't yet know if the lock/unlock presents a
> performance bottleneck. I've asked the graphics driver developers to try
> with the existing API and see if it is adequate.

OK seems like a good idea. You need a lot of GPIO
traffic for this to come into effect I believe, the cycles on the
io-port bus will be the major time consumer, right? Or are
these fast?

> My thinking was more
> along the lines of:
>
> gpio_lock_chip(struct gpio_chip *chip)
> gpio_direction_input_locked(gpio)
> val = gpio_get_value_locked(gpio)
> ...
> gpio_direction_output_locked(gpio
> gpio_set_value_locked(gpio, val)
> ...
> gpio_unlock_chip(struct gpio_chip *chip)
>
> I like the possibility of your suggestion, but I wonder if it will be
> flexible enough.

Argh, all these accessors with gpiod_* accesors already
being added this kernel cycle, it's going to be a *lot*
of duplicated APIs isn't it?

But will the above be flexible? It's just some big anonymous
lock and doesn't encourage fine-grained locking. It's like a
"big GPIO lock" and that's maybe not desireable.

Yours,
Linus Walleij
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