Re: [PATCH v2 0/4] gpiolib: some fixup patches

From: Alexandre Courbot
Date: Wed Feb 27 2013 - 23:57:32 EST

On Wed, Feb 27, 2013 at 4:52 PM, Grant Likely <grant.likely@xxxxxxxxxxxx> wrote:
> While you're working on that, I'd like you to keep the following in
> mind. I'm getting concerned with the level of overhead that the gpio
> access routines are incuring. They're doing a lot of checks right now
> when with GPIOs we want it to be as fast as possible for the case of
> mmio gpios. (i2c and spi gpios are always going to be slow, so I'm not
> so concerned here). gpio_get, gpio_set and gpio_direction all need to
> be fast.
> Basically, I think the model should be that if you've got a gpio_desc
> pointer, then you've got a valid gpio.

Yes, that's what I had in mind as well. Since the only way to obtain a
GPIO descriptor is through the gpiod_get() function(s), we can safely
issues descriptors are valid. There should be no gpiod_is_valid()

> A lot of the checks that are
> currently performed in the gpiod_ versions of functions can be moved
> to the gpio_ versions where a lookup has to be performed anyway. For
> example, right now gpiod_direction_output() is 61 lines long. Madness!
> :-)

There is certainly room to optimize things and avoid redundant tests
in the legacy GPIO API. Will be worth having a look at this once we
have a clean gpiod implementation.

> I've been playing with an idea of pulling in some basic MMIO gpio
> access directly into gpiolib so that when appropriate gpiolib itself
> can have a fast path for doing the register access and shadow register
> management.

Appealing. :)

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