Subject: Re: [v5] powerpc: gpio driver for mpc8349/8572/8610 and compatible

From: Nate Case <[email-masked]>
Date: Thu, 30 Oct 2008 09:39:35 -0500

On Thu, 2008-10-30 at 08:45 +0100, Peter Korsgaard wrote:
> I would write a dedicated driver for something like that instead of
> using gpiolib.
> 
> Gpiolib has quite some overhead compared to the actual work for
> changing a SoC gpio pin, but it also has some very nice
> advantages. For most stuff people use GPIO pins for, the overhead is
> not an issue (SPI chip selects, leds, keys, reset signals, ..). It's
> very handy that we nowadays have generic drivers that work with any
> GPIO (being SoC gpios or stuff on spi/i2c).

For what it's worth, I think it could be useful to have an optimally
fast gpiolib driver.  This would let you have truly generic bit-bang
drivers.  Take a look at drivers/spi for example -- there's ~7 bit-bang
drivers in there that use spi_bitbang.c for a library of functions.  I
am not familiar with these drivers, but at a glance it seems feasible
that with fast gpiolib implementations you could reduce all of those to
a single spi_bitbang.c driver.

Trent, it looks like the big performance wins for you were obtained by
deviating from the gpiolib interface?  In that case a generic SPI/JTAG
bit bang driver wouldn't benefit from the speed boosts in your MPC8572
GPIO driver (without explicitly calling mpc8572_gpio_lock and
_mpc8572_gpio_set, etc).  Is that correct?  In any case, I appreciate
what you did with your driver, so please don't give up on getting some
of your bits merged in :)

-- 
Nate Case <[email-masked]>