linux-kernel - Re: [PATCH RFC 1/2] gpio: Add a block GPIO API to gpiolib

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Message-ID: <50682402.8020402@antcom.de>
Date:	Sun, 30 Sep 2012 12:50:42 +0200
From:	Roland Stigge <stigge@...com.de>
To:	Stijn Devriendt <highguy@...il.com>
CC:	grant.likely@...retlab.ca, linus.walleij@...aro.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	w.sang@...gutronix.de, jbe@...gutronix.de,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	bgat@...lgatliff.com
Subject: Re: [PATCH RFC 1/2] gpio: Add a block GPIO API to gpiolib

On 30/09/12 11:35, Stijn Devriendt wrote:
> In our kernel tree we have similar code. If you like I can request
> permission
> to share. I can, however, already give you an update on the basic
> structure, perhaps
> it's useful now.
> 
> For the first part, the drivers need to implement a the gpio interface
> for groups.
> gpio_set_multi, gpio_get_multi, gpio_direction_input_multi,
> gpio_direction_output_multi. Each of them gets a 'u32 mask'.
> 
> Secondly, gpiolib gets some new code to handle groups:
> groups are requested via a list of gpio ids. Mind that order is respected:
> request( [1, 5, 2, 4] ) followed by a set(0x5) will translate to
> gpio_set_multi( 0x18 ). An opaque gpio_group struct is used to keep track.
> This means the gpiolib interface also has a u32 mask, but translation is
> done for the gpio-drivers.
> 
> There is some code to request groups via device-tree (again respecting
> order)
> and there are also platform driver structures.
> 
> gpiolib was also extended to export groups into sysfs, respecting policy
> (input, output, user-selectable) and to make softlinks to groups in other
> driver's subdir. (One driver we use this in is a power-sequencer with 2
> gpios selecting a margining profile, this driver then has the gpio_group
> exported in it's sysfs dir as .../profile, allowing H/W engineers to select
> the profile without voltage glitches)
> 
> There's also a separate driver, that does nothing more than exporting
> both individual pins and groups to userspace based on platform description
> or devicetree. This is probably less interesting for mainline, since we're
> abusing device-tree to do away with some init script that can do the same.
> 
> The rationale behind a 32bit mask is that typical processors can at most
> set one processor-word worth of GPIOs at once and there are probably
> few chips with over 32GPIOs on a single gpio_chip anyway.
> Nevertheless, in the era of 64bit, it's definitely possible to go for
> u64 instead.

Hi Stijn,

thank you for your notes!

Besides what I discussed with JC and Linus, I find the unsigned int
(i.e. u32 or u64, depending on the arch) quite appealing. It is a nice
compromise between my general bit mapped data model (variable size *u8
array) and the bool *values list. Even maps easily onto a single sysfs
entry for values, by abstracting a gpio list to an actual data word.

What do others think? JC? Linus? I'm considering this (unsigned int
data) a valid option.

One question: How did you solve the one-value-per-file in the sysfs
interface?

Thanks in advance!

Roland
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