Date: Tue, 3 May 2011 15:09:51 -0600
From: Grant Likely <grant.likely@secretlab.ca>
To: Jamie Iles <jamie@jamieiles.com>
Cc: linux-kernel@vger.kernel.org, linux@arm.linux.org.uk, tglx@linutronix.de,
        cbouatmailru@gmail.com, arnd@arndb.de, nico@fluxnic.net
Subject: Re: [PATCHv3 0/7] gpio: extend basic_mmio_gpio for different
 controllers
Message-ID: <20110503210950.GA2866@ponder.secretlab.ca>
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On Mon, Apr 11, 2011 at 12:21:47PM +0100, Jamie Iles wrote:
> Updated from v2 with change to use the set/dat registers for input/output
> register pair devices and removed some rebasing breakage.
> 
> Jamie Iles (7):
>   basic_mmio_gpio: remove runtime width/endianness evaluation
>   basic_mmio_gpio: convert to platform_{get,set}_drvdata()
>   basic_mmio_gpio: allow overriding number of gpio
>   basic_mmio_gpio: request register regions
>   basic_mmio_gpio: detect output method at probe time
>   basic_mmio_gpio: support different input/output registers
>   basic_mmio_gpio: support direction registers
> 
>  drivers/gpio/basic_mmio_gpio.c  |  390 +++++++++++++++++++++++++++++++--------
>  include/linux/basic_mmio_gpio.h |    1 +
>  2 files changed, 311 insertions(+), 80 deletions(-)

Hey Jamie,

Thanks a lot for putting this series together.

While on the topic of a generic mmio gpio driver, I've been thinking a
lot about things that Alan, Anton, and others have been doing, and I
took a good look at the irq_chip_generic work[1] that tglx (cc'd) put
together.

There are two things that stood out.  Alan pointed out (IIRC) that a
generic gpio driver should not require each bank to be encapsulated in
a separate struct platform_device, and after mulling over it a while I
agree.  It was also pointed out by Anton that often GPIO controllers
are embedded into other devices register addresses intertwined with
other gpio banks, or even other functions.

In parallel, tglx posted the irq_chip_generic patch[1] which has to
deal with pretty much the same set of issues.  I took a close look at
how he handled it for interrupt controllers, and I think it is
entirely appropriate to use the same pattern for creating a
gpio_mmio_generic library.

[1] http://comments.gmane.org/gmane.linux.ports.arm.kernel/113697

So, the direction I would like to go is to split the basic_mmio_gpio
drivers into two parts;
  - a platform_driver, and
  - a gpio_mmio_generic library.

The platform driver would be responsible for parsing pdata and/or
device tree node data, but would call into the gpio_mmio_generic
library for actually registering the gpio banks.

I envision the gpio_mmio_generic library would look something like
this:

First, a structure for storing the register offsets froma  base
address:

struct gpio_mmio_regs {
};

Next, a structure for each generic mmio gpio instance:

struct gpio_mmio_generic {
	spinlock_t	lock;

	/* initialized by user */
	unsigned long	reg_data;
	unsigned long	reg_set;
	unsigned long	reg_clr;
	unsigned long	reg_dir;

	void __iomem	*reg_base;

	/* Runtime register value caches; may be initialized by user */
	u32		data_cache;
	u32		dir_cache;

	/* Embedded gpio_chip.  Helpers functions set up accessors, but user
	 * can override before calling gpio_mmio_generic_add() */
	struct gpio_chip chip;
};

And then some helpers for initializing/adding/removing the embedded gpio_chip:

void gpio_mmio_generic_setup(struct gpio_mmio_generic *gmg, int register_width);
int gpio_mmio_generic_add(struct gpio_mmio_generic *gmg);
void gpio_mmio_generic_remove(struct gpio_mmio_generic *gmg);

gpio_mmio_generic_setup() sets up the common callback ops in the
embedded gpio_chip, but the decisions it makes could be overridden by
the user before calling gpio_mmio_generic_add().

I've not had time to prototype this yet, but I wanted to get it
written down and out onto the list for feedback since I probably won't
have any chance to get to it until after UDS.  Bonus points to anyone
who wants to take the initiative to hack it together before I get to
it.  Extra bonus points to anyone who also converts some of the other
gpio controllers to use it.  :-D

Thoughts?
g.
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