linux-kernel - [RFC PATCH 0/5] add gpio_chip

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Date:	Tue,  8 Apr 2014 20:20:10 +0200
From:	Javier Martinez Canillas <javier.martinez@...labora.co.uk>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Alexandre Courbot <acourbot@...dia.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Kevin Hilman <khilman@...aro.org>, linux-gpio@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>
Subject: [RFC PATCH 0/5] add gpio_chip_ops to hold GPIO operations

In the kernel there are basically two patterns to implement object
oriented code in C. You can either embedded a set of function pointers
in a struct along with other members or have a separate virtual function
table (vtable) structure that hold all the functions and only store a
pointer to that vtable on our particular object.

The struct gpio_chip uses the former approach, but I don't know if that
is a design decision or is just that this code predates the fact that
the separate structure pattern is now so popular. Since the having a
the operations on a different structure has a number of benefits:

- A clean separation between state (fields) and operations (functions).
- Size reduction of struct gpio_chip since will only hold one pointer.
- These functions are not supposed to change at runtime so the const
  qualifier can be used to prevent pointers modification during execution.
- Similar drivers for a chip family can reuse their function vtable.

There is a drawback though which is that now two memory accesses are
needed to execute a GPIO operation since an additional level of
indirection is introduced but that should be minimized due temporal and
spatial memory locality.

So this is an RFC patch-set to add a virtual table to be used by
GPIO chip controllers and consist of the following patches:

Javier Martinez Canillas (5):
  gpio: add a vtable to abstract GPIO controller operations
  gpiolib: set gpio_chip operations on add using a gpio_chip_ops
  gpio: omap: convert driver to use gpio_chip_ops
  gpio: twl4030: convert driver to use gpio_chip_ops
  gpio: switch to use struct struct gpio_chip_ops

 drivers/gpio/gpio-omap.c    | 19 ++++++++-----
 drivers/gpio/gpio-twl4030.c | 10 +++++--
 drivers/gpio/gpiolib.c      | 64 ++++++++++++++++++++---------------------
 include/linux/gpio/driver.h | 69 +++++++++++++++++++++++++++------------------
 4 files changed, 93 insertions(+), 69 deletions(-)

The patch-set is not a complete one though since only the GPIO OMAP
and GPIO TWL4030 drivers have been converted so I could test it on
my platform (DM3730 OMAP IGEPv2 board).

But I preferred to send an early RFC than changing every single driver
before discussing if doing the split is worth it or not.

To not break git bisect-ability, I added some patches that are
transitional changes. If you have a better suggestion on how to
handle that please let me know.

Thanks a lot and best regards,
Javier

-- 
1.9.0

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