[RFC] gpiolib: introduce descriptor-based GPIO interface

[Alexandre Courbot <acourbot@nvidia.com>]

Just a RFC to check if the direction seems good before going onward with more
serious work. This patch introduces a set of gpiod_* functions that work like
their gpio_* counterparts, but use opaque descriptors instead of integers.

The current integer-based GPIO namespace is bad because it allows GPIOs to be
used without being properly allocated, and poses portability concerns with the
maximum number of GPIOs of the system being a compile-time constant. Some
implementation details could also be improved by an opaque handler, e.g. the
GPIO descriptors could be moved from the current static array to a more flexible
list of GPIO chips containing their own descriptors.

With this patch GPIO handles can only be obtained by calling gpio_to_desc(),
which converts a valid GPIO number to its descriptor - proper lookup functions
that totally get rid of GPIO numbers are the next step if this first patch is
received positively.

This is only a first step towards a bigger gpiolib rework. Getting rid of the
integer handles for GPIOs will allow us to continue on with the following:
* Proper GPIO lookup functions a la regulator, pwm, etc.
* Getting rid of ARCH_NR_GPIO,
* Tearing out asm-generic/gpio.h into consumer/driver include files,
* ...

This API is only available if gpiolib is compiled in. One could however easily
implement a set of gpiod_* functions that would wrap around the old interface if
that was deemed necessary.

Comments and thoughts on potential issues are welcome, at this stage it is very
possible that I missed a few issues that will arise in later stages of this
project.

Alexandre Courbot (1):
  gpiolib: introduce descriptor-based GPIO interface

 drivers/gpio/gpiolib.c        | 302 ++++++++++++++++++++++--------------------
 include/asm-generic/gpio.h    |  74 ++++++++---
 include/linux/gpio/consumer.h |  45 +++++++
 3 files changed, 261 insertions(+), 160 deletions(-)
 create mode 100644 include/linux/gpio/consumer.h

-- 
1.8.0.1