[PATCH] pinctrl: elaborate a bit on arrangements in doc

[Linus Walleij]

On Wed, Jun 26, 2013 at 3:15 PM, Christian Ruppert
<christian.ruppert at abilis.com> wrote:

> In my experience, in hardware engineering terminology, GPIO/General
> Purpose I/O just means a physical pad macro cell which can be
> dynamically configured in different modes, e.g. as an input or as an
> output, as an open drain driver etc. This configuration is done through
> hardware signals and controlled by digital logic. This logic might
> either be a GPIO controller or some other hardware block, e.g. an I2C
> controller block.

This is what this patch is trying to hash out. Have you seen this
presentation I did a while back?
http://www.df.lth.se/~triad/papers/pincontrol.pdf

> Hardware GPIOs have nothing to do with the concept of GPIOs from the
> Linux kernel point of view

That is unfortunately a bit of HW engineering terminology problem.
It took some months before we came up with the three non-overlapping
(well) sets of "GPIO", "pin configuration" and "pin multiplexing" to sort
these things into three different buckets.

And this entire section in Documentation/pinctrl.txt is trying to explain
this to the kernel developer.

> In some cases, both modes are combined, e.g. one can imagine an SPI
> interface where the output levels are driven from an SPI controller
> hardware block and other parameters such as the drive strength or
> integrated pull-up/pull-down resistors are controlled through some
> independent mechanism.

This is the case on the Nomadik and I think many other pin controllers.

But on the U300, the output line is controlled by two things at the
time!

> The parameters controlled through that
> independent mechanism are sometimes referred to as the GPIO mode of the
> pin.

Yes...

Yours,
Linus Walleij