Re: An extremely simplified pinctrl bindings proposal

From: Stephen Warren
Date: Tue Feb 07 2012 - 00:33:50 EST

On 02/06/2012 11:05 AM, Mitch Bradley wrote:
> I like the general approach of simplifying the pinctrl thing, as the
> previous approach did not appear to be converging.
> One possible name would be "gpconfig" - for general purpose configuration.

Sounds reasonable

> The register access model in the strawman proposal is probably too
> simple. 32-bit memory mapped registers are certainly the most common
> subcase on ARM, but there are many other cases that occur in practice:
> * Registers that must be accessed with 8, 16, or 64-bit cycles.
> * Registers that have side effects on read, so read-mask-write must be
> avoided
> * Registers accessed via an index/data cycle pair, thus having locking
> requirements
> * Registers that must be read after being written, or otherwise
> requiring some sort of memory-ordering enforcement.
> * Time delays between pairs of writes
> * PCI configuration registers, which often have some combination of the
> above
> * Registers behind serial buses like I2C
> Both Open Firmware and ACPI have addressed this general problem. In
> addition to a numeric identifier for the register, you need to specify
> the access semantics. It's difficult to finitely enumerate all possible
> cases, but you can get to 99.9% with a modest number of access models,
> and then add new models as needed.

My thinking was that each driver that permitted execution of these
register write sequences on its registers would provide an extremely
simple driver to support this. Perhaps just one "op" function that
implemented each write, and took care of details such as timing delays
for the IO, indexed addressing, using I2C instead of memory-mapped,
perhaps register access width, etc.

Some of the items in the list above could be solved by explicit flags in
the data instead (register access width comes to mind for memory-mapped
devices at least).

Do you have any direct pointers to how OF and ACPI solved this kind of
thing; it'd be good background.

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