Re: [PATCH] mcspi: Add support for GPIO chip select lines

[Ben Gamari <bgamari.foss@gmail.com>]

On Thu, 23 Dec 2010 17:37:12 -0700, Grant Likely <grant.likely@secretlab.ca> wrote:
> On Thu, Dec 23, 2010 at 4:09 PM, Ben Gamari <bgamari.foss@gmail.com> wrote:
> > The reason I left this up to the board is it's easy to foresee cases
> > where you want a non-trivial mapping between logical CS numbers and CS
> > pin states. In my case, I using a 2-to-4 multiplexer as the source of
> > chip select.
> 
> Hi Ben,
> 
> I understand and appreciate the motivation.  However in practice, the
> gpio api is sufficient for pretty much any use case, even when the
> backing gpio controller driver ends up driving some oddball device
> with different constraints.  The big downside to using a callback is
> that it forces all users to do the extra work of implementing the
> callback.  With the gpio api, only the oddball cases have to do extra
> code (to adapt the custom device to the gpio api).

I understand your concerns, but I'm not sure how to satisfy them without
crippling the design's ability to accomodate my use-case. I can't pass a
GPIO line per spi_board_info since in my case of a multiplexed CS
configuration a single pin's state does not uniquely determine the
desired CS. The only other option I can think of is that we somehow
provide a list of GPIOs for each bus and map the CS numbers to
permutations of GPIO states. Unfortunately, I don't know of any suitable
structure to put this GPIO list in. Perhaps I'm missing something obvious?

- Ben