Re: Multiplex SPI Chip Selects

[Chris Packham <Chris.Packham@xxxxxxxxxxxxxxxxxxx>]

Hi Anthony,

On Wed, 2020-01-01 at 09:21 -0500, Anthony DeRosa wrote:
> Hi Chris,
> 
> I saw your thread[1] about multiplexing chip selects on the SPI bus
> and wondered what solution you settled on in the end. Looks like your
> patch was not integrated into the kernel, is that right? I have a
> similar problem where I need to use GPIOs to multiplex the CS0 pin on
> a raspberry pi. Any insights would be appreciated.
> 

For the simple case the existing cs-gpios support works well. So using
that if you can is my recommendation.

Unfortunately for me it's not that simple because I have 2 platforms
which need both the gpio CS and the native CS0. This is where things
get a bit tricky.

The existing code will invert the native CS0 (because SPI_CS_HIGH is
set) I think this is a straight corner-case bug and we should either
not invert the native CS0 or have a separate flag to indicate when the
native CS0 needs inverting.

For the 2 platforms I'm dealing with I have a complication in the HW
design where the gpio is used to steer the native CS0 (as opposed to
replacing it) which is what my original series attempted to deal with.
I ended up implementing a gpio-over-gpio controller that presented 2
virtual gpios which reflected the high-low state of a single physical
gpio (basically implemented Mark's suggestion from the thread). I
haven't attempted to upstream the driver because I consider it a SW fix
for quirk of our HW designs, but if there is interest I could at least
send it out.

Regards,
Chris

--
[1] - 
https://lwn.net/ml/devicetree/20190412050213.17698-1-chris.packham@xxxxxxxxxxxxxxxxxxx/