Re: SPI Device tree question from blog

[Chris Packham <Chris.Packham@xxxxxxxxxxxxxxxxxxx>]

(adding linux-spi+lkml)

Hi Lach,

On 17/04/23 12:36, Lachezar Temelkov wrote:
> Hi Chris,
>
> My name is Lach Temelkov and I saw your device tree implementation here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/spi/spi-mux.yaml
(note changed URL to point to the canonical upstream, content is the same)
> Since my Zynz Z7000 SoC does not have enough SPI, I am using one SPI 
> host and multiplexing the slave select pins as you do in the link above.
>
> However, in my application, one master  board with Zynq Z7000 sends 
> spi_clk, spi_mosi and spi_miso ( along with slave selects) to 8 other 
> IO boards, so I mux the spi_clk, mosi and miso as well.
>
> So, n the end,  I have 2 multiplexers( that i control from one 8 bit 
> gpio) , 1 mux  controls the slave select( as in the url above) and 
> another mux controlling which IO board the spi_clk, mosi and miso will go.
>
> This is done so that an EMI( electromagnetic interference) and 
> radiation does not occur- e.g, it prevents the spi _clock signals to 
> simultaneously go to 8 different boards- 12" appart. Also it helps 
> with signal integrity.
>
> Could you please  tell me ( a dts snippet maybe), how the device tree 
> would look like with 2 muxes. How can i combine them
>
> - mux 1: 4 bits - chooses  16 slave selects
> - mux 2 :3 bits - chooses 8 IO boards
>
> I really appreciate your input,
>
I developed the spi-mux support with boards that connected just the CS 
via a mux, the other lines were wired together. So it's not really 
something that I considered.

The complication for you is in the mux itself. The spi-mux code should 
work fine provided there is a mux driver that does what you need. I 
don't think the existing gpio-mux driver will handle having 1 state 
mapped to >1 gpio so you'd probably need to create your own mux driver 
that deals with that. You might be able to do some kind of multi-level 
mux but that probably won't play nice with the CS lines.