Re: Multiple SPI MCP23S17s sharing a CS line

[Linus Walleij <linus.walleij@linaro.org>]

Hi Phil,

I think the people I added to the To: line are better suited to answer
this question, they have used the MCP23* chips in practice more
than me and know how they work.

Some minor comments inline.

On Wed, Nov 27, 2019 at 12:20 PM Phil Elwell <phil@raspberrypi.org> wrote:

> We have a user asking for help to construct a Device Tree overlay to use
> the GPIOs exported by multiple MCP23S17s sharing an SPI Chip Select
> line. This is a special feature of the MCP devices whereby the SPI bus
> is treated rather like an I2C bus with each device assigned a unique
> address. His problem is in constructing gpiospecs to refer to all of the
> GPIOs.
>
> The mcp23s08 driver claims to support this feature, and there is a DT
> property ("microchip,spi-present-mask") to declare which addresses are
> used buy the devices.

It's an interesting hack and I kind of see why they are doing it.

>  I've spent an hour or so looking at the driver
> code and crawling through the kernel GPIO infrastructure, and I don't
> think it's possible. Here's my logic:
>
> 1. Although all devices that are found are presented as a single SPI
> device, they are each registered as independent gpio_chips.

So they are presented as a single SPI device, but they are
different physical packages (right?) so it is actually correct to
have several gpio_chips but incorrect that they are all
represented in a single device tree node.

Interestingly there is not a single device tree in the entire
kernel that uses the "*,spi-present-mask" attribute.

Could you provide an example?

I *THINK* the idea behind this attribute is just plain wrong
and cannot be made to work.

Instead the device should be represented as one SPI node
with subnodes for each separate physical device when this
attribute would be used.

mcp {
    compatible = "microchip,mcp23s08";
    microchip,spi-present-mask = <0x03>;
    mcp0: chip0 {
        reg = <0>;
      ....
    }
    mcp1: chip1 {
        reg = <1>;
      ....
    }
};

By introducing such child nodes it gets possible to reference
these chips by phandle <&mcp1 ...>;

Notice use of reg attribute to address subchip.

IIUC this needs to be figured out and both the DT bindings
and the driver need to be fixed to support this peculiar addressing
scheme.

Yours,
Linus Walleij