[DISCUSSION] spi multi chipselect support

[Sebastian Reichel <sebastian.reichel@collabora.co.uk>]

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Hi Mark,

Some SPI slave devices have more than one chip select [0]. Rob suggested
to use the following DT notation for this [1]:

spi-controller {
    slave@0 {
        reg = <0>, <1>;
    };
};

I think this makes sense from a DT point of view, but this needs
quite some changes in the SPI core. I tried to find out, how to
implement the suggestion in the last few days and I see two
possibilities:

1. Register two SPI Linux devices from one DT node. This needs the
   fewest changes in SPI core. But we loose the one-to-one mapping
   of DT nodes to devices. I have a feeling, that this will backfire.

2. Add support for multiple chip-select to spi_device and modify
   the transfer functions to make use of this. Unfortunately this
   would require changing all spi controller as far as I can see?

Can you please advise me how to proceed?

Thanks,

-- Sebastian

[0] For example microcontrollers with ISP (in-system programming)
    interface, like the NXP Kinetis K20 series or the MRF89XA,
    which has one chip-select for configuration register access
    and one for FIFO access
[1] https://patchwork.kernel.org/patch/10310109/

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