Re: [PATCH RESEND] spi: spidev: Allow matching DT compatible strings from ACPI

[Mark Brown <broonie-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

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On Tue, Jun 28, 2016 at 12:22:34PM +0300, Mika Westerberg wrote:
> On Mon, Jun 27, 2016 at 01:24:48PM +0100, Mark Brown wrote:

> > Please allow a reasonable time for review, especially for an invasive
> > change like this which needs a bunch of research to figure out how
> > sensible the infrastructure to shove DT into ACPI is.

> Sure.

The big thing I need to check out here is if this breaks the check for
directly listing spidev in DT so we end up with the same abstraction
failures that plague DT since people refuse to describe their hardware.

> The /dev/spi-0 then can be used analogous to /dev/i2c-0. You first need
> to program wanted chip select using those new ioctls. Then you can use
> the device as normal spidev (all existing file operations and ioctls
> still work). You can pick another chip select as needed. If there is an
> actual SPI device bound to a chip select, that cannot be used through
> /dev/spi-0.

The trouble with this is that unlike I2C using SPI devices involves
toggling signals that we don't definitively know are connected to the
bus.  It's therefore much less safe than the equivalent thing for DT.
If we went the extra step and required that a slave be listed in DT then
that is much less of a concern.

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