Re: spidev: Instantiating from DT as "spidev"

[Kyle Roeschley <kyle.roeschley-acOepvfBmUk@public.gmane.org>]

On Wed, Nov 29, 2017 at 05:32:55PM +0100, Geert Uytterhoeven wrote:
> 
> Implement something in sysfs like "new_device" for i2c, or "slave" for SPI.
> Then people can add a new device (e.g. "spidev") by writing to that
> virtual file.

That makes sense to me. However, the i2c "new_device" and SPI "slave" nodes are
simpler in that they only require the device name (and address for i2c). A SPI
device can have many properties specified in the device tree that we would
somehow need to get and parse. Two options I see would be:

1. Set device status to disabled in the DT and add the device when the name is
   written to the new_device sysfs node. But then, this is basically the same
   as just specifying "spidev" in the DT (minus the printks). I suppose we
   could also drop the compatible entry and require the user to specify a
   driver when writing to new_device.
2. Pass all parameters into the "new_device" node, e.g. by passing in and
   applying a DT overlay or snippet.

In my specific case [1], the device tree entires are simple, but I wouldn't
expect that for everyone. I've hacked up the first option and it works, but it
doesn't seem terribly useful. I noticed that you've been working with DT
overlays recently; what do you think of the second option?

[1] https://github.com/ni/linux/blob/nilrt/17.0/4.6/arch/arm/boot/dts/ni-76F2.dts#L108

-- 
Kyle Roeschley
Software Engineer
National Instruments
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