Re: SPI loopback tests

[Conor Dooley <conor@kernel.org>]

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On Tue, Mar 10, 2026 at 06:26:45PM +0000, Mark Brown wrote:
> On Tue, Mar 10, 2026 at 06:12:38PM +0000, Conor Dooley wrote:
> > On Tue, Mar 10, 2026 at 03:55:51PM +0000, Mark Brown wrote:
> > > On Tue, Mar 10, 2026 at 02:32:54PM +0100, Francesco Dolcini wrote:
> 
> > > > I am aware that you all DT maintainers shared in a pretty clear way your
> > > > view on the abuse of the spidev multiple times.
> 
> > > > What would you be your advice to handle the need of an SPI loopback
> > > > test? Am I missing something and there is a solution already available?
> 
> > > I'm not aware of any idiomatic way to deal with this with DT
> > > unfortunately.  Perhaps the DT people have some ideas here?
> 
> > I dunno, the suggestion seems fairly reasonable to me. I don't think
> > it's really abuse of anything, because the compatible would represent a
> > real hardware configuration. The only think that feels "abusive" is the
> > fake reg property you'd need.
> 
> In theory we should have a compatible specifically for loopback usage I
> suppose.  Depending on how the device does it's loopbacks there may
> actually be a genuine reg (if there's a chip select that's still
> selecting.

Okay, fair enough.

> > Not sure that there should be a "linux," vendor prefix though, there's
> > nothing linux-specific about doing loopback. Probably should be
> > vendor-less?
> 
> I think at this point it's old enough that it's just ABI and adding the
> option of having it vendorless would just cause people to ask why.

I'm not sure what you mean by "it's old enough", as this proposed loopback
compatible is new, right? Don't think this is any different to i2c-mux
or regulator-fixed etc etc. Unless you thought that I was saying that
"spidev" should be a compatible? Definitely was not suggesting that!

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