Re: [PATCH v2 0/4] iio: adc: ad4080: add support for AD4880 dual-channel ADC

[Andy Shevchenko]

On Sat, Feb 14, 2026 at 12:31:12PM -0600, David Lechner wrote:
> On 2/14/26 12:11 PM, Andy Shevchenko wrote:
> > On Sat, Feb 14, 2026 at 04:08:52PM +0000, Jonathan Cameron wrote:
> >> On Sun, 8 Feb 2026 14:50:23 +0200
> >> Andy Shevchenko <andriy.shevchenko@xxxxxxxxx> wrote:
> >>> On Fri, Feb 06, 2026 at 06:07:12PM +0200, Antoniu Miclaus wrote:

...

> >>> I believe there is a better approach, what you need is rather a flag
> >>> to SPI core to tell that this is the device with shared CS.
> >>
> >> Antoniu, this comment from Andy needs addressing before we move
> >> on. It seems fairly fundamental and I'm not seeing a reply to it on list.
> >>
> >> I'm not entirely sure what Andy is suggesting will work but this
> >> is perhaps a mismatch in really understanding what is going on here.
> >> Andy, how would a flag work given they seem to be separately addressable
> >> SPI buses. I think this isn't a shared SPI CS, but rather a device
> >> with two entirely separate SPI buses. I think the only reason
> >> we are bothering to implement it as a single device at all is the
> >> shared backend.
> > 
> > My understanding that there are two devices that for whatever reason share
> 
> It is the opposite. It is a _single_ device with _two_ CS lines.

Don't we have already support for that? This changes the picture even more towards
NAKing this. See below why.

> adc@0 {
> 	reg = <0>, <1>;
> 	...
> };
> 
> > the same CS line. Yes, I probably misread the idea behind, but I meant
> > some flag for SPI device that tells SPI core that the CS it wants is shared
> > (maybe a high bit in the cs field or so), then CS core won't complain on
> > validation about using the same cs number which is "already in use".
> 
> There was one existing user in the kernel of spi_new_ancillary_device()
> that looked like this, so it seemed the right way to approach it. However,
> code was added later that caused the primary SPI device to "claim" both
> CS lines for itself and probably broke the one existing user of
> spi_new_ancillary_device() (hard to tell without hardware to test).
> 
> The idea here was to unbreak that so we could use spi_new_ancillary_device()
> just as in the existing use case.
> 
> The patch for that could have been a bit more strict to only allow the
> spi_new_ancillary_device() to take CS 1 and fail otherwise, but users
> are going to notice if it isn't working right anyway, so I didn't ask
> for more checking.

> >> There is an argument that maybe we should be looking at how
> >> to do data muxing backends to support the more general case of two
> >> separate chips feeding into a single buffer, but that's a complex
> >> beast and I'm not sure if it is something we actually need.
> 
> I think it would actually be quite similar to what is done in this
> series.

TBH, the change sounds to me like a hack. It doesn't cover other potential ways
of the multi-cs devices come into play. Given that SPI core supports multi-cs
I don't see a good justification for this patch.

What did I miss?

> > Yeah, if possible I prefer to look at the (ASCII art) schematics
> > on how the HW looks like (connections with busses and CS lines).

-- 
With Best Regards,
Andy Shevchenko