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

[Andy Shevchenko]

On Mon, Feb 16, 2026 at 12:53:10PM -0600, David Lechner wrote:
> On 2/16/26 1:14 AM, Andy Shevchenko wrote:
> > On Sun, Feb 15, 2026 at 05:16:47PM -0600, David Lechner wrote:
> >> On 2/15/26 2:03 AM, Andy Shevchenko wrote:
> >>> 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.
> >>
> >> Yes, spi_new_ancillary_device() was introduced exactly for this sort
> >> of thing, which is why I think it makes sense to use it.
> >>
> >>>> 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?
> >>
> >> As far as I can tell, other than the one existing user of
> >> spi_new_ancillary_device(), other SPI multi-CS stuff is only used
> >> by SPI flash memory devices, not general SPI devices. There code
> >> that is being modified here was introduced to support the SPI
> >> flash memory devices, so that use case is already covered by
> >> existing code.
> > 
> > Right. And obvious question why can't we apply the same approach
> > to any SPI device? Like extending existing code to cover generic
> > cases.
> 
> spi_new_ancillary_device() was already accepted in the kernel as the
> solution for this sort of use case, so isn't it already the generic
> approach?

I don't think the single user functionality is considered generic.

> I can see that it could possibly be nice if the SPI core saw that
> there was more than one CS and called spi_new_ancillary_device()
> automatically and somehow passed that along with the main SPI device
> to the driver probe function. But since this is only the second user
> of spi_new_ancillary_device(), I don't think we have enough data
> points to be able to say if this is really what all peripheral drivers
> would want.

Also, if that one designed for the case, why is needed patching?

...

The  mentioned approach predates the SPI memory chip support being
integrated into SPI core. I think we should consider to kill
spi_new_ancillary_device() in favour of using the same mechanism
as being used for SPI mem chips.

-- 
With Best Regards,
Andy Shevchenko