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

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 8 Feb 2026 14:50:23 +0200
Andy Shevchenko <andriy.shevchenko@intel.com> wrote:

> On Fri, Feb 06, 2026 at 06:07:12PM +0200, Antoniu Miclaus wrote:
> > Add support for the AD4880, a dual-channel 20-bit 40MSPS SAR ADC from
> > the same family as AD4080.
> > 
> > The AD4880 has two independent ADC channels, each with its own SPI
> > configuration interface and LVDS data output. The driver uses
> > spi_new_ancillary_device() for the second channel's SPI and requires
> > two io-backend instances for the data interfaces.  
> 
> 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.

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.

Jonathan