RE: [PATCH v8 0/3] iio: adc: ad4080: add support for AD4880 dual-channel ADC

[Miclaus, Antoniu]

--
Antoniu Miclăuş

> -----Original Message-----
> From: Jonathan Cameron <jic23@xxxxxxxxxx>
> Sent: Sunday, April 12, 2026 9:34 PM
> To: Miclaus, Antoniu <Antoniu.Miclaus@xxxxxxxxxx>
> Cc: Lars-Peter Clausen <lars@xxxxxxxxxx>; Hennerich, Michael
> <Michael.Hennerich@xxxxxxxxxx>; David Lechner <dlechner@xxxxxxxxxxxx>;
> Sa, Nuno <Nuno.Sa@xxxxxxxxxx>; Rob Herring <robh@xxxxxxxxxx>; Krzysztof
> Kozlowski <krzk+dt@xxxxxxxxxx>; Conor Dooley <conor+dt@xxxxxxxxxx>;
> Olivier Moysan <olivier.moysan@xxxxxxxxxxx>; linux-iio@xxxxxxxxxxxxxxx;
> devicetree@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v8 0/3] iio: adc: ad4080: add support for AD4880 dual-
> channel ADC
> 
> [External]
> 
> On Sat, 28 Mar 2026 13:40:47 +0200
> Antoniu Miclaus <antoniu.miclaus@xxxxxxxxxx> wrote:
> 
> > Add support for the AD4880, a dual-channel 20-bit 40MSPS SAR ADC with
> > integrated fully differential amplifiers (FDA).
> >
> > Architecture notes:
> >
> > The AD4880 is modeled as a single IIO device rather than two independent
> > devices because the channels share power supplies, a voltage reference,
> > the CNV conversion clock, and a single interleaved data output stream.
> > Splitting them into separate IIO devices would make synchronized
> > dual-channel capture impossible from userspace.
> >
> > An MFD approach does not apply here either - the channels are not
> > functionally distinct sub-devices but identical ADC paths sharing a
> > common data interface.
> >
> > Each channel has fully independent configuration registers accessible
> > through separate SPI chip selects, so per-channel regmaps are used with
> > no locking between them. The data path has no software involvement at
> > runtime: the CNV clock triggers simultaneous conversions and the device
> > outputs an interleaved bitstream captured directly by the IIO backend
> > (FPGA). spi_new_ancillary_device() handles the configuration path;
> > the IIO backend handles the data path.
> >
> > The debugfs_reg_access callback is not exposed for the dual-channel
> > variant since the IIO framework provides a single (reg, val) interface
> > with no channel parameter, and exposing only one channel would be
> > misleading.
> >
> > The AD4880 is a fairly unique part - having separate SPI config
> > interfaces per channel with a shared interleaved data output is not
> > a common pattern.
> I tried applying this and it's not going in cleanly (I didn't check
> exactly why).  Please could you send a rebased version.  The togreg
> branch should be fine I think, but maybe sanity check it against
> my current testing branch as well.

The AD4880 driver has a cross-tree dependency on two SPI patches that are queued in spi/for-7.1:

- ffef4123043c ("spi: allow ancillary devices to share parent's chip selects")
- 463279e58811 ("spi: add devm_spi_new_ancillary_device()")

The driver uses devm_spi_new_ancillary_device() with multi-CS to create an ancillary SPI device for the second channel's configuration interface, so it won't build against togreg alone.

What approach do you suggest in this situation?

> 
> Whilst this driver is making a few more assumptions about the backend
> than I'd ideally like, I think it is reasonable to postpone any handling
> for truely separate backends until (maybe) someone needs it.
> 
> Thanks,
> 
> Jonathan
> 
> >
> > Changes in v8:
> >   - Drop fwnode_handle cleanup patch (now in jic23/testing)
> >   - Clarify backend buffer comment to describe FPGA architecture
> >     (two axi_ad408x IP instances with a packer block)
> >   - Make filter_type a per-channel array instead of a single variable
> >   - Restore debugfs_reg_access for AD4880 (uses channel 0 regmap),
> >     based on sashiko's review
> >
> > Antoniu Miclaus (3):
> >   iio: backend: add devm_iio_backend_get_by_index()
> >   dt-bindings: iio: adc: ad4080: add AD4880 support
> >   iio: adc: ad4080: add support for AD4880 dual-channel ADC
> >
> >  .../bindings/iio/adc/adi,ad4080.yaml          |  53 +++-
> >  drivers/iio/adc/ad4080.c                      | 251 ++++++++++++++----
> >  drivers/iio/industrialio-backend.c            |  53 ++--
> >  include/linux/iio/backend.h                   |   1 +
> >  4 files changed, 282 insertions(+), 76 deletions(-)
> >