Re: [PATCH RFC 0/8] AD9910 Direct Digital Synthesizer

[Nuno Sá]

On Sun, 2026-02-22 at 14:32 -0600, David Lechner wrote:
> On 2/22/26 4:01 AM, Rodrigo Alencar wrote:
> > On 26/02/21 02:16PM, David Lechner wrote:
> > > On 2/20/26 10:46 AM, Rodrigo Alencar via B4 Relay wrote:
> > > > This patch series adds support for the Analog Devices AD9910 DDS.
> > > > This is an RFC so that we can agree/discuss on the design that follows:
> > > > 
> 
> ...
> 
> > > > represents a distinct signal path into the DDS accumulator, so the driver
> > > > models them as separate IIO output channels (all IIO_ALTVOLTAGE type).
> > > 
> > > Generally IIO channels represent the physical input/output, not the
> > > internal channels.
> > 
> > That is part of the reason for this RFC. Dividing those top-level modes
> > into channels allows for better organization, as they can operate together,
> > i.e., phase or scale can be provided by single-tone profile, while
> > frequency is controlled by the digital ramp generator (see Mode Priority
> > section in the datasheet). Also, it allows to explore the most of standard
> > ABIs like, scale, frequency, phase, sampling_frequency and enable.
> > Putting everything into a single channel would make things a lot messy
> > to interface with.
> > 
> > > Ideally we would just have the one channel here with a mode selection
> > > attribute. Documentation can tell us which modes use which attributes.
> > > 
> > > > This per-channel separation allows userspace to configure each mode
> > > > independently through its own set of sysfs attributes, and to
> > > > enable/disable modes individually via IIO_CHAN_INFO_ENABLE, relying on
> > > > the hardware's own mode selection architecture.
> > > > 
> 
> Looking at Table 5 in the datasheet really helped me understand this better.
> I think this series could benefit from a documentation patch that explains
> more about how the driver works with some diagrams.
> 
> So really what we have here are a bunch of digital data generators rather
> than a bunch of altvotlage output channels. And the same data channels can be
> mixed and match as the source for up to 3 different components of the output
> (frequency, phase, amplitude) depending on the priority rules defined in
> Table 5.

More bellow... But note that all of the (or most of it) generators are going to
be feed into a DAC. Your output is altvoltage but maybe we can treat the
internals as voltage. Not sure.
 
> 
> Digital data sources are really more like a buffer in IIO terms than a
> channel. And before we added the IIO backend stuff, there wasn't really
> any other digital data source/sink that I am aware of other than buffers
> (but there are certainly a lot of odd corners of IIO that I haven't explored
> yet, so maybe I missed some).
> 
> In a recent discussion, the idea of possibly needing a way to provide
> some userspace interface to be able to tweak knobs of an IIO backend
> was also brought up.
> 
> Putting those ideas together, I'm wondering if we need some new channel
> type or even a whole new interface (e.g. a new sysfs directory like buffers
> and events) for managing these digital data sources/sinks that are not an
> IIO buffer.
> 

But what would be that channel? In the end of the day, we typically have voltage or
current DACs and a DDS primary function is indeed to generate alternating waveforms
that you then typically feed into a DAC (and in some cases from the DAC into a
power amplifier). So the DDS is just part of the data/signal path. Anyways, not sure
on the new type and I think we already have the "blocks" in IIO for dealing with this:

. frequency
. phase
. amplitude (raw + scale + offset)

But you're right that maybe it's time to think in a better way to fit them together. 
Maybe a new type (as buffers or events) can make sense where the above are treated as, example, scan
elements. Maybe it's overcomplicating, not sure. It surely needs  discussion and thinking :).

And spoiler alert, as you might have guessed already, the parallel port stuff is to be
used with DMA buffers (and IIO backends). At least, that was the plan IIRC. But Rodrigo
can confirm it.

> I think we've seen enough of these already to know that things like a
> "tone generator" and a "ramp generator" are going to be common and could
> share some standard attributes. 
> 

I tend to agree. For example, there already some DACs (with dithering) that make use of a similar
interface (but with a custom prefix). Though the end goal is different, the interface is not that
far off:


https://elixir.bootlin.com/linux/v6.19.3/source/Documentation/ABI/testing/sysfs-bus-iio-dac-ltc2688

Anyways, I knew this one would be an interesting one for upstream :)

- Nuno Sá