Re: [RFC] iio: adc: support for multi-device aggregation

[David Lechner <dlechner@baylibre.com>]

On 5/16/26 5:37 AM, Jonathan Cameron wrote:
> On Fri, 15 May 2026 13:31:02 -0300
> Jonathan Santos <jonath4nns@gmail.com> wrote:
> 
>> On 05/08, Nuno Sá wrote:
>>> On Thu, May 07, 2026 at 06:28:58PM -0300, Jonathan Santos wrote:  
>>>> Hi all,
>>>>
>>>> We have a request to support multiple devices tied together in a single evaluation
>>>> board. The goal is to be able to read them simultaneously via the IIO framework,
>>>> while also controlling them individually. Currently we have two ADC devices that
>>>> would benefit from this, but there might be more in the future.
>>>>
>>>> This is the scenario:
>>>>
>>>> +---------------+                                                 
>>>> |     ADC 0     |                                                 
>>>> |               |                                                 
>>>> |        SYNC_IN|---+---------------------------+                 
>>>> |          DRDY0|---|------------------------+  |                 
>>>> |               |   |                        |  |   +------------+
>>>> |          SCLK0|---|------+                 |  |   |    HOST    |
>>>> |           SDI0|---|------|--+              |  |   |            |
>>>> |            CS0|---|------|--|-----------+  |  +-->|ADC_SYNC    |

							Is this arrow
							supposed to be
							going the other
							direction?

>>>> |          DOUT0|---|------|--|--------+  |  |      |            |
>>>> |               |   |      |  |        |  |  |      |            |
>>>> +---------------+   |      +--|--------|--|--|----->|SCLK        |
>>>>                     |      |  +--------|--|--|----->|MOSI        |
>>>> +---------------+   |      |  |        |  |  |      |            |
>>>> |     ADC 1     |   |      |  |        |  |  |      |            |
>>>> |               |   |      |  |        |  |  +----->|DRDY0       |
>>>> |        SYNC_IN|---+      |  |        |  +-------->|CS0         |
>>>> |          DRDY1|---|------|--|----+   +----------->|MISO0       |
>>>> |               |   |      |  |    |                |            |
>>>> |          SCLK1|---|------+  |    |                |            |
>>>> |           SDI1|---|------|--+    +--------------->|DRDY1       |
>>>> |            CS1|---|------|--|-------------------->|CS1         |
>>>> |          DOUT1|---|------|--|-------------------->|MISO1       |
>>>> |               |   |      |  |                     |            |
>>>> +---------------+   |      |  |                     | .          |
>>>>                     |      |  |                     | .          |
>>>>        ...          |      |  |                     | .          |
>>>>                     |      |  |                     |            |
>>>> +---------------+   |      |  |            +------->|DRDYN       |
>>>> |     ADC N     |   |      |  |            | +----->|CSN         |
>>>> |               |   |      |  |            | | +--->|MISON       |
>>>> |        SYNC_IN|---+      |  |            | | |    |            |
>>>> |          DRDYN|----------|--|------------+ | |    +------------+
>>>> |               |          |  |              | |                  
>>>> |          SCLKN|----------+  |              | |                  
>>>> |           SDIN|-------------+              | |                  
>>>> |            CSN|----------------------------+ |                  
>>>> |          DOUTN|------------------------------+                  
>>>> |               |                                                 
>>>> +---------------+                                                                                                    
>>>>   

Would there actually be 4 DRDY inputs on the host system? I assume that
they would be the SPI offload trigger. But there can only be one trigger.
So I would expect all DRDY going into an N-input AND gate and the output
of that gate used as the trigger.

> Took me a while to get to this. 
> 
> Another datapoint to perhaps look at is daisychain SPI devices. That's
> moderately common on high end ADCs with applications such as electric car battery
> monitoring as they need to scale to 100s of devices and separate CS for each would
> be a pain.  
> 
> For those we just handled it in the ADC driver.
> 
> If we need to have a device specific aggregator driver to make any scheme work
> then it might be better to just make it a driver problem.
> 
>>>
>>> Do we have any FPGA IP for high speed transfers? If so, it would be nice
>>> to have it in the above diagram.
>>>   
>>
>> We only use the SPI-engine offload with the new multilane data feature.
>>
>>>> To summarize, the devices share SPI pins such as SCLK and MOSI, but have individual
>>>> chip-selects and MOSIs (we can consider individual SPI interfaces). The ideia
>>>> is to allow users to aggregate these devices so they can be read simultaneously
>>>> from the user space.
>>>>
>>>> I found a similar case here involving the AD4880 (ad4080 driver), which consists
>>>> of two independent ADC channels, each with its own SPI interface for configuration.
>>>> In that instance, the ancillary device feature was used because it was considered
>>>> the approach of a single device with independent channels rather than independent
>>>> devices connected together. Additionally, the backend handled the buffered data
>>>> aggregation.
>>>>
>>>> However, I would like to discuss a more generic approach to support device aggregation
>>>> across different drivers. Marcelo suggested a while ago to consider the components
>>>> framework. This would allow us to create a virtual device responsible for 
>>>> aggregating and controlling the sub-devices in a standard yet flexible manner.  
>>>
>>> component might fit here but it has it's limitations and I fear (one of
>>> the reasons I did not used for the backend stuff) is that it looks too geared for DRM. But yeah,
>>> in theory is more or less what we have here with the distinction (I
>>> think) that the type of devices are actually different :).
>>>   
>>
>> Yes, they are different, but i did not find something more similar. Here
>> the goal is to define a standard way of aggregating multiple devices
>> from the same driver.
>>
>>>>
>>>> The aggregate driver could either be an extension to the main driver (e.g. ad7768-1.c),
>>>> or a separate file (e.g. ad7768-1-agreegator.c).   
>>>
>>> I guess we could support this in the main driver (more on this below).
>>>   
>>>>
>>>> Here's an example of how the devicetree would look like: 
>>>> (includes the multiple data lanes feature)
>>>>
>>>> spi {
>>>>     #address-cells = <1>;
>>>>     #size-cells = <0>;
>>>>
>>>>     /* AD7768-1 physical devices */
>>>>     adaq7768_1_0: adaq7768-1@0 {
>>>> 	compatible = "adi,adaq7768-1";
>>>> 	reg = <0>;  /* CS0 - First physical device */
>>>>         spi-tx-lane-map = <0>;
>>>>         spi-rx-lane-map = <0>;
>>>> 	/* other properties */
>>>>     };
>>>>
>>>>     adaq7768_1_1: adaq7768-1@1 {
>>>>         compatible = "adi,adaq7768-1";
>>>>         reg = <1>;  /* CS1 - Second physical device */
>>>>         spi-tx-lane-map = <0>;
>>>>         spi-rx-lane-map = <1>;
>>>>         /* other properties */
>>>>     };
>>>>
>>>>     adaq7768_1_2: adaq7768-1@2 {
>>>>         compatible = "adi,adaq7768-1";
>>>>         reg = <2>;  /* CS2 - Third physical device */
>>>>         spi-tx-lane-map = <0>;
>>>>         spi-rx-lane-map = <2>;
>>>>         /* other properties */
>>>>     };
>>>>
>>>>     adaq7768_1_3: adaq7768-1@3 {
>>>>         compatible = "adi,adaq7768-1";
>>>>         reg = <3>;  /* CS3 */
>>>>         spi-tx-lane-map = <0>;
>>>>         spi-rx-lane-map = <3>;
>>>>         /* other properties */
>>>>     };
>>>>
>>>>     /* AD7768-1 aggregator/virtual device */
>>>>     quad_adaq7768: ad7768-1-aggregator@4 {
>>>>         compatible = "adi,ad7768-1-aggregator";
>>>>         reg = <4>; /* ? */
>>>>
>>>>         adaq7768-components = <&adaq7768_1_0>, <&adaq7768_1_1>, <&adaq7768_1_2>, <&adaq7768_1_3>;
>>>>           
>>>
>>> I guess we can avoid the dummy device! The one having the components
>>> with be the main/controller device but I guess we would still need a custom
>>> property for the other nodes in case they need to do something specific
>>> for this arrangement.
>>>   
>>
>> Yeah, defining one device as the controller looks cleaner, but we still
>> have that problem of the main "owning" or using the CS from the other
>> devices (if they are registered separately).
>>
> Long ago I did some thinking about the fidlier problem of aggregating multiple
> unrelated device driver buffer outputs - was for sensor fusions stuff.
> In the end I concluded it was easier to just do it in user space.  As long
> as they ran off the same trigger then the data alignment problem wasn't too hard.
> 
> Obviously here things are a bit different as the SPI offload IP is doing the
> data marshalling.
> 
>>>   
>>>>     };
>>>>         
>>>> };
>>>>
>>>> Is it ok to proceed with component helper for this purpose or do we have something
>>>> better? If yes, I have some following questions:
>>>>   
>>>> -> How to read all devices simultaneously in buffer mode given we can't assert   
>>>> all CS from the virtual device?  
>>>
>>> Isn't this also an HW question? Not sure how that can be done
>>> simultaneously without some kind of HW synchronization. In SW, I'm not
>>> seeing other way other than  N SPI transfers and put them together in the buffer. 
>>>   
>>
>> In HW we have that multiple data lane feature that receives the data
>> from eache SDI lane and put them in order (for FIFO mode and offload mode).
>> If we are not using offload, we could set N SPI transfers and then
>> aggregate them into one buffer. But how to do that in offload? We cannot
>> control the CS mask from userspace.
> 
> If it's not doing hardware offload is it worth the pain?  Easy enough to do
> it in userspace - particularly if we have timestamps as a bit of nearest
> timestamp maths allows easy buffer alignment.
> 

For this type of SPI controller where it can read multiple data
channels at the same time, the .dts should look like this:

spi {
    #address-cells = <1>;
    #size-cells = <0>;

    adc@0 {
	compatible = "adi,adaq7768-1";
	reg = <0>, <1>, <2>, <3>;
        
        spi-rx-bus-width = <1>, <1>, <1>, <1>;

	/* other properties */
    };
};

The reg property is the 4 CS lines and spi-rx-bus-width having 4 items means
that the controller has four SDI lines (the value <1> could be different if
each data channel/device was dual or quad SPI).

This goes along with what jic23 suggested about making it similar to
what we do with daisy-chaining.

The only tricky part with that is when devices don't share all of the
same of another property. For example, in the wiring diagram above, I
see that the DRDY lines are not connected together. But I would assume
that everything else is the same (all chips share the same power supplies,
etc.).

We would need to modify devicetree bindings a bit in cases where a property
needed a per-chip value. But I think that this happens rarely enough, that we
can just do that on an as-needed basis rather than trying to make every property
support multi-device use.



>>
>>> thou>   
>>>> -> Should the physical devices be registered in IIO during probe, or should only  
>>>> the aggregator be exposed to control attributes and general configuration?  
>>>
>>> Good question but it would likely make for a better/simpler interface if only
>>> one device was registered (with multiple channels - depending on the
>>> number of devices). Similar to backends. I guess the idea is also to
>>> only have one IIO buffer for all the channels?
>>>   
>>
>> Yes, the ideia is to have a single buffer to allow reading them
>> simultaneously from the userspace.
>>
>>> Or, IIUC, at the very least, only the aggregator could expose a buffer.
>>> But again, linking the other device channels to the buffer is not really
>>> doable without major changes in the core.
>>>
>>> Something that also just come to my mind! What about the IIO inkernel
>>> interface and things like 
>>>
>>> industrialio-buffer-cb.c
>>> industrialio-hw-consumer.c
>>>
>>> Maybe they have some limitations but something that we can work on? Not
>>> sure though...
>>>   
>>
>> The Inkernel is interesting, I will see what can be done to cover this
>> case.
> 
> How common do we think this requirement is?  If it's just a couple of
> devices I'd go a similar route to the daisy chaining case and do
> something in the specific driver.  Maybe the only point of generalization
> is the dt-bindings if we got that way.  Designing that to maybe let
> us do a generic answer in the long term might be the right compromise
> in not adding complexity to the core until it's proven to be needed.
> 
> Now if you have customers asking for it on 10 different devices today then
> sure let's look at it sooner!  Even then it might be some library code that
> the drivers use rather than big changes to the data flow in the IIO core.
> 
> Jonathan
> 
>>
>>> - Nuno Sá
>>>   
>>>>
>>>> Regards,
>>>> Jonathan S.
>>>>   
>>
>