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

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

[Jonathan Santos]

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    |
|          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|------------------------------+                  
|               |                                                 
+---------------+                                                                                                    

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.

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). 

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>;
        
    };
        
};

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?

-> Should the physical devices be registered in IIO during probe, or should only
the aggregator be exposed to control attributes and general configuration?

Regards,
Jonathan S.

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

[Andy Shevchenko]

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.

> 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.

This paragraph contradicts itself. The they share the bus. The bus is serial
and can't do at all what you are describing. Try to rephrase, or forget about
this, it's simply impossible.

-- 
With Best Regards,
Andy Shevchenko

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

[Andy Shevchenko]

On Fri, May 08, 2026 at 10:27:54AM +0300, Andy Shevchenko wrote:
> On Thu, May 07, 2026 at 06:28:58PM -0300, Jonathan Santos wrote:
> > 
> > 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.
> 
> > 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.
> 
> This paragraph contradicts itself. The they share the bus. The bus is serial
> and can't do at all what you are describing. Try to rephrase, or forget about
> this, it's simply impossible.

Ah, this is semi-shared bus... Interesting how the host controller looks like
for this? It's not a regular SPI.

-- 
With Best Regards,
Andy Shevchenko

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

[Nuno Sá]

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    |
> |          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|------------------------------+                  
> |               |                                                 
> +---------------+                                                                                                    
> 

Do we have any FPGA IP for high speed transfers? If so, it would be nice
to have it in the above diagram.

> 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 :).

> 
> 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.


>     };
>         
> };
> 
> 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. 

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?

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...

- Nuno Sá

> 
> Regards,
> Jonathan S.
> 

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

[Jonathan Santos]

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    |
> > |          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|------------------------------+                  
> > |               |                                                 
> > +---------------+                                                                                                    
> > 
> 
> 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).

> 
> >     };
> >         
> > };
> > 
> > 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.

> 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.

> - Nuno Sá
> 
> > 
> > Regards,
> > Jonathan S.
> > 

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

[Jonathan Santos]

On 05/08, Andy Shevchenko wrote:
> On Fri, May 08, 2026 at 10:27:54AM +0300, Andy Shevchenko wrote:
> > On Thu, May 07, 2026 at 06:28:58PM -0300, Jonathan Santos wrote:
> > > 
> > > 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.
> > 
> > > 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.
> > 
> > This paragraph contradicts itself. The they share the bus. The bus is serial
> > and can't do at all what you are describing. Try to rephrase, or forget about
> > this, it's simply impossible.
> 
> Ah, this is semi-shared bus... Interesting how the host controller looks like
> for this? It's not a regular SPI.
>

Yes, the only pins they don't share is the SDI and CS. The main
difference in the controller is the multiple SDI lanes, but that is
something we support in the SPI driver. Because of this feature, the 
controller can read all lanes simultaneously and put them in order on the
DMA buffer.

> -- 
> With Best Regards,
> Andy Shevchenko
> 

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

[Jonathan Cameron]

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    |
> > > |          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|------------------------------+                  
> > > |               |                                                 
> > > +---------------+                                                                                                    
> > >   
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.

> 
> > 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.
> > >   
>