Re: [PATCH v1 2/2] iio: adc: Add the NXP SAR ADC support for the s32g2/3 platforms

["Nuno Sá" <noname.nuno@gmail.com>]

On Tue, 2025-09-09 at 18:22 +0200, Daniel Lezcano wrote:
> On 09/09/2025 11:29, Nuno Sá wrote:
> > Hi *Daniel* (sorry for that :)),
> > 
> > On Mon, 2025-09-08 at 08:58 -0500, David Lechner wrote:
> > > On 9/8/25 7:16 AM, Daniel Lezcano wrote:
> > > > On 05/09/2025 23:54, David Lechner wrote:
> > > > > On 9/5/25 3:58 PM, Daniel Lezcano wrote:
> > > > > > On 05/09/2025 17:25, David Lechner wrote:
> > > > > > > On 9/5/25 4:44 AM, Daniel Lezcano wrote:
> > > > > > > > On 04/09/2025 19:49, David Lechner wrote:
> > > > > > > > > On 9/4/25 12:40 PM, Daniel Lezcano wrote:
> > > > > > 
> > > > > > [ ... ]
> 
> [ ... ]
> 
> > > Unfortunately, not really. Until the last few years, there wasn't really
> > > any users of these APIs. I added
> > > devm_iio_dmaengine_buffer_setup_with_handle()
> > > for the SPI offloading work I did recently. The only reason it had to be
> > > added is because we needed to get the DMA handle from a different
> > > devicetree
> > > node from the ADC's node. Since this device has dmas and dma-names in
> > > the devicetree, then if devm_iio_dmaengine_buffer_setup[_ext]() doesn't
> > > work
> > > with that, then it might have other problems (assumptions made for a
> > > specific
> > > use case) than just not calling dma_slave_config().
> > > 
> > > I think maybe Nuno and certainly I are guilty of trying to offer you
> > > advice
> > > without looking deeply enough into what you already submitted. :-/
> > > 
> > 
> > Yes, I pretty much just asked for (at least) some discussion about this and
> > see
> > if we could use what we already have in IIO.
> > 
> > > I see now that what you are doing with the DMA looks more like other SoC
> > > ADCs
> > > (AT91/STM32/AM335x) which is quite different from how the
> > > iio_dmaengine_buffer
> > > stuff works, e.g. cyclic vs. not. So unless you are interested in evolving
> > 
> > Supporting cyclic should be fairly easy (Paul left it almost prepared for
> > it)
> > and do I have some patches already. I'm just waiting on having something
> > accepted on the ADI DMA IP driver (dmaengine) before sending the IIO patches
> > I
> > already have.
> > 
> > > the iio_dmaengine_buffer code to be more general to handle this case as
> > > well,
> > > it might not be the right tool for the job currently.
> > 
> > I think for the STM (IIRC) case they "open" coded it because the IIO DMA
> > support
> > we had at the time (if any) was more "rudimentary" - (no real zero copy
> > interface with userspace for example). But yeah, it seems there are some
> > gaps
> > for your usecase so as David said, you would need to be interested in
> > evolving
> > the IIO DMA API to accommodate your needs. Again, if nicely integrated you
> > would
> > gain some nice "improvements" in performance (not having to use the fileio
> > interface for reading the buffers) for "free".
> > 
> > As for dma_slave_config(), you're right and we have no usecase needing that
> > setup and TBH, I did not looked or have any idea (atm) on how to do it. That
> > said, I'll be here to help and contribute if you decide to try and follow
> > the
> > IIO DMA buffer API.
> 
> I would be glad to help improving the IIO DMA but I don't have enough 
> bandwidth ATM. I was hoping to get this driver merged for v6.18 which is 
> the next LTS AFAICT. Is it something possible by taking into account all 
> the comments without improving the DMA code ?
> 

No personal problem with that. But ultimately that is up to Jonathan :)

> Do you have a pointer to Paul's series and the patches you mentioned 
> above ? I can give a try when having some spare time
> 
> 

Here is the DMABUF work from Paul [1]. But I do not think this is of much use
for you (as an in kernel consumer) but it is still interesting :).

My own patches are in here [2] but you have out of tree "noise" in there [2]. At
ADI tree we had some legacy "high speed" implementation that we're still
supporting (eventually it will go away and we will sync completely with the
upstream solution). I was hopping for this [3] to land first before sending the
IIO bits but I'm having some issues with lack of bandwidth (I guess) from the
DMA maintainer. They are not really dependent on each other so I might send the
IIO stuff soon enough.

One last comment about dma_slave_config(). That should be easy to go around with
devm_iio_dmaengine_buffer_setup_with_handle(). I mean, we canl do all the DMA
chan request in the consumer driver (which would allow to easily call
dma_slave_config() when needed) and pass the DMA handle to IIO with the API
David introduced. In the future, given enough users, we could introduce an
helper in the IIO code and it would be easy to "convert" the driver. Now, if you
do need to do something special in the DMA termination callback, we would
definitely need to add the mechanism for that. In the ADI tree we do have a way
for a custom DMA submit operation [4] but we never had a proper upstreamable
user for it :(

Anyways, just some thoughts if you or someone else ever have the time for this
:)


[1]: https://lore.kernel.org/linux-iio/20240620122726.41232-1-paul@crapouillou.net/
[2]: https://github.com/analogdevicesinc/linux/commit/a1f64e5e7927d1d96da08245cce35ba9e08a5f52
[3]: https://lore.kernel.org/dmaengine/20250811-dev-axi-dmac-fixes-v1-0-8d9895f6003f@analog.com/
[4]: https://github.com/analogdevicesinc/linux/blob/main/drivers/iio/buffer/industrialio-buffer-dma.c#L1214

- Nuno Sá