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

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Message-ID: <a3373804-08a4-4526-a432-c21a74ea3d6b@baylibre.com>
Date: Thu, 4 Sep 2025 12:49:21 -0500
From: David Lechner <dlechner@...libre.com>
To: Daniel Lezcano <daniel.lezcano@...aro.org>,
 Nuno Sá <noname.nuno@...il.com>, jic23@...nel.org,
 nuno.sa@...log.com, andy@...nel.org, robh@...nel.org, conor+dt@...nel.org,
 krzk+dt@...nel.org
Cc: linux-iio@...r.kernel.org, s32@....com, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org, chester62515@...il.com, mbrugger@...e.com,
 ghennadi.procopciuc@....nxp.com
Subject: Re: [PATCH v1 2/2] iio: adc: Add the NXP SAR ADC support for the
 s32g2/3 platforms

On 9/4/25 12:40 PM, Daniel Lezcano wrote:
> 
> Hi Nuno,
> 
> On 03/09/2025 13:20, Nuno Sá wrote:
>> On Wed, 2025-09-03 at 12:27 +0200, Daniel Lezcano wrote:
>>> From: Stefan-Gabriel Mirea <stefan-gabriel.mirea@....com>
>>>
>>> The NXP S32G2 and S32G3 platforms integrate a successive approximation
>>> register (SAR) ADC. Two instances are available, each providing 8
>>> multiplexed input channels with 12-bit resolution. The conversion rate
>>> is up to 1 Msps depending on the configuration and sampling window.
>>>
>>> The SAR ADC supports raw, buffer, and trigger modes. It can operate
>>> in both single-shot and continuous conversion modes, with optional
>>> hardware triggering through the cross-trigger unit (CTU) or external
>>> events. An internal prescaler allows adjusting the sampling clock,
>>> while per-channel programmable sampling times provide fine-grained
>>> trade-offs between accuracy and latency. Automatic calibration is
>>> performed at probe time to minimize offset and gain errors.
>>>
>>> The driver is derived from the BSP implementation and has been partly
>>> rewritten to comply with upstream requirements. For this reason, all
>>> contributors are listed as co-developers, while the author refers to
>>> the initial BSP driver file creator.
>>>
>>> All modes have been validated on the S32G274-RDB2 platform using an
>>> externally generated square wave captured by the ADC. Tests covered
>>> buffered streaming via IIO, trigger synchronization, and accuracy
>>> verification against a precision laboratory signal source.
>>>
>>> Co-developed-by: Alexandru-Catalin Ionita <alexandru-catalin.ionita@....com>
>>> Signed-off-by: Alexandru-Catalin Ionita <alexandru-catalin.ionita@....com>
>>> Co-developed-by: Ciprian Costea <ciprianmarian.costea@....com>
>>> Signed-off-by: Ciprian Costea <ciprianmarian.costea@....com>
>>> Co-developed-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@....nxp.com>
>>> Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@....nxp.com>
>>> Signed-off-by: Stefan-Gabriel Mirea <stefan-gabriel.mirea@....com>
>>> Co-developed-by: Daniel Lezcano <daniel.lezcano@...aro.org>
>>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
>>> ---
> 
> [ ... ]
> 
>> The above needs some discussion at the very least. Have you considered the IIO
>> DMA buffer interface? It should be extendable to accommodate any particularity
>> of your usecase (or we should at least discuss it).
>>
>> With it, you also gain a userspace interface where you can actually share DMA
>> buffers in a zero copy fashion. You can also share these buffers with USB
>> gadgets. For instance, with libiio, you would be able to fetch samples from your
>> host machine (through USB) in a very fast way (zero copy between IIO and USB).
>>
>> Setting up DMA to then "having" to push it to a SW buffer and needing a syscall
>> to retrieve the data seems counter-productive.
> 
> I've read a bit about the DMA engine. It is unclear how to use it and there are very few examples in the different drivers to refer to.
> 
> This proposed driver supports the RAW, BUFFER and TRIGGERED.
> 
> Shall I create an IIO device with the modes:
> 
> indio_dev->modes =
>     INDIO_DIRECT_MODE |

Only INDIO_DIRECT_MODE needs to be set here.

>     INDIO_BUFFER_HARDWARE |
>     INDIO_BUFFER_TRIGGERED
> 
> And then use:
> 
> devm_iio_triggered_buffer_setup()

Yes, use this and it will add INDIO_BUFFER_TRIGGERED to the flags.

> 
> and
> 
> devm_iio_dmaengine_buffer_setup_with_handle

Likewise, this will add INDIO_BUFFER_HARDWARE.

And you likely only need to call devm_iio_dmaengine_buffer_setup() which will
save some boilerplate code.

> 
> ?
> 
> 
>