Re: [PATCH 3/3] iio: adc: ti-ads112c14: add support for buffered read

[David Lechner <dlechner@baylibre.com>]

On 7/23/26 6:25 PM, Jonathan Cameron wrote:
> On Tue, 21 Jul 2026 18:23:58 -0500
> David Lechner <dlechner@baylibre.com> wrote:
> 
>> On 7/19/26 7:55 PM, Jonathan Cameron wrote:
>>> On Tue, 14 Jul 2026 19:19:32 -0500
>>> "David Lechner (TI)" <dlechner@baylibre.com> wrote:
>>>   
>>>> Add support for buffered reads using a triggered buffer.
>>>>
>>>> The device has a continuous conversion mode, but that can only be used
>>>> with one channel at a time since there is nothing like a sequencer to
>>>> support that in hardware. Instead, we use single-shot reads like we do
>>>> for direct reads to be able to read multiple channels.
>>>>
>>>> Support for continuous conversion mode could be added in the future if
>>>> needed via a 2nd buffer.  
>>>
>>> How about enabling that if only one channel is requested?  I vaguely recall
>>> us doing that for another driver (though I might be dreaming :)  I did
>>> see your comment in the cover letter about it affecting timing and that
>>> making life complex.  Fine to leave considering this for another day
>>> but maybe don't suggest a particular solution here.
>>>   
>> Actually, my latest thought it to do it by trigger type. This chip has
>> a DRDY interrupt that can be used as the trigger for the continuous
>> conversion mode but we need some software trigger to the single-shot
>> mode. It think this would take care of the timing issues as well.
>>
>> The logic would be that if the self trigger (DRDY) is selected in
>> the trigger/current_trigger attribute, then buffered read will use
>> continuous mode and fail if more than one channel is enabled. If
>> another (software) trigger is enabled, then use single-shot mode
>> and allow multiple channels.
> 
> Hmm. Bit unintuitive but can't really be helped.
> 
>>
>> The timing issue is that (or will be since I haven't sent the relevant
>> patches yet) there is a settling delay before the first sample. For
>> single-shot mode, every sample is considered the first sample (because
>> it is "single"), so the chip applies this settling time on every sample.
>> So it might be good enough to just document that as a quirk of software
>> triggers for this particular chip since they would be using single-shot
>> mode?
>>
> This is where it gets messy if settling time is reported. I guess acceptable
> given it doesn't make that much sense for an external trigger anyway.
> In theory if the settling time has to happen each single shot, we could
> just merge it into sampling frequency (as no longer depends on whether
> channel changes or not) but then we have to make that dependent on whether
> it is the devices own trigger or not. Messy. I guess one to paper over
> as a quirk / corner case and document as you say.
Yes, this is the direction I am leaning as well. The input chopping feature
that we have been talking about has a similar issue. In addition to doubling
the sample period, it also includes the settling time (2x as well) minus a
few clock cycles. Even messier.