linux-kernel - Re: [PATCH 6/6] iio: adc: ti-adc128s052: Support ROHM BD79104

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Message-ID: <a35ab4b1-4d6a-4b95-963a-96b2ab4c05e9@gmail.com>
Date: Tue, 1 Apr 2025 15:33:15 +0300
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 Lars-Peter Clausen <lars@...afoo.de>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Nuno Sa <nuno.sa@...log.com>,
 David Lechner <dlechner@...libre.com>,
 Javier Carrasco <javier.carrasco.cruz@...il.com>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] iio: adc: ti-adc128s052: Support ROHM BD79104

On 31/03/2025 14:22, Jonathan Cameron wrote:
> On Mon, 31 Mar 2025 11:03:58 +0300
> Matti Vaittinen <mazziesaccount@...il.com> wrote:
> 
>> The ROHM BD79104 ADC has identical SPI communication logic as the
>> ti-adc128s052. Eg, SPI transfer should be 16 clk cycles, conversion is
>> started when the CS is pulled low, and channel selection is done by
>> writing the channel ID after two zero bits. Data is contained in
>> big-endian format in the last 12 bits.
> 
> Nicely found match.  Sometimes these are tricky to spot.
> 
>>
>> The BD79104 has two input voltage pins. Data sheet uses terms "vdd" and
>> "iovdd". The "vdd" is used also as an analog reference voltage. Hence
>> the driver expects finding these from the device-tree, instead of having
>> the "vref" only as TI's driver.
>>
>> NOTE: The TI's data sheet[1] does show that the TI's IC does actually
>> have two voltage inputs as well. Pins are called Va (analog reference)
>> and Vd (digital supply pin) - but I keep the existing driver behaviour
>> for the TI's IC "as is", because I have no HW to test changes, and
>> because I have no real need to touch it.
>>
>> NOTE II: The BD79104 requires SPI MODE 3.
>>
>> NOTE III: I used evaluation board "BD79104FV-EVK-001" made by ROHM. With
>> this board I had to drop the SPI speed below the 20M which is mentioned
>> in the data-sheet [2]. This, however, may be a limitation of the EVK
>> board, not the component itself.
>>
>> [1]: https://www.ti.com/lit/ds/symlink/adc128s052.pdf
>>
>> [2]:
>> https://fscdn.rohm.com/en/products/databook/datasheet/ic/data_converter/dac/bd79104fv-la-e.pdf
>>
> Prefer Datasheet tags with # [1]
> after them for the cross references.
> 
> Those belong here in the tag block (no blank lines)
>> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
> 
> One request for an additional cleanup precursor patch given you are
> touching the relevant code anyway.   It's a small one that you can
> test so hope you don't mind doing that whilst here.
> 
> I'm relying on the incredibly small chance anyone has a variable
> regulator wired up to the reference that they are modifying at runtime.
> I have seen that done (once long ago on a crazy dev board for a really
> noisy humidity sensor) when the reference was VDD but not on a separate
> reference pin.  That means we almost certainly won't break the existing
> parts and can't have a regression on your new one so we should be fine
> to make the change.

The change you ask for is indeed small. I have no real objections 
against implementing it (and I actually wrote it already) - but I am 
still somewhat hesitant. As you say, (it seems like) the idea of the 
original code is to allow changing the vref at runtime. It looks to me 
this might've been intentional choice. I am not terribly happy about 
dropping the working functionality, when the gained simplification isn't 
particularly massive.

Because of this, I am thinking of adding the patch dropping the 
functionality as an RFC. Leaving that floating on the list for a while 
would at least have my ass partially covered ;)

I'd rather not delayed the support for the BD79104 though. So - would it 
be okay if I didn't implement the clean-up as a precursory patch, but 
did it as a last patch of the series? That will make it a tad more 
complex to review, but it'd allow taking the BD79104 changes in while 
leaving the RFC to float on a list. (Also, I'm not sure if you can push 
an RFC in next without taking it in for the cycle?)

Yours,
	-- Matti