linux-kernel - Re: [PATCH v4 07/10] iio: adc: ti-ads7924: Respect device tree config

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Message-ID: <7c79ce3a-0dc4-4aa4-941a-e05be9a34fb8@gmail.com>
Date: Sun, 2 Mar 2025 15:10:12 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: David Lechner <dlechner@...libre.com>,
 Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 Hugo Villeneuve <hvilleneuve@...onoff.com>,
 Lars-Peter Clausen <lars@...afoo.de>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Daniel Scally <djrscally@...il.com>,
 Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
 Sakari Ailus <sakari.ailus@...ux.intel.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Danilo Krummrich <dakr@...nel.org>,
 Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
 Chen-Yu Tsai <wens@...e.org>, Jernej Skrabec <jernej.skrabec@...il.com>,
 Samuel Holland <samuel@...lland.org>, Nuno Sa <nuno.sa@...log.com>,
 Javier Carrasco <javier.carrasco.cruz@...il.com>,
 Guillaume Stols <gstols@...libre.com>,
 Olivier Moysan <olivier.moysan@...s.st.com>,
 Dumitru Ceclan <mitrutzceclan@...il.com>,
 Trevor Gamblin <tgamblin@...libre.com>,
 Matteo Martelli <matteomartelli3@...il.com>,
 Alisa-Dariana Roman <alisadariana@...il.com>,
 Ramona Alexandra Nechita <ramona.nechita@...log.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
 linux-renesas-soc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-sunxi@...ts.linux.dev
Subject: Re: [PATCH v4 07/10] iio: adc: ti-ads7924: Respect device tree config

On 02/03/2025 05:27, Jonathan Cameron wrote:
> On Wed, 26 Feb 2025 08:39:11 +0200
> Matti Vaittinen <mazziesaccount@...il.com> wrote:
> 
>> On 26/02/2025 02:09, David Lechner wrote:
>>> On 2/24/25 12:34 PM, Matti Vaittinen wrote:
>>>> The ti-ads7924 driver ignores the device-tree ADC channel specification
>>>> and always exposes all 4 channels to users whether they are present in
>>>> the device-tree or not. Additionally, the "reg" values in the channel
>>>> nodes are ignored, although an error is printed if they are out of range.
>>>>
>>>> Register only the channels described in the device-tree, and use the reg
>>>> property as a channel ID.
>>>>
>>>> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
>>>>
>>>> ---
>>>> Revision history:
>>>> v3 => v4:
>>>>    - Adapt to 'drop diff-channel support' changes to ADC-helpers
>>>>    - select ADC helpers in the Kconfig
>>>> v2 => v3: New patch
>>>>
>>>> Please note that this is potentially breaking existing users if they
>>>> have wrong values in the device-tree. I believe the device-tree should
>>>> ideally be respected, and if it says device X has only one channel, then
>>>> we should believe it and not register 4. Well, we don't live in the
>>>> ideal world, so even though I believe this is TheRightThingToDo - it may
>>>> cause havoc because correct device-tree has not been required from the
>>>> day 1. So, please review and test and apply at your own risk :)
>>>
>>> The DT bindings on this one are a little weird. Usually, if we don't
>>> use any extra properties from adc.yaml, we leave out the channels. In
>>> this case it does seem kind of like the original intention was to work
>>> like you are suggesting, but hard to say since the driver wasn't actually
>>> implemented that way. I would be more inclined to actually not make the
>>> breaking change here and instead relax the bindings to make channel nodes
>>> optional and just have the driver ignore the channel nodes by dropping
>>> the ads7924_get_channels_config() function completely. This would make
>>> the driver simpler instead of more complex like this patch does.
>>
>> I have no strong opinion on this. I see this driver says 'Supported' in
>> MAINTAINERS. Maybe Hugo is able to provide some insight?
>>
> This seems to be ABI breakage.  Never something we can take if there is
> a significant chance of anyone noticing.  Here it looks like risk
> is too high.

Ok. I'll just drop this patch then. Thanks David & Jonathan :)

Yours,
	-- Matti