linux-kernel - Re: [PATCH 1/4] dt-bindings: Add Raspberry Pi's RP1 ADC

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Message-ID: <20250930-overlord-scion-d60cff2ef1f5@spud>
Date: Tue, 30 Sep 2025 19:58:16 +0100
From: Conor Dooley <conor@...nel.org>
To: Phil Elwell <phil@...pberrypi.com>
Cc: Stanimir Varbanov <svarbanov@...e.de>,
	Guenter Roeck <linux@...ck-us.net>, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-rpi-kernel@...ts.infradead.org,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	linux-hwmon@...r.kernel.org, Jean Delvare <jdelvare@...e.com>,
	Rob Herring <robh@...nel.org>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Andrew Lunn <andrew@...n.ch>,
	Stefan Wahren <wahrenst@....net>,
	Saenz Julienne <nsaenz@...nel.org>,
	Andrea della Porta <andrea.porta@...e.com>,
	Jonathan Bell <jonathan@...pberrypi.com>,
	Dave Stevenson <dave.stevenson@...pberrypi.com>
Subject: Re: [PATCH 1/4] dt-bindings: Add Raspberry Pi's RP1 ADC

On Tue, Sep 30, 2025 at 11:47:58AM +0100, Phil Elwell wrote:
> Hi Stanimir, Guenter,
> 
> On Tue, 30 Sept 2025 at 11:21, Stanimir Varbanov <svarbanov@...e.de> wrote:
> >
> > Hi,
> >
> > On 9/25/25 11:37 PM, Guenter Roeck wrote:
> > > On Thu, Sep 25, 2025 at 08:40:54PM +0100, Conor Dooley wrote:
> > >> On Thu, Sep 25, 2025 at 03:04:13AM +0300, Stanimir Varbanov wrote:
> > >>> Document dt-bindings for Raspberry Pi's RP1 ADC.
> > >>>
> > >>> Signed-off-by: Stanimir Varbanov <svarbanov@...e.de>
> > >>> ---
> > >>>  .../bindings/hwmon/raspberrypi,rp1-adc.yaml   | 46 +++++++++++++++++++
> > >>>  1 file changed, 46 insertions(+)
> > >>>  create mode 100644 Documentation/devicetree/bindings/hwmon/raspberrypi,rp1-adc.yaml
> > >>>
> > >>> diff --git a/Documentation/devicetree/bindings/hwmon/raspberrypi,rp1-adc.yaml b/Documentation/devicetree/bindings/hwmon/raspberrypi,rp1-adc.yaml
> > >>> new file mode 100644
> > >>> index 000000000000..5266b253fd2b
> > >>> --- /dev/null
> > >>> +++ b/Documentation/devicetree/bindings/hwmon/raspberrypi,rp1-adc.yaml
> > >>> @@ -0,0 +1,46 @@
> > >>> +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > >>> +%YAML 1.2
> > >>> +---
> > >>> +$id: http://devicetree.org/schemas/hwmon/raspberrypi,rp1-adc.yaml#
> > >>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > >>> +
> > >>> +title: Rasberry Pi RP1 ADC device
> > >>> +
> > >>> +maintainers:
> > >>> +  - Stanimir Varbanov <svarbanov@...e.de>
> > >>> +
> > >>> +description: |
> > >>> +  The RP1 ADC is a five input successive-approximation ADC with 12-bit
> > >>> +  resolution (ENOB 9.5-bit) at 500kSPS. It has four external inputs
> > >>> +  and one internal temperature sensor.
> > >>> +
> > >>> +properties:
> > >>> +  compatible:
> > >>> +    const: raspberrypi,rp1-adc
> > >>> +
> > >>> +  reg:
> > >>> +    maxItems: 1
> > >>> +
> > >>> +  clocks:
> > >>> +    maxItems: 1
> > >>> +
> > >>> +  vref-supply:
> > >>> +    description:
> > >>> +      Reference voltage regulator 3.3V.
> > >>
> > >> Looks like you're missing the io-channels-cells property that allows
> > >> this device to be a provider of adc channels to other devices.
> > >>
> > > Only makes sense if the driver is implemented as iio driver.
> > > Which would be fine with me, assuming this is a generic ADC.
> > > The iio -> hwmon bridge can then be used to instantiate a
> > > hwmon device if needed.
> > >
> >
> > According to the RP1 peripheral document the information about ADC is
> > limited and I cannot be 100% sure that this is generic ADC, but it looks
> > like it is. On RPi5 board the ADC inputs are not configurable, but that
> > could change on another board.
> >
> > I personally don't have objections to implement it as IIO driver.
> >
> > Phil, are you fine with implementing the driver as IIO?
> 
> The problem with adding unused functionality, apart from the effort
> involved, is that testing it is harder. Will the IIO driver be
> inherently better/simpler because some of the hwmon support gets
> picked up by the generic IIO/HWMON bridge?
> 
> Ultimately we'll make whatever changes are considered necessary in
> order to get this simple driver accepted, but it would be nice to feel
> there was some real world benefit now for the work, not on Pi 6/7/etc.

tbh, I don't care how the driver side of things is done, I would just
like for the binding documentation to be complete. If there's no usecase
at all where the device provides io channels, then omit the property.

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