Re: [PATCH 1/1] dt-bindings: iio: adc: add missing vref-supply

From: Alexander Stein <alexander.stein@ew.tq-group.com>
To: Marco Felsch <m.felsch@pengutronix.de>
Cc: Jonathan Cameron <jic23@kernel.org>,
	Cai Huoqing <cai.huoqing@linux.dev>,
	Haibo Chen <haibo.chen@nxp.com>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Shawn Guo <shawnguo@kernel.org>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Fabio Estevam <festevam@gmail.com>,
	devicetree@vger.kernel.org, linux-iio@vger.kernel.org,
	NXP Linux Team <linux-imx@nxp.com>,
	Pengutronix Kernel Team <kernel@pengutronix.de>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/1] dt-bindings: iio: adc: add missing vref-supply
Date: Fri, 03 Feb 2023 15:20:41 +0100	[thread overview]
Message-ID: <3214924.aeNJFYEL58@steina-w> (raw)
In-Reply-To: <20230203141217.et55oxbtxcilg3ig@pengutronix.de>

Am Freitag, 3. Februar 2023, 15:12:17 CET schrieb Marco Felsch:
> Hi,
> 
> On 23-02-03, Alexander Stein wrote:
> 
> ...
> 
> > > > > > > +  vref-supply:
> > > > > > > +    description: External ADC reference voltage supply on VREFH
> > > > > > > pad.
> > > > > > 
> > > > > > Please add it to the list of required properties, we can remove it
> > > > > > as
> > > > > > soon as the driver has support for the internal reference
> > > > > > voltages.
> > > > > 
> > > > > I was thinking in doing so before as well. But DT describes the
> > > > > hardware, and this ADC apparently would be functioning without a
> > > > > reference voltage on that pad, using a different one. What the
> > > > > driver
> > > > > actual does is a different matter.>
> > > > 
> > > > I have also thought about it first but than I checked the RM which
> > > > says
> > > > that "multi-reference selection" is chip dependent.
> > 
> > Nice for pointing this out. I wasn't aware that there are differences.
> > 
> > > Oh goody. So is it detectable?
> > 
> > That's my problem. I didn't find any source of information which chips do
> > support multiple references and which don't.
> > Marco, do you have some information on this?
> 
> You can download the RM from the NXP website but you need an account for
> it:
> https://www.nxp.com/products/processors-and-microcontrollers/arm-processors/
> i-mx-applications-processors/i-mx-8-applications-processors/i-mx-8-family-ar
> m-cortex-a53-cortex-a72-virtualization-vision-3d-graphics-4k-video:i.MX8
> 
> Or is this the wrong model? The naming scheme is quite confusing to me.

That's i.MX8 (imx8qm), the bindings are for i.MX8X (imx8qxp/imx8dxp). But I 
assume the ADC is similar/identical.

> > > If we are going to stick to a single compatible rather than adding them
> > > for
> > > the variants with and without this feature, should probably add a note
> > > at
> > > least to say it is required for some parts.
> > 
> > That's a good idea. I'm okay with that, until there is more information
> > available.
> 
> According the RM there is a bit which can be read: Multi Vref
> Implemented (MVI).

Ah, nice. So there is a hardware feature. From the RM I have available it is 
set for both imx8qm and imx8qxp. Given that I will not mark this as required, 
but add a comment regarding this feature bit.

Best regards,
Alexander

> Regards,
>   Marco

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