Re: [RFC PATCH v1 0/4] Add support for Allwinner GPADC on D1/T113s/R329 SoCs

From: Jonathan Cameron <jic23@kernel.org>
To: Maxim Kiselev <bigunclemax@gmail.com>
Cc: "Andre Przywara" <andre.przywara@arm.com>,
	linux-iio@vger.kernel.org, "Lars-Peter Clausen" <lars@metafoo.de>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Chen-Yu Tsai" <wens@csie.org>,
	"Jernej Skrabec" <jernej.skrabec@gmail.com>,
	"Samuel Holland" <samuel@sholland.org>,
	"Conor Dooley" <conor@kernel.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Philipp Zabel" <p.zabel@pengutronix.de>,
	"Heiko Stuebner" <heiko.stuebner@vrull.eu>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Cosmin Tanislav" <demonsingur@gmail.com>,
	"Mike Looijmans" <mike.looijmans@topic.nl>,
	"Haibo Chen" <haibo.chen@nxp.com>,
	"ChiYuan Huang" <cy_huang@richtek.com>,
	"Ramona Bolboaca" <ramona.bolboaca@analog.com>,
	"Ibrahim Tilki" <Ibrahim.Tilki@analog.com>,
	"Caleb Connolly" <caleb.connolly@linaro.org>,
	"William Breathitt Gray" <william.gray@linaro.org>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Leonard Göhrs" <l.goehrs@pengutronix.de>,
	"AngeloGioacchino Del Regno"
	<angelogioacchino.delregno@collabora.com>,
	"Hugo Villeneuve" <hvilleneuve@dimonoff.com>,
	"ChiaEn Wu" <chiaen_wu@richtek.com>,
	devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-sunxi@lists.linux.dev, linux-kernel@vger.kernel.org,
	linux-riscv@lists.infradead.org
Subject: Re: [RFC PATCH v1 0/4] Add support for Allwinner GPADC on D1/T113s/R329 SoCs
Date: Sun, 28 May 2023 16:25:07 +0100	[thread overview]
Message-ID: <20230528162507.144cfdf2@jic23-huawei> (raw)
In-Reply-To: <20230528162257.7e8932b9@jic23-huawei>

On Sun, 28 May 2023 16:22:57 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> On Wed, 24 May 2023 14:36:28 +0300
> Maxim Kiselev <bigunclemax@gmail.com> wrote:
> 
> > Hi Andre,
> > 
> > thanks for you comments
> >   
> > > This may sound kind of obvious, but wouldn't it be easier to model this
> > > with one compatible string, and have the number of channels as a DT
> > > property?    
> > 
> > Yes, I completely agree that using separate config for each SoCs is looks
> > overcomplicated because the only difference is the number of channels.
> > I thought about a DT property with channels number but I didn't find
> > another ADC driver with the same approach (except i2c ADC's with child nodes).  
> If you are 100% sure that that devices are either
> 1) Detectable at runtime
> 2) Identical in functionality.
> 
> So that in neither case will any changes on driver support expose differences
> in the future then a single compatible is fine.
> 
> The back up is that you use fallback compatibles - list more than one.
> Whilst it doesn't matter (as no differences found) the driver can use
> the first one.  If differences become apparent later, others may be used.
> 
> I'm not however keen on a simple channel count parameter.  If you want
> to go that way, it's better to provide the fine control of individual channel
> child nodes (see Documentation/devicetree/bindings/iio/adc/adc.yaml)
> 
> That way the control is on which channels are wired to something useful, rather
> than whether the device can read them or not (which is pointless if no one
> wired them up.

Another thing to note is that with generic compatibles you loose the ability
to have the dt-schmema validate that sets of parameters make sense. If it's
just the channels available that may not matter however.

> 
> 
> >   
> > > Or, alternatively, using iio/multiplexer/io-channel-mux.yaml, since it's
> > > only one ADC anyway?    
> > I'm sorry, I didn't quite understand what you're suggesting.  
> 
> That's normally only used for a separate MUX where we need a separate driver
> to handle it.  If used on a device like this it would expose additional complexity
> to userspace with no benefits in generality etc.
> 
> >   
> > > And btw: it seems that the T507 (the H616 die with a different pinout) has
> > > the same IP, with four channels:
> > > http://dl.linux-sunxi.org/T507/    
> > 
> > Oh, thanks for pointing that. I'll add it to the list in the next version.  
> 