Re: [PATCH 1/3] dt-bindings: iio: adc: stm32: add support for diff channels

From: Jonathan Cameron <Jonathan.Cameron-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
To: Fabrice Gasnier <fabrice.gasnier-qxv4g6HH51o@public.gmane.org>
Cc: Jonathan Cameron
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	devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
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	benjamin.gaignard-qxv4g6HH51o@public.gmane.org
Subject: Re: [PATCH 1/3] dt-bindings: iio: adc: stm32: add support for diff channels
Date: Mon, 23 Oct 2017 14:09:56 +0100	[thread overview]
Message-ID: <20171023140956.00002613@huawei.com> (raw)
In-Reply-To: <0726f676-0440-afbe-f275-ec7fdaa29c9f-qxv4g6HH51o@public.gmane.org>

On Mon, 23 Oct 2017 10:06:27 +0200
Fabrice Gasnier <fabrice.gasnier-qxv4g6HH51o@public.gmane.org> wrote:

> On 10/21/2017 09:23 PM, Jonathan Cameron wrote:
> > On Sat, 21 Oct 2017 18:54:01 +0100
> > Jonathan Cameron <jic23-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> >   
> >> On Tue, 17 Oct 2017 15:15:43 +0200
> >> Fabrice Gasnier <fabrice.gasnier-qxv4g6HH51o@public.gmane.org> wrote:
> >>  
> >>> STM32H7 ADC channels may be configured either as single-ended or
> >>> differential.
> >>> Add 'st,adc-diff-channels' property to support differential channels.
> >>> Differential channels are defined as a pair of positive and negative
> >>> inputs: vinp & vinn.
> >>>
> >>> Signed-off-by: Fabrice Gasnier <fabrice.gasnier-qxv4g6HH51o@public.gmane.org>    
> >> Hmm. Fair enough.  Sometimes we support both types of channels
> >> and leave it to userspace, but in many cases that makes little sense
> >> - particularly if like I think is going on here, we aren't combining channels
> >> that can be separately read but rather the negative pin is simply unused
> >> when we are in single channel mode... (did I understand that right?)  
> > I clearly didn't understand.  This is same as other devices
> > in that we are picking a pair from the channels that can otherwise be
> > used as single end channels. (I think).   
> 
> Hi Jonathan,
> 
> Indeed, we're picking a pair from the channels that can be used as
> single channels otherwise.
> Also, it doesn't makes sense to leave the choice to userspace: when
> these channels are used in differential mode, both input should be
> biased. This is quite dependent on hardware (board), not really a
> runtime choice.
> Hope this clarifies ?
> 
> > 
> > For this binding I think I'd like to see an example added showing how
> > this works...  
> 
> You're right, I should have added that. Please let me know if I can add
> bellow example in v2:
Looks good to me.

Jonathan
> 
> Example to setup differential channels 1, 2 & 3 (with resp. 0, 6 & 7
> negative inputs):
> 	adc: adc@40022000 {
> 		compatible = "st,stm32h7-adc-core";
> 		...
> 		adc1: adc@0 {
> 			compatible = "st,stm32h7-adc";
> 			...
> 			st,adc-diff-channels = <1 0>, <2 6>, <3 7>;
> 		};
> 	};
> 
> Thanks for reviewing,
> Best Regards,
> Fabrice
> 
> > 
> > Thanks,
> > 
> > Jonathan  
> >>
> >> Jonathan
> >>  
> >>> ---
> >>>  Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt | 6 ++++++
> >>>  1 file changed, 6 insertions(+)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt b/Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt
> >>> index 48bfcaa3..c3c8de1 100644
> >>> --- a/Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt
> >>> +++ b/Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt
> >>> @@ -62,6 +62,12 @@ Required properties:
> >>>  - st,adc-channels: List of single-ended channels muxed for this ADC.
> >>>    It can have up to 16 channels on stm32f4 or 20 channels on stm32h7, numbered
> >>>    from 0 to 15 or 19 (resp. for in0..in15 or in0..in19).
> >>> +- st,adc-diff-channels: List of differential channels muxed for this ADC.
> >>> +  Depending on part used, some channels can be configured as differential
> >>> +  instead of single-ended (e.g. stm32h7). List here positive and negative
> >>> +  inputs pairs as <vinp vinn>, <vinp vinn>,... vinp and vinn are numbered
> >>> +  from 0 to 19 on stm32h7)
> >>> +  Note: At least one of "st,adc-channels" or "st,adc-diff-channels" is required.
> >>>  - #io-channel-cells = <1>: See the IIO bindings section "IIO consumers" in
> >>>    Documentation/devicetree/bindings/iio/iio-bindings.txt
> >>>      
> >>
> >> --
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