linux-kernel - Re: [PATCH 2/8] iio: adc: axp20x_adc: add support for AXP813 ADC

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Message-ID: <20171212151232.00006fd0@huawei.com>
Date:   Tue, 12 Dec 2017 15:12:32 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Quentin Schulz <quentin.schulz@...e-electrons.com>
CC:     Jonathan Cameron <jic23@...nel.org>, <sre@...nel.org>,
        <robh+dt@...nel.org>, <mark.rutland@....com>, <wens@...e.org>,
        <linux@...linux.org.uk>, <maxime.ripard@...e-electrons.com>,
        <lee.jones@...aro.org>, <knaack.h@....de>, <lars@...afoo.de>,
        <pmeerw@...erw.net>, <linux-pm@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-iio@...r.kernel.org>, <icenowy@...c.io>,
        <linux-sunxi@...glegroups.com>,
        <thomas.petazzoni@...e-electrons.com>
Subject: Re: [PATCH 2/8] iio: adc: axp20x_adc: add support for AXP813 ADC

On Mon, 11 Dec 2017 09:18:55 +0100
Quentin Schulz <quentin.schulz@...e-electrons.com> wrote:

> Hi Jonathan,
> 
> On 10/12/2017 17:36, Jonathan Cameron wrote:
> > On Mon,  4 Dec 2017 15:12:48 +0100
> > Quentin Schulz <quentin.schulz@...e-electrons.com> wrote:
> >   
> >> The X-Powers AXP813 PMIC is really close to what is already done for
> >> AXP20X/AXP22X.
> >>
> >> There are two pairs of bits to set the rate (one for Voltage and Current
> >> measurements and one for TS/GPIO0 voltage measurements) instead of one.  
> > 
> > This would normally imply we need to split the device into two logical
> > IIO devices.  However, that only becomes relevant if we are using
> > buffered output which this driver doesn't support.  
> > > It'll be nasty to deal with this if we add that support down the line  
> > though.  Up to you though as it's more likely to be your problem than
> > anyone else's :)
> >   
> 
> I have no plans for supporting buffered output for the AXPs at the
> moment. But that's an interesting (and important) limitation to raise.
> Wouldn't be more of a hack to have two IIO devices representing the
> actual same IP?

We have thought about allowing multiple buffers from a single IIO device
but that makes for some horrible changes to the ABI - so as things stand
the only option is two devices for one IP.  Ultimately they aren't really
two devices - in the same way we have triggers separating registered on
the IIO bus (often many of them).  Just two different elements of the same IP.

> 
> > For now you could elect to support the different sampling frequencies
> > if you wanted to but just providing controls for each channel.
> >   
> 
> I guess that you're offering to use IIO_CHAN_INFO_SAMP_FREQ in
> info_mask_separate for each channel?
Yes
> 
> > Given the driver doesn't currently expose these at all (I think)
> > this is all rather immaterial ;)  
> 
> I'm not giving the user the option to chose the sampling frequency for
> now. I have no plans to do it either, but I think it would be rather
> simple to later add support for setting frequency sampling since we only
> need to add a sysfs entry (with IIO_CHAN_INFO_SAMP_FREQ) that does not
> exist yet. Don't you think? Am I missing something?
No should be straight forward as long as we keep clear of the buffered
interfaces with their limitations.

> 
> Thanks,
> Quentin