linux-kernel - Re: [PATCH v1 3/7] iio: adc: ina2xx: Remove unneeded dummy read to clear CNVR flag

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Date:   Sun, 17 Dec 2017 10:09:22 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Stefan Brüns <stefan.bruens@...h-aachen.de>
Cc:     <linux-iio@...r.kernel.org>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Maciej Purski <m.purski@...sung.com>,
        <linux-kernel@...r.kernel.org>, "Andrew F . Davis" <afd@...com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Hartmut Knaack <knaack.h@....de>
Subject: Re: [PATCH v1 3/7] iio: adc: ina2xx: Remove unneeded dummy read to
 clear CNVR flag

On Wed, 13 Dec 2017 00:48:40 +0100
Stefan Brüns <stefan.bruens@...h-aachen.de> wrote:

> On Tuesday, December 12, 2017 9:15:30 PM CET Jonathan Cameron wrote:
> > On Sun, 10 Dec 2017 21:53:42 +0100
> > 
> > Stefan Brüns <stefan.bruens@...h-aachen.de> wrote:  
> > > On Sunday, December 10, 2017 6:27:33 PM CET Jonathan Cameron wrote:  
> > > > On Fri, 8 Dec 2017 18:41:48 +0100
> > > > 
> > > > Stefan Brüns <stefan.bruens@...h-aachen.de> wrote:  
> > > > > Although the datasheet states the CNVR flag is cleared by reading the
> > > > > BUS_VOLTAGE register, it is actually cleared by reading any of the
> > > > > voltage/current/power registers.
> > > > > 
> > > > > The behaviour has been confirmed by TI support:
> > > > > http://e2e.ti.com/support/amplifiers/current-shunt-monitors/f/931/p/64
> > > > > 7053
> > > > > /2378282
> > > > > 
> > > > > Signed-off-by: Stefan Brüns <stefan.bruens@...h-aachen.de>  
> > > > 
> > > > I haven't checked the code thoroughly so there may well be something
> > > > stopping it but have you checked the case where the only channel enabled
> > > > is
> > > > the timestamp?
> > > > 
> > > > Obviously it makes little sense, but IIRC there is nothing in the core
> > > > preventing that happening.  
> > > 
> > > The timestamp is completely unrelated to the status register, so I fail to
> > > understand your question. Can you please clarify?  
> > 
> > If you only have a timestamp, the trigger will still fire (I think)
> > but you'll do no reading at all from the device.  If configured in this,
> > admittedly odd, way you should just get a stream of timestamps with no
> > data.  
> 
> If there are reads depends on the mode - if running asynchronously, it will 
> just stream out 64 bits of timestamp each interval. In synchronous mode, the 
> driver will read the status register (low bits of bus voltage register for 
> INA219, msk register for INA226), which implicitly clears the CNVR flag.
>  
> > > This only removes a redundant read.  
> > 
> > The question is whether it is redundant if we have no non timestamp
> > registers enabled.  
> 
> According to the documentation, INA219 and 226 had to be treated differently. 
> As it turned out, both actually behave the same way regarding the CNVR flag, 
> so we just poll the status register, which for both devices clears the flag.
Ah, fine then. I thought we were talking about having to read a channel
not just the register we are anyway polling for status.

Applied to the togreg branch of iio.git.

Thanks,

Jonathan
> 
> Regards,
> 
> Stefan
> 
>