linux-kernel - Re: [PATCH] iio: adc: sc27xx: Change to polling mode to read data

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Date:   Mon, 12 Aug 2019 10:37:44 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        freeman.liu@...soc.com,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-iio@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iio: adc: sc27xx: Change to polling mode to read data

On Sun, 11 Aug 2019 at 16:03, Jonathan Cameron <jic23@...nel.org> wrote:
>
> On Tue, 6 Aug 2019 15:39:45 +0800
> Baolin Wang <baolin.wang@...aro.org> wrote:
>
> > Hi Jonathan,
> >
> > On Mon, 5 Aug 2019 at 21:50, Jonathan Cameron <jic23@...nel.org> wrote:
> > >
> > > On Mon, 29 Jul 2019 10:19:48 +0800
> > > Baolin Wang <baolin.wang@...aro.org> wrote:
> > >
> > > > Hi Jonathan,
> > > >
> > > > On Sun, 28 Jul 2019 at 01:27, Jonathan Cameron <jic23@...nel.org> wrote:
> > > > >
> > > > > On Thu, 25 Jul 2019 14:33:50 +0800
> > > > > Baolin Wang <baolin.wang@...aro.org> wrote:
> > > > >
> > > > > > From: Freeman Liu <freeman.liu@...soc.com>
> > > > > >
> > > > > > On Spreadtrum platform, the headphone will read one ADC channel multiple
> > > > > > times to identify the headphone type, and the headphone identification is
> > > > > > sensitive of the ADC reading time. And we found it will take longer time
> > > > > > to reading ADC data by using interrupt mode comparing with the polling
> > > > > > mode, thus we should change to polling mode to improve the efficiency
> > > > > > of reading data, which can identify the headphone type successfully.
> > > > > >
> > > > > > Signed-off-by: Freeman Liu <freeman.liu@...soc.com>
> > > > > > Signed-off-by: Baolin Wang <baolin.wang@...aro.org>
> > > > >
> > > > > Hi,
> > > > >
> > > > > My concerns with this sort of approach is that we may be sacrificing power
> > > > > efficiency for some usecases to support one demanding one.
> > > > >
> > > > > The maximum sleep time is 1 second (I think) which is probably too long
> > > > > to poll a register for in general.
> > > >
> > > > 1 second is the timeout time, that means something wrong when reading
> > > > the data taking 1 second, and we will poll the register status every
> > > > 500 us.
> > > > From the testing, polling mode takes less time than interrupt mode
> > > > when reading ADC data multiple times, so polling mode did not
> > > > sacrifice power
> > > > efficiency.
> > >
> > > Hmm.  I'll go with a probably on that, depends on interrupt response
> > > latency etc so isn't entirely obvious.  Faster response doesn't necessarily
> > > mean lower power.
> > >
> > > >
> > > > > Is there some way we can bound that time and perhaps switch between
> > > > > interrupt and polling modes depending on how long we expect to wait?
> > > >
> > > > I do not think the interrupt mode is needed any more, since the ADC
> > > > reading is so fast enough usually. Thanks.
> > > The reason for interrupts in such devices is usually precisely the opposite.
> > >
> > > You do it because things are slow enough that you can go to sleep
> > > for a long time before the interrupt occurs.
> > >
> > > So question becomes whether there are circumstances in which we are
> > > running with long timescales and would benefit from using interrupts.
> >
> > From our testing, the ADC version time is usually about 100us, it will
> > be faster to get data if we poll every 50us in this case. But if we
> > change to use interrupt mode, it will take millisecond level time to
> > get data. That will cause problems for those time sensitive scenarios,
> > like headphone detection, that's the main reason we can not use
> > interrupt mode.
> >
> > For those non-time-sensitive scenarios, yes, I agree with you, the
> > interrupt mode will get a better power efficiency. But ADC driver can
> > not know what scenarios asked by consumers, so changing to polling
> > mode seems the easiest way to solve the problem, and we've applied
> > this patch in our downstream kernel for a while, we did not see any
> > other problem.
> >
> > Thanks for your comments.
>
> OK. It's not ideal but sometimes such is life ;)

Thanks for your understanding :)

>
> So last question - fix or not?  If a fix, can I have a fixes tag
> please.

This is a bigger patch, I am afraid it can not be merged into stable
kernel, and original code can work at most scenarios. So I think no
need add stable tag for this patch. Thanks.

-- 
Baolin Wang
Best Regards