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

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 6 Aug 2019 15:39:45 +0800
Baolin Wang <baolin.wang@linaro.org> wrote:

> Hi Jonathan,
> 
> On Mon, 5 Aug 2019 at 21:50, Jonathan Cameron <jic23@kernel.org> wrote:
> >
> > On Mon, 29 Jul 2019 10:19:48 +0800
> > Baolin Wang <baolin.wang@linaro.org> wrote:
> >  
> > > Hi Jonathan,
> > >
> > > On Sun, 28 Jul 2019 at 01:27, Jonathan Cameron <jic23@kernel.org> wrote:  
> > > >
> > > > On Thu, 25 Jul 2019 14:33:50 +0800
> > > > Baolin Wang <baolin.wang@linaro.org> wrote:
> > > >  
> > > > > From: Freeman Liu <freeman.liu@unisoc.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@unisoc.com>
> > > > > Signed-off-by: Baolin Wang <baolin.wang@linaro.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 ;)

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

Thanks,

Jonathan

>