Re: [Outreachy kernel] Re: [PATCH v3 3/4] staging: iio: adc: ad7192: get_filter_freq code optimization

From: Stefano Brivio <sbrivio@redhat.com>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Deepak R Varma <mh12gx2825@gmail.com>,
	outreachy-kernel@googlegroups.com,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Daniel Baluta <daniel.baluta@gmail.com>,
	kieran.bingham@ideasonboard.com,
	Lars-Peter Clausen <lars@metafoo.de>,
	Michael Hennerich <Michael.Hennerich@analog.com>,
	Jonathan Cameron <jic23@kernel.org>,
	Hartmut Knaack <knaack.h@gmx.de>,
	Peter Meerwald <pmeerw@pmeerw.net>,
	linux-iio <linux-iio@vger.kernel.org>
Subject: Re: [Outreachy kernel] Re: [PATCH v3 3/4] staging: iio: adc: ad7192: get_filter_freq code optimization
Date: Mon, 23 Mar 2020 13:15:31 +0100	[thread overview]
Message-ID: <20200323131531.1d6c8f93@elisabeth> (raw)
In-Reply-To: <CAHp75VfF+oDWsOF+_=U6YBeP60XLF=SLYzExa=5WjLSgoMUw9A@mail.gmail.com>

On Mon, 23 Mar 2020 11:28:52 +0200
Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

> On Mon, Mar 23, 2020 at 2:49 AM Stefano Brivio <sbrivio@redhat.com> wrote:
> > On Mon, 23 Mar 2020 01:44:20 +0200
> > Andy Shevchenko <andy.shevchenko@gmail.com> wrote:  
> > > On Sun, Mar 22, 2020 at 9:57 PM Deepak R Varma <mh12gx2825@gmail.com> wrote:  
> > > >
> > > > Current implementation of the function ad7192_get_available_filter_freq
> > > > repeats calculation of output data rate a few times. We can simplify
> > > > these steps by refactoring out the calculation of fADC. This would also
> > > > addresses the checkpatch warning of line exceeding 80 character.  
> > >
> > > I'm not sure you did an equivalent changes. I believe in the original
> > > code precision is better. Consider low clock frequencies when 10 bit
> > > right shift may hide some bits of the division.  
> >
> > Note that those bits are eventually "hidden" in the same way later,  
> 
> Even if mathematically (arithmetically) evaluation is correct, we have
> to remember that computers are bad with floating point and especially
> kernel, which uses integer arithmetic. That said, it's easy to get
> off-by-one error (due to precision lost) if we do big division before
> (not so big) multiplication.

That's exactly the point I was trying to explain below: swapping steps
in a sequence of DIV_ROUND_CLOSEST() (*not* of arithmetic divisions),
*should* not affect quantisation ("off-by-one") error.

I'm not entirely sure in this case, so a quick "demonstration" in
Python or suchlike as you suggested would be nice to have, indeed.

> > despite the different sequence, due to DIV_ROUND_CLOSEST() being used
> > at every step (both before and after the change) without other
> > operations occurring.  
> 
> By the way, where AD7192_SINC3_FILTER and AD7192_SINC4_FILTER
> multiplications disappear and why?

Those were in fact divisions (multiplications of the divisor). Overall,
these steps are now arranged in a way closer to how they are presented
in the datasheet mentioned here (up to "Chop Enabled" paragraph, page
26).

-- 
Stefano