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

From: "Ardelean, Alexandru" <alexandru.Ardelean@analog.com>
To: "mh12gx2825@gmail.com" <mh12gx2825@gmail.com>,
	"sbrivio@redhat.com" <sbrivio@redhat.com>,
	"Caprioru, Mircea" <Mircea.Caprioru@analog.com>
Cc: "kieran.bingham@ideasonboard.com"
	<kieran.bingham@ideasonboard.com>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
	"jic23@kernel.org" <jic23@kernel.org>,
	"outreachy-kernel@googlegroups.com"
	<outreachy-kernel@googlegroups.com>,
	"Hennerich, Michael" <Michael.Hennerich@analog.com>,
	"lars@metafoo.de" <lars@metafoo.de>,
	"andy.shevchenko@gmail.com" <andy.shevchenko@gmail.com>,
	"daniel.baluta@gmail.com" <daniel.baluta@gmail.com>,
	"pmeerw@pmeerw.net" <pmeerw@pmeerw.net>,
	"knaack.h@gmx.de" <knaack.h@gmx.de>
Subject: Re: [Outreachy kernel] Re: [PATCH v3 3/4] staging: iio: adc: ad7192: get_filter_freq code optimization
Date: Tue, 24 Mar 2020 08:06:34 +0000	[thread overview]
Message-ID: <4385dc0509f035bbd14d85b81527997f7a2538e8.camel@analog.com> (raw)
In-Reply-To: <20200323175223.GC22110@deeUbuntu>

On Mon, 2020-03-23 at 23:22 +0530, DEEPAK VARMA wrote:
> [External]
> 
> On Mon, Mar 23, 2020 at 01:15:31PM +0100, Stefano Brivio wrote:
> > 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).
> > 
> 
> Thank you Andy and Stefano for your comments. Its very thoughtful. I am
> not much familiar with Python so far, but thinking on evaluating your
> suggestion in a sample c program. I will share the outcome shortly.

+adding Mircea Caprioru

Umm, this math-cleanup looks pretty dangerous.
If possible, I wouldn't change it.
At least without some testing on HW.

Maybe doing math-simulations in Python scripts would also work, but not sure.

> 
> Deepak.
> 
> 
> > -- 
> > Stefano
> > 