Date: Mon, 13 May 2024 15:25:55 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Devarsh Thakkar <devarsht@...com>
Cc: Jani Nikula <jani.nikula@...el.com>, mchehab@...nel.org,
	hverkuil-cisco@...all.nl, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, benjamin.gaignard@...labora.com,
	sebastian.fricke@...labora.com, akpm@...ux-foundation.org,
	gregkh@...uxfoundation.org, adobriyan@...il.com,
	p.zabel@...gutronix.de, airlied@...il.com, daniel@...ll.ch,
	dri-devel@...ts.freedesktop.org, laurent.pinchart@...asonboard.com,
	praneeth@...com, nm@...com, vigneshr@...com, a-bhatia1@...com,
	j-luthra@...com, b-brnich@...com, detheridge@...com,
	p-mantena@...com, vijayp@...com, andrzej.p@...labora.com,
	nicolas@...fresne.ca
Subject: Re: [PATCH v7 6/8] math.h Add macros to round to closest specified
 power of 2

On Mon, May 13, 2024 at 04:55:58PM +0530, Devarsh Thakkar wrote:
> On 13/05/24 14:29, Andy Shevchenko wrote:
> > On Sat, May 11, 2024 at 11:11:14PM +0530, Devarsh Thakkar wrote:
> >> On 10/05/24 20:45, Jani Nikula wrote:

[...]

> >>> Moreover, I think the naming of round_up() and round_down() should have
> >>> reflected the fact that they operate on powers of 2. It's unfortunate
> >>> that the difference to roundup() and rounddown() is just the underscore!
> >>> That's just a trap.
> >>>
> >>> So let's perhaps not repeat the same with round_closest_up() and
> >>> round_closest_down()?
> >>
> >> Yes the naming is inspired by existing macros i.e. round_up, round_down
> >> (which round up/down to next pow of 2) and DIV_ROUND_CLOSEST (which
> >> round the divided value to closest value) and there are already a lot of
> >> users for these API's :
> >>
> >>   linux-next git:(heads/next-20240509) ✗ grep -nr round_up drivers | wc
> >>     730    4261   74775
> >>
> >>   linux-next git:(heads/next-20240509) ✗ grep -nr round_down drivers | wc
> >>     226    1293   22194
> >>
> >>  linux-next git:(heads/next-20240509) ✗ grep -nr DIV_ROUND_CLOSEST
> >> drivers | wc
> >>    1207    7461  111822
> > 
> > Side note, discover `git grep ...`: it's much much faster on Git index,
> > than classic one on a working copy.
> > 
> >> so I thought to align with existing naming convention assuming
> >> developers are already familiar with this.
> >>
> >> But if a wider consensus is to go with a newer naming convention then I
> >> am open to it, although a challenge there would be to keep it short. For
> >> e.g. this one is already 3 words, if we go with more explicit
> >> "round_closest_up_pow_2" it looks quite long in my opinion :) .
> > 
> > You need properly name the macros. Again, round_up() / roundup() and
> > roundup_pow_of_two() are three _different_ macros, and it's not clear
> > why you can't use one of them in your case.
> 
> I can't use any of these because these macros either round up or round down,
> whereas I want to round to closest value for the argument specified by the
> user, be it achieved either by rounding up or rounding down depending upon
> whichever makes the answer closer to the user-specified argument.
> 
> To make it clear, I have already included the examples in the macro
> description [2], copying it here, maybe I can put the same examples in the
> commit message too to avoid confusions :
> 
> round_closest_up(17, 4) = 16
> round_closest_up(15, 4) = 16
> round_closest_up(14, 4) = 16
> 
> round_closest_down(17, 4) = 16
> round_closest_down(15, 4) = 16
> round_closest_down(14, 4) = 12
> 
> Coming back to naming, this is as per existing convention used for naming
> round_up, round_down (notice the `_` being used for macros working with pow of
> 2) and DIV_ROUND_CLOSEST (notice the work `closest` used to specify the answer
> to be nearest to specified value). Naming is a bit subjective, but I
> personally don't think it is a good idea to go away with the existing naming
> convention or go with longer names.
> 
> > The patch that changes those to a new one are doubtful to begin with.
> > I.e. need a careful review on the arithmetics side of the change
> > including HW capabilities of handling "closest" results.
> 
> This is already tested from my side, in-fact I have posted some of the results
> in cover-letter with these macros [1] :
> 
> Regarding hardware capabilities, it uses existing round_up, round_down macros
> underneath which are optimized to handle pow of 2 after modifying the user
> provided argument using addition/subtraction and division, so I don't think it
> should generally a problem with the hardware.
> And I see other macros DIV_ROUND_CLOSEST [3] already using similar operations
> i.e. addition/subtraction and division so don't think it should be a problem
> to keep similar other macros in the same file.

Okay, thank you for elaborating. So, what I would expect in the new version of
the series is that:
- align naming (with the existing round*() macros)
- add examples into commit message of the math.h patch
- add test cases (you need to create lib/math_kunit.c for that)
- elaborate in the commit message of the IPU3 change what you posted above
  (some kind of a summary)

> [1]:
> https://gist.github.com/devarsht/de6f5142f678bb1a5338abfd9f814abd#file-v7_jpeg_encoder_crop_validation-L204
> [2]: https://lore.kernel.org/all/20240509183952.4064331-1-devarsht@ti.com/
> [3]: https://elixir.bootlin.com/linux/latest/source/include/linux/math.h#L86

-- 
With Best Regards,
Andy Shevchenko

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