linux-kernel - Re: [PATCH v4 1/6] drm/format-helper: Add drm_fb_xrgb8888_to_gray8

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Date:   Mon, 14 Feb 2022 15:07:03 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     linux-fbdev@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Javier Martinez Canillas <javierm@...hat.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Noralf Trønnes <noralf@...nnes.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maxime Ripard <maxime@...no.tech>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH v4 1/6] drm/format-helper: Add
 drm_fb_xrgb8888_to_gray8_line()

On Mon, Feb 14, 2022 at 01:54:59PM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 14.02.22 um 13:47 schrieb Ville Syrjälä:
> > On Mon, Feb 14, 2022 at 01:12:48PM +0100, Thomas Zimmermann wrote:
> >> Hi
> >>
> >> Am 14.02.22 um 11:38 schrieb Andy Shevchenko:
> >>> On Mon, Feb 14, 2022 at 10:03:53AM +0100, Thomas Zimmermann wrote:
> >>>> Am 11.02.22 um 16:41 schrieb Andy Shevchenko:
> >>>
> >>> ...
> >>>
> >>>>>> IMO *always* prefer a for loop over while or do-while.
> >>>>>>
> >>>>>> The for (i = 0; i < N; i++) is such a strong paradigm in C. You
> >>>>>> instantly know how many times you're going to loop, at a glance. Not so
> >>>>>> with with the alternatives, which should be used sparingly.
> >>>>>
> >>>>> while () {}  _is_ a paradigm, for-loop is syntax sugar on top of it.
> >>>>
> >>>> Naw, that's not true.
> >>>
> >>> In the section 3.5 "Loops - While and For" in "The C Programming
> >>> Language" 2nd by K&R, the authors said:
> >>
> >> Year of publication: 1988 . It's not the most up-to-date reference for C
> >> programming.
> >>
> >>>
> >>> 	The for statement ... is equivalent to ... while..."
> >>>
> >>> They said that for is equivalent to while, and not otherwise.
> >>
> >> Even leaving readability aside, it's not equivalent. You can declare
> >> variables as part of the for statement. (I know it's not the kernel's
> >> style.) Also, 'continue' statements are not well-suited in for loops,
> >> because it's non-obvious if the loop's update statement is being
> >> executed. (It isn't.)
> > 
> > It is.
> > 
> > 'continue' is just shorthand for 'goto end_of_loop_body'.
> 
> Well, indeed. lol
> 
> Fun fact: I actually had to look this up and still got it wrong. Let me 
> just count it under proving-my-point: continue in a for statement is a 
> bad idea and for isn't equivalent to while.

Nah. We use 'continue' a *lot* in for loops in kms/atomic code.
I'd be surprised if you can find many loops without a 'continue'.

Looking at the loc stats I was a bit surprised to see more 'break'
but then I realized switch() is bloating up those numbers quite
a bit.

-- 
Ville Syrjälä
Intel