On Fri, 11 Feb 2022, Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> On Fri, Feb 11, 2022 at 02:05:56PM +0200, Jani Nikula wrote:
>> On Fri, 11 Feb 2022, Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
>> > Am 11.02.22 um 12:12 schrieb Andy Shevchenko:
>> >> On Fri, Feb 11, 2022 at 11:40:13AM +0100, Javier Martinez Canillas wrote:
>> >>> On 2/11/22 11:28, Andy Shevchenko wrote:
>> >>>> On Fri, Feb 11, 2022 at 10:19:22AM +0100, Javier Martinez Canillas wrote:
>
> ...
>
>> >>>>> +static void drm_fb_xrgb8888_to_gray8_line(u8 *dst, const u32 *src, unsigned int pixels)
>> >>>>> +{
>> >>>>> + unsigned int x;
>> >>>>> +
>> >>>>> + for (x = 0; x < pixels; x++) {
>> >>>>> + u8 r = (*src & 0x00ff0000) >> 16;
>> >>>>> + u8 g = (*src & 0x0000ff00) >> 8;
>> >>>>> + u8 b = *src & 0x000000ff;
>> >>>>> +
>> >>>>> + /* ITU BT.601: Y = 0.299 R + 0.587 G + 0.114 B */
>> >>>>> + *dst++ = (3 * r + 6 * g + b) / 10;
>> >>>>> + src++;
>> >>>>> + }
>> >>>>
>> >>>> Can be done as
>> >>>>
>> >>>> while (pixels--) {
>> >>>> ...
>> >>>> }
>> >>>>
>> >>>> or
>> >>>>
>> >>>> do {
>> >>>> ...
>> >>>> } while (--pixels);
>> >>>>
>> >>>
>> >>> I don't see why a while loop would be an improvement here TBH.
>> >>
>> >> Less letters to parse when reading the code.
>> >
>> > It's a simple refactoring of code that has worked well so far. Let's
>> > leave it as-is for now.
>>
>> 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.
And while() is just syntax sugar for goto. :p
The for loop written as for (i = 0; i < N; i++) is hands down the most
obvious counting loop pattern there is in C.
>> And yes, the do-while suggested above is buggy, and you actually need to
>> stop and think to see why.
>
> It depends if pixels can be 0 or not and if it's not, then does it contain last
> or number.
>
> The do {} while (--pixels); might be buggy iff pixels may be 0.
Yeah. And how long does it take to figure that out?
BR,
Jani.
--
Jani Nikula, Intel Open Source Graphics Center
