From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 89560C433EF for ; Fri, 11 Feb 2022 17:28:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1352122AbiBKR2T (ORCPT ); Fri, 11 Feb 2022 12:28:19 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:46656 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1351129AbiBKR2Q (ORCPT ); Fri, 11 Feb 2022 12:28:16 -0500 Received: from mga07.intel.com (mga07.intel.com [134.134.136.100]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6F5DE2C9; Fri, 11 Feb 2022 09:28:13 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1644600493; x=1676136493; h=date:from:to:cc:subject:message-id:references: mime-version:in-reply-to; bh=kFSqnDXKLmnzkeqj34XrRG8BjVnVtFyc7BDAxOa/Lkg=; b=XfeKUXs7b8/0GIEmvuDV8d1xCtrfHycn62P/QwSDKPxHCt57/ov9KmBH HyXYWVxJutWA2koDVU9zf+CvR1Asrd8CaPdlGqx4qZMv9oFiNNOPVRbOX iT2ArNJe3RJg0W+7UNYBsms6zoHxtBMs8BuzSN8iPy748C7lJNduiVHna WEUh9Xo7nnhYMmVQmWKZ7glEk/oiNfTDc0bm6riT/qHWoZOdPHqvD+Le9 BtP5cYgAU4tZ1ZW93kXH1dEaPqm1dG1X9SMR0hKiU4/7T7uoTSQQfrN8A 7ltt+2+zdprZ7GUZ+g1HjV6VM/gEltBNIG3d/2h8tKHSYPL84jJA7oQBa g==; X-IronPort-AV: E=McAfee;i="6200,9189,10255"; a="313054504" X-IronPort-AV: E=Sophos;i="5.88,361,1635231600"; d="scan'208";a="313054504" Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 Feb 2022 09:28:13 -0800 X-IronPort-AV: E=Sophos;i="5.88,361,1635231600"; d="scan'208";a="772150602" Received: from smile.fi.intel.com ([10.237.72.61]) by fmsmga006-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 Feb 2022 09:28:09 -0800 Received: from andy by smile.fi.intel.com with local (Exim 4.95) (envelope-from ) id 1nIZhQ-003Wrs-CU; Fri, 11 Feb 2022 19:27:12 +0200 Date: Fri, 11 Feb 2022 19:27:12 +0200 From: Andy Shevchenko To: Jani Nikula Cc: Thomas Zimmermann , Javier Martinez Canillas , linux-fbdev@vger.kernel.org, David Airlie , Daniel Vetter , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, Noralf =?iso-8859-1?Q?Tr=F8nnes?= , Geert Uytterhoeven , Maxime Ripard , Sam Ravnborg Subject: Re: [PATCH v4 1/6] drm/format-helper: Add drm_fb_xrgb8888_to_gray8_line() Message-ID: References: <20220211091927.2988283-1-javierm@redhat.com> <20220211091927.2988283-2-javierm@redhat.com> <4fa465d9-4fac-4199-9a04-d8e09d164308@redhat.com> <7560cd10-0a7c-3fda-da83-9008833e3901@suse.de> <87pmnt7gm3.fsf@intel.com> <87fsop74lu.fsf@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87fsop74lu.fsf@intel.com> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 11, 2022 at 06:25:17PM +0200, Jani Nikula wrote: > On Fri, 11 Feb 2022, Andy Shevchenko wrote: > > On Fri, Feb 11, 2022 at 02:05:56PM +0200, Jani Nikula wrote: > >> On Fri, 11 Feb 2022, Thomas Zimmermann 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? Okay, I made a mistake to drop the explanation. So, I (mistakenly) assumed that people know this difference between post-decrement and pre-decrement (note, while-loop here is not what is problematic). -- With Best Regards, Andy Shevchenko