linux-kernel - Re: [PATCH v4 1/2] drm/format-helper: Add conversion from XRGB8888 to BGR888

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Message-ID: <Z7yKQoWOEofPvbXC@smile.fi.intel.com>
Date: Mon, 24 Feb 2025 17:03:30 +0200
From: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
To: Aditya Garg <gargaditya08@...e.com>
Cc: "maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>,
	"mripard@...nel.org" <mripard@...nel.org>,
	"tzimmermann@...e.de" <tzimmermann@...e.de>,
	"airlied@...il.com" <airlied@...il.com>,
	"simona@...ll.ch" <simona@...ll.ch>,
	Kerem Karabay <kekrby@...il.com>,
	Atharva Tiwari <evepolonium@...il.com>,
	Aun-Ali Zaidi <admin@...eit.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 1/2] drm/format-helper: Add conversion from XRGB8888
 to BGR888

On Mon, Feb 24, 2025 at 05:00:50PM +0200, andriy.shevchenko@...ux.intel.com wrote:
> On Mon, Feb 24, 2025 at 02:54:07PM +0000, Aditya Garg wrote:
> > This conversion helper mimics the existing drm_fb_xrgb8888_to_rgb888 helper
> 
> Not really. See below.
> 
> > > On 24 Feb 2025, at 7:59 PM, andriy.shevchenko@...ux.intel.com wrote:
> > > On Mon, Feb 24, 2025 at 01:38:32PM +0000, Aditya Garg wrote:

...

> > >> +static void drm_fb_xrgb8888_to_bgr888_line(void *dbuf, const void *sbuf, unsigned int pixels)
> > > 
> > > Okay the xrgb8888 is the actual pixel format independently on
> > > the CPU endianess.
> > > 
> > >> +{
> > >> + u8 *dbuf8 = dbuf;
> > >> + const __le32 *sbuf32 = sbuf;
> > > 
> > > But here we assume that sbuf is __le32.
> > > And I think we may benefit from the __be32 there.
> > 
> > So, like drm_fb_xrgb8888_to_rgb888, we are using __le32
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/drm_format_helper.c?h=v6.14-rc4#n657
> 
> The rgb888 != bgr888, that's where the byte swapping happens. So, one should
> use __be32 if the other has already been using __le32.

But in both cases it's actually the 24-bit format in 4-byte packets.

I would rewrite both to remove these types as they are just confusing. But it's
probably not your call. So, if you want to stick with __le32, fine, not my call
either :-)

> > >> + unsigned int x;
> > >> + u32 pix;
> > >> +
> > >> + for (x = 0; x < pixels; x++) {
> > >> + pix = le32_to_cpu(sbuf32[x]);
> > >> + /* write red-green-blue to output in little endianness */
> > >> + *dbuf8++ = (pix & 0x00ff0000) >> 16;
> > >> + *dbuf8++ = (pix & 0x0000ff00) >> 8;
> > >> + *dbuf8++ = (pix & 0x000000ff) >> 0;
> > > 
> > > pix = be32_to_cpu(sbuf[4 * x]) >> 8;
> > > put_unaligned_le24(pix, &dbuf[3 * x]);
> > 
> > Again, 
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/drm_format_helper.c?h=v6.14-rc4#n664
> 
> As per above.
> 
> > >> + }
> > > 
> > > Or, after all, this __le32 magic might be not needed at all. Wouldn't the below
> > > be the equivalent
> > > 
> > > static void drm_fb_xrgb8888_to_bgr888_line(void *dbuf, const void *sbuf, unsigned int pixels)
> > > {
> > > unsigned int x;
> > > u32 pix;
> > > 
> > > for (x = 0; x < pixels; x++) {
> > > /* Read red-green-blue from input in big endianess and... */
> > > pix = get_unaligned_be24(sbuf + x * 4 + 1);
> > > /* ...write it to output in little endianness. */
> > > put_unaligned_le24(pix, dbuf + x * 3);
> > > }
> > > }
> > > 
> > > The comments can even be dropped as the code quite clear about what's going on.


-- 
With Best Regards,
Andy Shevchenko