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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z72acJ0MoSOK5_RI@smile.fi.intel.com>
Date: Tue, 25 Feb 2025 12:24:48 +0200
From: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Aditya Garg <gargaditya08@...e.com>,
	"maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>,
	"mripard@...nel.org" <mripard@...nel.org>,
	"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 Tue, Feb 25, 2025 at 08:37:32AM +0100, Thomas Zimmermann wrote:
> Am 24.02.25 um 15:29 schrieb andriy.shevchenko@...ux.intel.com:
> > 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.
> 
> No, please. XRGB is the logical order. The raw physical byte order for DRM
> formats is always* little endian, hence reversed from the logical one. sbuf
> points to raw memory and is therefore __le32. DRM-format byte order is
> impossible to understand, I know. But that code is correct.

Okay, so it's only about the colour (top-level) layout, the input and output
data is always in little endian?

> *) White lie: there's a DRM format flag signalling physical big endianess.
> That isn't the case here. So nothing here should ever indicate big
> endianess.

But should it indicate the little? To me sounds like neither...

> > > +	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]);
> > 
> > > +	}
> > 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.
> > 
> > > +}
> > But it's up to you. I don't know which solution gives better code generation
> > either.

-- 
With Best Regards,
Andy Shevchenko