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

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Message-ID: <Z7yCLxBN4Cl4btQm@smile.fi.intel.com>
Date: Mon, 24 Feb 2025 16:29:03 +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 01:38:32PM +0000, Aditya Garg wrote:
> From: Kerem Karabay <kekrby@...il.com>
> 
> Add XRGB8888 emulation helper for devices that only support BGR888.

...

> +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.

> +	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