This conversion helper mimics the existing drm_fb_xrgb8888_to_rgb888 helper
> On 24 Feb 2025, at 7:59 PM, andriy.shevchenko@xxxxxxxxxxxxxxx wrote:
>
> On Mon, Feb 24, 2025 at 01:38:32PM +0000, Aditya Garg wrote:
>> From: Kerem Karabay <kekrby@xxxxxxxxx>
>>
>> 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.
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
>
>> + 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
>
>> + }
>
> 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.
These comments are literally rewritten :
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/drm_format_helper.c?h=v6.14-rc4#n663
>
>> +}
>
> But it's up to you. I don't know which solution gives better code generation
> either.
I don't really mind any code change tbh, but I’d prefer that as an improvement to existing code, and not a part of this patchset.
