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

[Aditya Garg <gargaditya08@live.com>]

> On 24 Feb 2025, at 8:30 PM, andriy.shevchenko@linux.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@linux.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.
> 
>>>> + 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.
>> 
>> 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.
> 
> Right, but see my argumentation above.

Guess what, I’ll just wait for Thomas to reply here. He knows more DRM than me.
> 
> -- 
> With Best Regards,
> Andy Shevchenko