On Tue, Feb 25, 2025 at 08:37:32AM +0100, Thomas Zimmermann wrote:
> Am 24.02.25 um 15:29 schrieb andriy.shevchenko@xxxxxxxxxxxxxxx:
> > 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
