[PATCH libdrm 3/3] util: Add SMPTE pattern support for C4 format

[Geert Uytterhoeven]

Hi Ilia,

On Mon, Mar 7, 2022 at 10:23 PM Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Mon, Mar 7, 2022 at 3:53 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> > diff --git a/tests/util/pattern.c b/tests/util/pattern.c
> > index 953bf95492ee150c..42d75d700700dc3d 100644
> > --- a/tests/util/pattern.c
> > +++ b/tests/util/pattern.c
> > @@ -608,6 +608,46 @@ static void fill_smpte_rgb16fp(const struct util_rgb_info *rgb, void *mem,
> >  static unsigned int smpte_middle[7] = { 6, 7, 4, 7, 2, 7, 0 };
> >  static unsigned int smpte_bottom[8] = { 8, 9, 10, 7, 11, 7, 12, 7 };
> >
> > +static void write_pixel_4(uint8_t *mem, unsigned int x, unsigned int pixel)
> > +{
> > +       if (x & 1)
> > +               mem[x / 2] = (mem[x / 2] & 0xf0) | (pixel & 0x0f);
> > +       else
> > +               mem[x / 2] = (mem[x / 2] & 0x0f) | (pixel << 4);
> > +}
>
> The standard layout is MSB? i.e. first pixel goes in the upper bits of
> the first byte? It's been ages since I've dealt with C4 (or perhaps I
> never even touched it), but this seems a bit surprising.

Exactly. All register documentation I've ever seen shows the MSB on
the left, i.e. for bytes:

     MSB                         LSB
    +---+---+---+---+---+---+---+---+
    | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
    +---+---+---+---+---+---+---+---+

IBM used to count bits in the reverse order, but still had MSB left:

     MSB                         LSB
    +---+---+---+---+---+---+---+---+
    | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
    +---+---+---+---+---+---+---+---+

If the reverse ordering of pixels is ever needed, a new fourcc code can
be introduced.  Note that the fbdev API has support for both orderings
(see fb_bitfield.msb_right), but no driver ever sets msb_right = 1,
hence the fbdev core doesn't support it yet.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds