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

[Geert Uytterhoeven]

Hi Ilia,

On Tue, Mar 8, 2022 at 8:57 AM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> 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.

Turns out I was wrong: fbdev ordering follows native ordering, and
there's also FBINFO_FOREIGN_ENDIAN  :-(

I'll reply to the thread that introduced the format.
https://lore.kernel.org/all/8d3c0cc370b0214244b01a64c588e5e506531716.1646683502.git.geert@linux-m68k.org/

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