Re: [PATCH v2 05/10] drm/fourcc: Add DRM_FORMAT_C[124]

From: Geert Uytterhoeven
Date: Tue Mar 15 2022 - 04:57:41 EST

Hi Pekka,

On Tue, Mar 15, 2022 at 9:46 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> On Tue, 15 Mar 2022 08:51:31 +0100
> Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > On Tue, Mar 15, 2022 at 8:33 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > > On Tue, 15 Mar 2022 09:15:08 +1100 (AEDT)
> > > Finn Thain <fthain@xxxxxxxxxxxxxx> wrote:
> > > > On Mon, 14 Mar 2022, Geert Uytterhoeven wrote:
> > > > > On Mon, Mar 14, 2022 at 4:05 PM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> > > > > > On Mon, 14 Mar 2022 14:30:18 +0100
> > > > > > Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > > > On Mon, Mar 7, 2022 at 9:53 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > > > > Introduce fourcc codes for color-indexed frame buffer formats with
> > > > > > > > two, four, and sixteen colors, and provide a mapping from bit per
> > > > > > > > pixel and depth to fourcc codes.
> > > > > > > >
> > > > > > > > As the number of bits per pixel is less than eight, these rely on
> > > > > > > > proper block handling for the calculation of bits per pixel and
> > > > > > > > pitch.
> > > > > > > >
> > > > > > > > Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > > > > > >
> > > > > > > > --- a/include/uapi/drm/drm_fourcc.h
> > > > > > > > +++ b/include/uapi/drm/drm_fourcc.h
> > > > > > > > @@ -99,7 +99,10 @@ extern "C" {
> > > > > > > > #define DRM_FORMAT_INVALID 0
> > > > > > > >
> > > > > > > > /* color index */
> > > > > > > > -#define DRM_FORMAT_C8 fourcc_code('C', '8', ' ', ' ') /* [7:0] C */
> > > > > > > > +#define DRM_FORMAT_C1 fourcc_code('C', '1', ' ', ' ') /* [7:0] C0:C1:C2:C3:C4:C5:C6:C7 1:1:1:1:1:1:1:1 eight pixels/byte */
> > > > > > > > +#define DRM_FORMAT_C2 fourcc_code('C', '2', ' ', ' ') /* [7:0] C0:C1:C2:C3 2:2:2:2 four pixels/byte */
> > > > > > > > +#define DRM_FORMAT_C4 fourcc_code('C', '4', ' ', ' ') /* [7:0] C0:C1 4:4 two pixels/byte */
> > > > > > > > +#define DRM_FORMAT_C8 fourcc_code('C', '8', ' ', ' ') /* [7:0] C 8 one pixel/byte */
> > > > > > > >
> > > > > > > > /* 8 bpp Red */
> > > > > > > > #define DRM_FORMAT_R8 fourcc_code('R', '8', ' ', ' ') /* [7:0] R */
> > > > > > >
> > > > > > > After replying to Ilia's comment[1], I realized the CFB drawing
> > > > > > > operations use native byte and bit ordering, unless
> > > > > > > FBINFO_FOREIGN_ENDIAN is set.
> > > > > > > While Amiga, Atari, and Sun-3 use big-endian bit ordering,
> > > > > > > e.g. Acorn VIDC[2] uses little endian, and SH7760[3] is configurable
> > > > > > > (sh7760fb configures ordering to match host order).
> > > > > > > BTW, ssd130{7fb,x}_update_rect() both assume little-endian, so I
> > > > > > > guess they are broken on big-endian.
> > > > > > > Fbtest uses big-endian bit ordering, so < 8 bpp is probably broken
> > > > > > > on little-endian.
> > > > > > >
> > > > > > > Hence the above should become:
> > > > > > >
> > > > > > > #define DRM_FORMAT_C1 fourcc_code('C', '1', ' ', ' ') /*
> > > > > > > [7:0] C7:C6:C5:C4:C3:C2:C1:C0 1:1:1:1:1:1:1:1 eight pixels/byte */
> > > > > > > #define DRM_FORMAT_C2 fourcc_code('C', '2', ' ', ' ') /*
> > > > > > > [7:0] C3:C2:C1:C0 2:2:2:2 four pixels/byte */
> > > > > > > #define DRM_FORMAT_C4 fourcc_code('C', '4', ' ', ' ') /*
> > > > > > > [7:0] C1:C0 4:4 two pixels/byte */
> > > > > > >
> > > > > > > The same changes should be made for DRM_FORMAT_[RD][124].
> > > > > > >
> > > > > > > The fbdev emulation code should gain support for these with and without
> > > > > > > DRM_FORMAT_BIG_ENDIAN, the latter perhaps only on big-endian platforms?
> > > > > > >
> > > > > > > [1] https://lore.kernel.org/r/CAKb7UvgEdm9U=+RyRwL0TGRfA_Qc7NbhCWoZOft2DKdXggtKYw@xxxxxxxxxxxxxx/
> > > > > > > [2] See p.30 of the VIDC datasheet
> > > > > > > http://chrisacorns.computinghistory.org.uk/docs/Acorn/Misc/Acorn_VIDC_Datasheet.pdf
> > > > > > > [3] See p.1178 of the SH7660 datasheet
> > > > > > > https://datasheet.octopart.com/HD6417760BL200AV-Renesas-datasheet-14105759.pdf
> > > > > >
> > > > > > why would CPU endianess affect the order of bits in a byte?
> > > > >
> > > > > It doesn't, but see below.
> > > > >
> > > > > > Do you mean that bit 0 one machine is (1 << 0), and on another machine
> > > > > > bit 0 is (1 << 7)?
> > > > >
> > > > > No, I mean that in case of multiple pixels per byte, the display
> > > > > hardware pumps out pixels to the CRTC starting from either the MSB
> > > > > or the LSB of the first display byte. Which order depends on the
> > > > > display hardware, not on the CPU.
> > > > >
> > > > > > In C, we have only one way to address bits of a byte and that is with
> > > > > > arithmetic. You cannot take the address of a bit any other way, can you?
> > > > > >
> > > > > > Can we standardise on "bit n of a byte is addressed as (1 << n)"?
> > > > >
> > > > > BIT(n) in Linux works the same for little- and big-endian CPUs.
> > > > > But display hardware may use a different bit order.
> > > >
> > > > Perhaps some of this confusion could be avoided if you describe the
> > > > problem in terms of the sequence of scan-out of pixels, rather than in
> > > > terms of the serialization of bits. The significance of bits within each
> > > > pixel and the ordering of pixels within each memory word are independent,
> > > > right?
> > >
> > > Yes, that might help.
> >
> > Display:
> >
> > P0 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15
> >
> > P15 P14 P13 P12 P11 P10 P9 P8 P7 P6 P5 P4 P3 P2 P1 P0
>
> Hi Geert,
>
> does this mean the display hardware emits even rows from left to right
> and odd rows from right to left?

No, it means I should have my morning coffee first, and remove all
temporary cruft before pressing send :-(

The above paragraph should have read:

Display (16 pixels):

P0 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15

> I'm guessing P stands for "pixel".

Exactly.

> > Memory:
> >
> > 1 bpp (MSB first):
> >
> > bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
> > ---- ---- ---- ---- ---- ---- ---- ----
> > byte 0: P0 P1 P2 P3 P4 P5 P6 P7
> > byte 1: P8 P9 P10 P11 P12 P13 P14 P15
> >
> > 1 bpp (LSB first):
> >
> > bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
> > ---- ---- ---- ---- ---- ---- ---- ----
> > byte 0: P7 P6 P5 P4 P3 P2 P1 P0
> > byte 1: P15 P14 P13 P12 P11 P10 P9 P8
> >
> > 2 bpp (MSB first):
> >
> > bits7-6 bits5-4 bits3-2 bits1-0
> > ------- ------- ------- -------
> > byte 0: P0 P1 P2 P3
> > byte 1: P4 P5 P6 P7
> > byte 2: P8 P9 P10 P11
> > byte 3: P12 P13 P14 P15
> >
> > 2 bpp (LSB first):
> >
> > bits7-6 bits5-4 bits3-2 bits1-0
> > ------- ------- ------- -------
> > byte 0: P3 P2 P1 P0
> > byte 1: P7 P6 P5 P4
> > byte 2: P11 P10 P9 P8
> > byte 3: P15 P14 P13 P12
> >
> > 4 bpp (MSB first):
> >
> > bits7-4 bits3-0
> > ------- -------
> > byte 0: P0 P1
> > byte 1: P2 P3
> > byte 2: P4 P5
> > byte 3: P6 P7
> > byte 4: P8 P9
> > byte 5: P10 P11
> > byte 6: P12 P13
> > byte 7: P14 P15
> >
> > 4 bpp (LSB first):
> >
> > bits7-4 bits3-0
> > ------- -------
> > byte 0: P1 P0
> > byte 1: P3 P2
> > byte 2: P5 P4
> > byte 3: P7 P6
> > byte 4: P9 P8
> > byte 5: P11 P10
> > byte 6: P13 P12
> > byte 7: P15 P14
>
> I think I can guess what you meant there, and it looks understandable
> to me. These tables are actually very clear, and leave only one thing
> undefined: when multiple bits form a pixel, in which order do the bits
> form the value. I recall you said fbdev allows for both orderings but
> only one order is ever used if I understood right.

Indeed. The third ordering is the ordering of the bits in a pixel.
As fb_bitfield.msb_right is always false, no hardware ever supported by
fbdev used the other ordering, so we only have to care about:

1 bpp: P = [ bitN ]
2 bpp: P = [ bitN bitN-1 ]
4 bpp: P = [ bitN bitN-1 bitN-2 bitN-3 ]

> > > Also, when drm_fourcc.h is describing pixel formats, it needs to
> > > consider only how a little-endian CPU accesses them. That's how pixel
> > > data in memory is described. Display hardware plays no part in that.
> > > It is the driver's job to expose the pixel formats that match display
> > > hardware behaviour.
> >
> > But if the "CPU format" does not match the "display support",
> > all pixel data must be converted?
>
> Of course. If the driver author does not want to convert pixel data in
> flight, then the author should not let the driver expose a format that
> needs conversion.

... in which case we need a DRM fourcc code for the format?

BTW, Atari and Amiga use bitplanes for bpp <= 8, so they need
conversion anyway.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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