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

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Tue, 15 Mar 2022 08:51:31 +0100
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> Hi Pekka,
> 
> On Tue, Mar 15, 2022 at 8:33 AM Pekka Paalanen <ppaalanen@gmail.com> wrote:
> > On Tue, 15 Mar 2022 09:15:08 +1100 (AEDT)
> > Finn Thain <fthain@linux-m68k.org> wrote:  
> > > On Mon, 14 Mar 2022, Geert Uytterhoeven wrote:  
> > > > On Mon, Mar 14, 2022 at 4:05 PM Pekka Paalanen <ppaalanen@gmail.com> wrote:  
> > > > > On Mon, 14 Mar 2022 14:30:18 +0100
> > > > > Geert Uytterhoeven <geert@linux-m68k.org> wrote:  
> > > > > > On Mon, Mar 7, 2022 at 9:53 PM Geert Uytterhoeven <geert@linux-m68k.org> 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@linux-m68k.org>  
> > > > > >  
> > > > > > > --- 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@mail.gmail.com/
> > > > > > [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?

I suppose that would practically eliminate the horizontal blanking
period in CRT timings. If so, I think that might be best represented as
a new format modifier.

I'm guessing P stands for "pixel".

> 
> 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.

> > 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.


Thanks,
pq

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