Hi Geert,
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?
