On Tue, Oct 25, 2011 at 09:34, Rabin Vincent [off-list ref] wrote:

I'm trying to figure out the best way to implement fb_setcolreg() for
FB_VISUAL_TRUECOLOR.

FB_VISUAL_TRUECOLOR implies the colormap is read-only.
Perhaps you meant FB_VISUAL_DIRECTCOLOR?

14 or so drivers copy/paste the CNVT_TOHW from skeletonfb.c:

 #define CNVT_TOHW(val,width) ((((val)<<(width))+0x7FFF-(val))>>16)
 ...
 red = CNVT_TOHW(red, info->var.red.length);

Another 18 or so drivers copy/paste a chan_to_field() function which has
a straightforward shift:

 static inline u_int chan_to_field(u_int chan, struct fb_bitfield *bf)
 {
       chan &= 0xffff;
       chan >>= 16 - bf->length;
       return chan << bf->offset;
 }
 ...
 val = chan_to_field(red, &info->var.red);

Both methods return similar values, except that with CNVT_TOHW() the
extreme values seem to have a smaller range than the other values, while
with the chan_to_field() shift all the values have an equal range.

What's the reason behind choosing one over the other?  Could/should a
common function be provided for all the drivers to use?

The idea is to use the full dynamic range of the 16-bit values. I.e. 'all ones'
in the hardware-specific value should map to 'all ones' in the 16-bit expanded
value. So the macros are preferred.

Gr{oetje,eeting}s,

                        Geert

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