On Tue, Oct 25, 2011 at 09:34, Rabin Vincent <rabin@xxxxxx> 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@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
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