I'm trying to figure out the best way to implement fb_setcolreg() for
FB_VISUAL_TRUECOLOR.
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?
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
On Tue, Oct 25, 2011 at 01:15:41PM +0200, Geert Uytterhoeven wrote:
On Tue, Oct 25, 2011 at 09:34, Rabin Vincent [off-list ref] wrote:
quoted
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?
I was referring to the pseudo palette usage for FB_VISUAL_TRUECOLOR, which is
where most of the drivers use chan_to_field(), for example in s3c2410fb.c:
switch (info->fix.visual) {
case FB_VISUAL_TRUECOLOR:
/* true-colour, use pseudo-palette */
if (regno < 16) {
u32 *pal = info->pseudo_palette;
val = chan_to_field(red, &info->var.red);
quoted
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.
I'm not sure I follow. In the pseudo palette, where the bf->length is less
than 16, both CNVT_TOHW() and the plain shift return the same values for the
all-ones case.