fb_setcolreg(), CNVT_TOHW() and chan_to_field()
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?
Quoted 2 lines · Rabin Vincent
I'm trying to figure out the best way to implement
fb_setcolreg()forFB_VISUAL_TRUECOLOR.
FB_VISUAL_TRUECOLOR implies the colormap is read-only. Perhaps you meant FB_VISUAL_DIRECTCOLOR?
Quoted 18 lines
14 or so drivers copy/paste the
CNVT_TOHWfromskeletonfb.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_intchan_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);
Quoted 6 lines
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 thechan_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 -- [email protected]
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
Quoted 6 lines · Geert Uytterhoeven
Quoted 2 lines · Rabin Vincent
I'm trying to figure out the best way to implement
fb_setcolreg()forFB_VISUAL_TRUECOLOR.
FB_VISUAL_TRUECOLORimplies the colormap is read-only. Perhaps you meantFB_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 23 lines
Quoted 23 lines
14 or so drivers copy/paste the
CNVT_TOHWfromskeletonfb.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_intchan_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 thechan_to_field()shift all the values have an equal range.
Quoted 6 lines
Quoted 2 lines
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.