On Mon, 2012-08-06 at 16:11 -0700, Steven Fuerst wrote: > We use __fls() to find the most significant bit. Using that, the > loop can be avoided. A second trick is to use the mod(32) > behaviour of the rotate instructions on x86 to expand the range > of the unsigned int to float conversion to the full 32 bits. > > The routine is now exact up to 2^24. Above that, we truncate which > is equivalent to rounding towards zero. > > Signed-off-by: Steven Fuerst <svfuerst@xxxxxxxxx> > --- > drivers/gpu/drm/radeon/r600_blit.c | 52 +++++++++++++++++++++--------------- > 1 file changed, 30 insertions(+), 22 deletions(-) > > diff --git a/drivers/gpu/drm/radeon/r600_blit.c b/drivers/gpu/drm/radeon/r600_blit.c > index 3c031a4..f0ce441 100644 > --- a/drivers/gpu/drm/radeon/r600_blit.c > +++ b/drivers/gpu/drm/radeon/r600_blit.c > @@ -489,29 +489,37 @@ set_default_state(drm_radeon_private_t *dev_priv) > ADVANCE_RING(); > } > > -static uint32_t i2f(uint32_t input) > +/* 23 bits of float fractional data */ > +#define I2F_FRAC_BITS 23 > +#define I2F_MASK ((1 << I2F_FRAC_BITS) - 1) > + > +/* > + * Converts unsigned integer into 32-bit IEEE floating point representation. > + * Will be exact from 0 to 2^24. Above that, we round towards zero > + * as the fractional bits will not fit in a float. (It would be better to > + * round towards even as the fpu does, but that is slower.) > + * This routine depends on the mod(32) behaviour of the rotate instructions > + * on x86. The radeon driver works on other architectures than x86. It sounds (and looks, looking at ror32() in include/linux/bitops.h) like this change will break those, which is a no go. > + /* > + * Use a rotate instead of a shift because that works both leftwards > + * and rightwards due to the mod(32) beahviour. This means we don't > + * need to check to see if we are above 2^24 or not. > + */ > + fraction = ror32(x, msb - I2F_FRAC_BITS) & I2F_MASK; Seems like you could write this as fraction = ror32(x, (msb - I2F_FRAC_BITS) & 31) & I2F_MASK; to avoid that, and remove the mentions of relying on the mod(32) behaviour. -- Earthling Michel Dänzer | http://www.amd.com Libre software enthusiast | Debian, X and DRI developer _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
