Re: [PATCH v2 1/4] drm/komeda: Add a new helper drm_color_ctm_s31_32_to_qm_n()

[Ilia Mirkin]

On Mon, Oct 14, 2019 at 9:16 PM james qian wang (Arm Technology China)
<james.qian.wang@xxxxxxx> wrote:
> On Mon, Oct 14, 2019 at 11:58:48AM -0400, Ilia Mirkin wrote:
> > On Fri, Oct 11, 2019 at 1:43 AM james qian wang (Arm Technology China)
> > <james.qian.wang@xxxxxxx> wrote:
> > > + *
> > > + * Convert and clamp S31.32 sign-magnitude to Qm.n 2's complement.
> > > + */
> > > +uint64_t drm_color_ctm_s31_32_to_qm_n(uint64_t user_input,
> > > +                                     uint32_t m, uint32_t n)
> > > +{
> > > +       u64 mag = (user_input & ~BIT_ULL(63)) >> (32 - n);
> > > +       bool negative = !!(user_input & BIT_ULL(63));
> > > +       s64 val;
> > > +
> > > +       /* the range of signed 2s complement is [-2^n+m, 2^n+m - 1] */
> >
> > This implies that n = 32, m = 0 would actually yield a 33-bit 2's
> > complement number. Is that what you meant?
>
> Yes, since m doesn't include sign-bit So a Q0.32 is a 33bit value.

This goes counter to what the wikipedia page says [
https://en.wikipedia.org/wiki/Q_(number_format) ]:

(reformatted slightly for text-only consumption):

"""
For example, a Q15.1 format number:

- requires 15+1 = 16 bits
- its range is [-2^14, 2^14 - 2^-1] = [-16384.0, +16383.5] = [0x8000,
0x8001 ... 0xFFFF, 0x0000, 0x0001 ... 0x7FFE, 0x7FFF]
- its resolution is 2^-1 = 0.5
"""

This suggests that the proper way to represent a standard 32-bit 2's
complement integer would be Q32.0.

  -ilia