On Fri, 6 May 2022 20:05:39 -0300
Igor Torrente <igormtorrente@gmail.com> wrote:

> Hi Pekka,
> 
> On 4/27/22 04:55, Pekka Paalanen wrote:
> > On Tue, 26 Apr 2022 21:53:19 -0300
> > Igor Torrente <igormtorrente@gmail.com> wrote:
> >   
> >> Hi Pekka,
> >>
> >> On 4/21/22 07:58, Pekka Paalanen wrote:  
> >>> On Mon,  4 Apr 2022 17:45:15 -0300
> >>> Igor Torrente <igormtorrente@gmail.com> wrote:
> >>>      
> >>>> Adds this common format to vkms.
> >>>>
> >>>> This commit also adds new helper macros to deal with fixed-point
> >>>> arithmetic.
> >>>>
> >>>> It was done to improve the precision of the conversion to ARGB16161616
> >>>> since the "conversion ratio" is not an integer.
> >>>>
> >>>> V3: Adapt the handlers to the new format introduced in patch 7 V3.
> >>>> V5: Minor improvements
> >>>>
> >>>> Signed-off-by: Igor Torrente <igormtorrente@gmail.com>
> >>>> ---
> >>>>    drivers/gpu/drm/vkms/vkms_formats.c   | 70 +++++++++++++++++++++++++++
> >>>>    drivers/gpu/drm/vkms/vkms_plane.c     |  6 ++-
> >>>>    drivers/gpu/drm/vkms/vkms_writeback.c |  3 +-
> >>>>    3 files changed, 76 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
> >>>> index 8d913fa7dbde..4af8b295f31e 100644
> >>>> --- a/drivers/gpu/drm/vkms/vkms_formats.c
> >>>> +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> >>>> @@ -5,6 +5,23 @@
> >>>>    
> >>>>    #include "vkms_formats.h"
> >>>>    
> >>>> +/* The following macros help doing fixed point arithmetic. */
> >>>> +/*
> >>>> + * With Fixed-Point scale 15 we have 17 and 15 bits of integer and fractional
> >>>> + * parts respectively.
> >>>> + *  | 0000 0000 0000 0000 0.000 0000 0000 0000 |
> >>>> + * 31                                          0
> >>>> + */
> >>>> +#define FIXED_SCALE 15  
> >>>
> >>> I think this would usually be called a "shift" since it's used in
> >>> bit-shifts.  
> >>
> >> Ok, I will rename this.
> >>  
> >>>      
> >>>> +
> >>>> +#define INT_TO_FIXED(a) ((a) << FIXED_SCALE)
> >>>> +#define FIXED_MUL(a, b) ((s32)(((s64)(a) * (b)) >> FIXED_SCALE))
> >>>> +#define FIXED_DIV(a, b) ((s32)(((s64)(a) << FIXED_SCALE) / (b)))  
> >>>
> >>> A truncating div, ok.
> >>>      
> >>>> +/* This macro converts a fixed point number to int, and round half up it */
> >>>> +#define FIXED_TO_INT_ROUND(a) (((a) + (1 << (FIXED_SCALE - 1))) >> FIXED_SCALE)  
> >>>
> >>> Yes.
> >>>      
> >>>> +/* Convert divisor and dividend to Fixed-Point and performs the division */
> >>>> +#define INT_TO_FIXED_DIV(a, b) (FIXED_DIV(INT_TO_FIXED(a), INT_TO_FIXED(b)))  
> >>>
> >>> Ok, this is obvious to read, even though it's the same as FIXED_DIV()
> >>> alone. Not sure the compiler would optimize that extra bit-shift away...
> >>>
> >>> If one wanted to, it would be possible to write type-safe functions for
> >>> these so that fixed and integer could not be mixed up.  
> >>
> >> Ok, I will move to a function.  
> > 
> > That's not all.
> > 
> > If you want it type-safe, then you need something like
> > 
> > struct vkms_fixed_point {
> > 	s32 value;
> > };
> > 
> > And use `struct vkms_fixed_point` (by value) everywhere where you pass
> > a fixed point value, and never as a plain s32 type. Then it will be
> > impossible to do incorrect arithmetic or conversions by accident on
> > fixed point values.
> > 
> > Is it worth it? I don't know, since it's limited into this one file.
> > 
> > A simple 'typedef s32 vkms_fixed_point' does not work, because it does
> > not prevent computing with vkms_fixed_point as if it was just a normal
> > s32. Using a struct prevents that.  
> 
> ohhh. Got it!
> 
> > 
> > I wonder if the kernel doesn't already have something like this
> > available in general...  
> 
> After some time searching I found `include/drm/drm_fixed.h`[1].
> 
> It seems fine. There are minor things to consider:
> 
> 1. It doesn't have a `FIXED_TO_INT_ROUND` equivalent.
> 2. We can use `fixed20_12` for rgb565 but We have to use s64 for YUV
> formats or add a `sfixed20_12` with s32.
> 
> In terms of consistency, do you think worth using this "library" given
> that we may need to use two distinct ways to represent the fixed point
> soonish? Or it's better to implement `sfixed20_12`? Or just continue 
> with the
> current macros?
> 
> [1] - https://elixir.bootlin.com/linux/latest/source/include/drm/drm_fixed.h

I think that is something the kernel people should weigh in on.

The one thing I would definitely avoid is ending up using multiple
fixed point formats in VKMS.

In the mean time, your current macros seem good enough, if there is no
community interest in better type safety nor sharing the fixed point
code.


Thanks,
pq