Re: [PATCH 1/9] drm: Add helpers for x16 fixed point values

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 19 Jun 2024, Imre Deak <imre.deak@intel.com> wrote:
> On Wed, Jun 19, 2024 at 01:10:09PM +0300, Jani Nikula wrote:
>> On Fri, 14 Jun 2024, Imre Deak <imre.deak@intel.com> wrote:
>> > Add helpers to convert between x16 fixed point and integer/fraction
>> > values. Also add the format/argument macros required to printk x16
>> > fixed point variables.
>> >
>> > These are needed by later patches dumping the Display Stream Compression
>> > configuration in DRM core and in the i915 driver to replace the
>> > corresponding bpp_x16 helpers defined locally in the driver.
>> >
>> > Signed-off-by: Imre Deak <imre.deak@intel.com>
>> > ---
>> >  drivers/gpu/drm/display/drm_dp_helper.c |  5 +++--
>> >  include/drm/drm_fixed.h                 | 23 +++++++++++++++++++++++
>> >  2 files changed, 26 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
>> > index 79a615667aab1..806f9c9764995 100644
>> > --- a/drivers/gpu/drm/display/drm_dp_helper.c
>> > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
>> > @@ -35,6 +35,7 @@
>> >  #include <drm/display/drm_dp_helper.h>
>> >  #include <drm/display/drm_dp_mst_helper.h>
>> >  #include <drm/drm_edid.h>
>> > +#include <drm/drm_fixed.h>
>> >  #include <drm/drm_print.h>
>> >  #include <drm/drm_vblank.h>
>> >  #include <drm/drm_panel.h>
>> > @@ -4151,9 +4152,9 @@ int drm_dp_bw_overhead(int lane_count, int hactive,
>> >  	int symbol_cycles;
>> >  
>> >  	if (lane_count == 0 || hactive == 0 || bpp_x16 == 0) {
>> > -		DRM_DEBUG_KMS("Invalid BW overhead params: lane_count %d, hactive %d, bpp_x16 %d.%04d\n",
>> > +		DRM_DEBUG_KMS("Invalid BW overhead params: lane_count %d, hactive %d, bpp_x16 " DRM_X16_FMT "\n",
>> >  			      lane_count, hactive,
>> > -			      bpp_x16 >> 4, (bpp_x16 & 0xf) * 625);
>> > +			      DRM_X16_ARGS(bpp_x16));
>> >  		return 0;
>> >  	}
>> >  
>> > diff --git a/include/drm/drm_fixed.h b/include/drm/drm_fixed.h
>> > index 81572d32db0c2..0fe2a7f50d54e 100644
>> > --- a/include/drm/drm_fixed.h
>> > +++ b/include/drm/drm_fixed.h
>> > @@ -214,4 +214,27 @@ static inline s64 drm_fixp_exp(s64 x)
>> >  	return sum;
>> >  }
>> >  
>> > +static inline int drm_x16_from_int(int val_int)
>> > +{
>> > +	return val_int << 4;
>> > +}
>> > +
>> > +static inline int drm_x16_to_int(int val_x16)
>> > +{
>> > +	return val_x16 >> 4;
>> > +}
>> > +
>> > +static inline int drm_x16_to_int_roundup(int val_x16)
>> > +{
>> > +	return (val_x16 + 0xf) >> 4;
>> > +}
>> > +
>> > +static inline int drm_x16_to_frac(int val_x16)
>> > +{
>> > +	return val_x16 & 0xf;
>> > +}
>> 
>> Sad trombone about the completely different naming scheme compared to
>> the rest of the file.
>> 
>> Not saying the existing naming is great, but neither is this. And
>> there's no way to unify except by renaming *both* afterwards.
>> 
>> We could devise a scheme now that could be used for the existing stuff
>> later, without renaming the new stuff.
>
> Based on [1]'s short variant, we could have:
>
> dfixed*(fixed20_12 v)  -> drm_uq12*(drm_uq20_12_t v)
> drm_fixp*(s64 v)       -> drm_q32*(s64 v)
> drm_x16*(int v)        -> drm_q4*(int v)

I like it. If you're brave, add them with a generic prefix instead of
drm_ from the start, say fp_q4. ;)

BR,
Jani.

>
> Or instead of uq12/q32/q4 using ufp12/fp32/fp4.
>
> [1] https://en.wikipedia.org/wiki/Q_(number_format)
>
>> *shrug*
>> 
>> BR,
>> Jani.
>> 
>> 
>> 
>> > +
>> > +#define DRM_X16_FMT		"%d.%04d"
>> > +#define DRM_X16_ARGS(val_x16)	drm_x16_to_int(val_x16), (drm_x16_to_frac(val_x16) * 625)
>> > +
>> >  #endif
>> 
>> -- 
>> Jani Nikula, Intel

-- 
Jani Nikula, Intel