Re: [PATCH 3/3] drm/i915/icl: Add Multi-segmented gamma support

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Apr 29, 2019 at 05:00:21PM +0530, Sharma, Shashank wrote:
> 
> On 4/26/2019 11:59 PM, Ville Syrjälä wrote:
> > On Fri, Apr 26, 2019 at 11:31:51PM +0530, Shashank Sharma wrote:
> >> ICL introduces a new gamma correction mode in display engine, called
> >> multi-segmented-gamma mode. This mode allows users to program the
> >> darker region of the gamma curve with sueprfine precision. An
> >> example use case for this is HDR curves (like PQ ST-2084).
> >>
> >> If we plot a gamma correction curve from value range between 0.0 to 1.0,
> >> ICL's multi-segment has 3 different sections:
> >> - superfine segment: 9 values, ranges between 0 - 1/(128 * 256)
> >> - fine segment: 257 values, ranges between 0 - 1/(128)
> >> - corase segment: 257 values, ranges between 0 - 1
> >>
> >> This patch:
> >> - Changes gamma LUTs size for ICL/GEN11 to 262144 entries (8 * 128 * 256),
> >>    so that userspace can program with highest precision supported.
> >> - Changes default gamma mode (non-legacy) to multi-segmented-gamma mode.
> >> - Adds functions to program/detect multi-segment gamma.
> >>
> >> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> >> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> >>
> >> Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >> Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
> >> Signed-off-by: Uma Shankar <uma.shankar@intel.com>
> >> ---
> >>   drivers/gpu/drm/i915/i915_pci.c    |   3 +-
> >>   drivers/gpu/drm/i915/intel_color.c | 155 ++++++++++++++++++++++++++++-
> >>   2 files changed, 156 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/i915_pci.c b/drivers/gpu/drm/i915/i915_pci.c
> >> index ffa2ee70a03d..83698951760b 100644
> >> --- a/drivers/gpu/drm/i915/i915_pci.c
> >> +++ b/drivers/gpu/drm/i915/i915_pci.c
> >> @@ -749,7 +749,8 @@ static const struct intel_device_info intel_cannonlake_info = {
> >>   	GEN(11), \
> >>   	.ddb_size = 2048, \
> >>   	.has_logical_ring_elsq = 1, \
> >> -	.color = { .degamma_lut_size = 33, .gamma_lut_size = 1024 }
> >> +	.color = { .degamma_lut_size = 33, .gamma_lut_size = 262144 }
> >> +
> >>   
> >>   static const struct intel_device_info intel_icelake_11_info = {
> >>   	GEN11_FEATURES,
> >> diff --git a/drivers/gpu/drm/i915/intel_color.c b/drivers/gpu/drm/i915/intel_color.c
> >> index ca341a9e47e6..d1fb79a5d764 100644
> >> --- a/drivers/gpu/drm/i915/intel_color.c
> >> +++ b/drivers/gpu/drm/i915/intel_color.c
> >> @@ -41,6 +41,7 @@
> >>   #define CTM_COEFF_ABS(coeff)		((coeff) & (CTM_COEFF_SIGN - 1))
> >>   
> >>   #define LEGACY_LUT_LENGTH		256
> >> +#define ICL_MULTISEG_LUT_LENGTH		(256 * 128 * 8)
> >>   /*
> >>    * Extract the CSC coefficient from a CTM coefficient (in U32.32 fixed point
> >>    * format). This macro takes the coefficient we want transformed and the
> >> @@ -58,6 +59,12 @@
> >>   
> >>   #define ILK_CSC_POSTOFF_LIMITED_RANGE (16 * (1 << 12) / 255)
> >>   
> >> +enum icl_ms_gamma_segments {
> >> +	ICL_MS_GAMMA_SEG_SUPERFINE,
> >> +	ICL_MS_GAMMA_SEG_FINE,
> >> +	ICL_MS_GAMMA_SEG_COARSE,
> >> +};
> >> +
> >>   static const u16 ilk_csc_off_zero[3] = {};
> >>   
> >>   static const u16 ilk_csc_coeff_identity[9] = {
> >> @@ -767,6 +774,149 @@ static void glk_load_luts(const struct intel_crtc_state *crtc_state)
> >>   	}
> >>   }
> >>   
> >> +/* ilk+ "12.4" interpolated format (high 10 bits) */
> >> +static u32 ilk_lut_12p4_ldw(const struct drm_color_lut *color)
> >> +{
> >> +	return (color->red >> 6) << 20 | (color->green >> 6) << 10 |
> >> +		(color->blue >> 6);
> >> +}
> >> +
> >> +/* ilk+ "12.4" interpolated format (low 6 bits) */
> >> +static u32 ilk_lut_12p4_udw(const struct drm_color_lut *color)
> >> +{
> >> +	return (color->red & 0x3f) << 24 | (color->green & 0x3f) << 14 |
> >> +		(color->blue & 0x3f);
> >> +}
> >> +
> >> +static void
> >> +icl_program_gamma_gcmax(const struct intel_crtc_state *crtc_state)
> >> +{
> >> +	struct intel_crtc *crtc = to_intel_crtc(crtc_state->base.crtc);
> >> +	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
> >> +	enum pipe pipe = crtc->pipe;
> >> +
> >> +	/*
> >> +	 * Program the max register to clamp values > 1.0.
> >> +	 * ToDo: Extend the ABI to be able to program values
> >> +	 * from 1.0
> >> +	 */
> >> +	I915_WRITE(PREC_PAL_GC_MAX(pipe, 0), (1 << 16));
> >> +	I915_WRITE(PREC_PAL_GC_MAX(pipe, 1), (1 << 16));
> >> +	I915_WRITE(PREC_PAL_GC_MAX(pipe, 2), (1 << 16));
> > This one I think we want to program based on the provide LUT. It's the
> > last entry that still gets used in interpolation for <1.0 values.
> > Or at least that's the way it works with the 12p4 mode IIRC. I don't
> > actually remember how it goes with the multi segment mode.
> I am a bit skeptical if its a good idea to program from the LUT, as you 
> know, the drm_color_lut elements are 16bit per channel, and in order to 
> program 1.0 we need 17 bits. So even if userspace wants, it cant set the 
> value of 1.0 as of now. Also, wouldn't it would be good for the 
> interpolation to have the accurate max value (1.0) of the curve ?

If userspace doesn't want the max to be 1.0 then we will do the wrong
thing here. But I suppose that wouldn't be a very common thing to do.
The better arguemnt for doing what I suggest is that we already do it
like that on the other platforms with interpolated gamma (gen4/vlv/chv).

-- 
Ville Syrjälä
Intel
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