On Wed, Sep 30, 2015 at 03:31:34PM +0100, Rob Bradford wrote:
Hi Shashank, some feedback below that you would be great to get addressed before your next version.
On Wed, 2015-09-16 at 23:07 +0530, Shashank Sharma wrote:
BDW/SKL/BXT platforms support various Gamma correction modes which are:
- Legacy 8-bit mode
- 10-bit mode
- 10-bit Split Gamma mode
- 12-bit mode
This patch does the following:
- Adds the core function to program Gamma correction values for BDW/SKL/BXT platforms
- Adds Gamma correction macros/defines
Signed-off-by: Shashank Sharma shashank.sharma@intel.com Signed-off-by: Kausal Malladi kausalmalladi@gmail.com
drivers/gpu/drm/i915/i915_reg.h | 17 +- drivers/gpu/drm/i915/intel_color_manager.c | 270 +++++++++++++++++++++++++++++
*snip*
+static u32 bdw_write_10bit_gamma_precision(u32 red, u32 green, u32 blue) +{
- u32 word;
- u8 blue_int, green_int, red_int;
- u16 blue_fract, green_fract, red_fract;
- blue_int = _GAMMA_INT_PART(blue);
- if (blue_int > GAMMA_INT_MAX)
blue = BDW_MAX_GAMMA;- green_int = _GAMMA_INT_PART(green);
- if (green_int > GAMMA_INT_MAX)
green = BDW_MAX_GAMMA;- red_int = _GAMMA_INT_PART(red);
- if (red_int > GAMMA_INT_MAX)
red = BDW_MAX_GAMMA;- blue_fract = _GAMMA_FRACT_PART(blue);
- green_fract = _GAMMA_FRACT_PART(green);
- red_fract = _GAMMA_FRACT_PART(red);
- blue_fract >>= BDW_10BIT_GAMMA_MSB_SHIFT;
- green_fract >>= BDW_10BIT_GAMMA_MSB_SHIFT;
- red_fract >>= BDW_10BIT_GAMMA_MSB_SHIFT;
- /* Red (29:20) Green (19:10) and Blue (9:0) */
- word = red_fract;
- word <<= BDW_GAMMA_SHIFT;
- word = word | green_fract;
- word <<= BDW_GAMMA_SHIFT;
- word = word | blue_fract;
- return word;
+}
I think the above function, and perhaps others in this series have the same flaw with respect to maximum colour value.
In our discussions we agreed that we would follow the "GL style" where maximum colour (i.e. 255 in 8-bit) would be represented by 1.0f. 1.0f when converted to fixed point 8.24 notation is 1 << 24.
I observed that with my test code that a linear ramp (where the last entry is set to 1.0) gives me black for white. I tracked it down to this function.
In order to map 1.0 to the maximum value for the table entry in the hardware this function needs to be changed. One way to achieve this would be change the test "blue_int > GAMMA_INT_MAX" to be "blue_int >= GAMMA_INT_MAX" but since GAMMA_INT_MAX = 1 then it might be clearer to say blue_int > 0.
BDW_MAX_GAMMA also looks wrong. I think it should be (1 << 24) - 1 not the 0x10000 (see below). As well as correct clamping it is also necessary to scale as Daniel suggested on IRC:
" 13:54 < danvet> robster, so we'd need to rescale in the kernel from 1.0 to 0.1111111111b ? 13:55 < danvet> well substract 0.0000000001b for all values > 0.5 probably since float math in the kernel is evil
Or just clamp to '(1<<bits)-1'. I think the end result is the same actually.