[PATCH 3/4] drm: lcdif: Switch to limited range for RGB to YUV conversion

[Laurent Pinchart]

On Wed, Sep 28, 2022 at 02:37:04AM +0200, Marek Vasut wrote:
> On 9/28/22 02:21, Laurent Pinchart wrote:
> 
> Hi,
> 
> [...]
> 
> >>> -		/* CSC: BT.601 Full Range RGB to YCbCr coefficients. */
> >>> -		writel(CSC0_COEF0_A2(0x096) | CSC0_COEF0_A1(0x04c),
> >>> +		/* CSC: BT.601 Limited Range RGB to YCbCr coefficients. */
> >>> +		writel(CSC0_COEF0_A2(0x081) | CSC0_COEF0_A1(0x042),
> >>>    		       lcdif->base + LCDC_V8_CSC0_COEF0);
> >>> -		writel(CSC0_COEF1_B1(0x7d5) | CSC0_COEF1_A3(0x01d),
> >>> +		writel(CSC0_COEF1_B1(0x7da) | CSC0_COEF1_A3(0x019),
> >>>    		       lcdif->base + LCDC_V8_CSC0_COEF1);
> >>> -		writel(CSC0_COEF2_B3(0x080) | CSC0_COEF2_B2(0x7ac),
> >>> +		writel(CSC0_COEF2_B3(0x070) | CSC0_COEF2_B2(0x7b6),
> >>>    		       lcdif->base + LCDC_V8_CSC0_COEF2);
> >>> -		writel(CSC0_COEF3_C2(0x795) | CSC0_COEF3_C1(0x080),
> >>> +		writel(CSC0_COEF3_C2(0x7a2) | CSC0_COEF3_C1(0x070),
> >>>    		       lcdif->base + LCDC_V8_CSC0_COEF3);
> >>> -		writel(CSC0_COEF4_D1(0x000) | CSC0_COEF4_C3(0x7ec),
> >>> +		writel(CSC0_COEF4_D1(0x010) | CSC0_COEF4_C3(0x7ee),
> >>>    		       lcdif->base + LCDC_V8_CSC0_COEF4);
> >>>    		writel(CSC0_COEF5_D3(0x080) | CSC0_COEF5_D2(0x080),
> >>>    		       lcdif->base + LCDC_V8_CSC0_COEF5);
> >>
> >> Would it make sense to use the same coeffs as csc2_coef_bt601_lim in
> >> drivers/media/platform/nxp/imx-pxp.c , since the block is most likely
> >> identical ?
> > 
> > The coefficients in this patch have been computed to distribute the
> > error in such a way that the sum of all lines stays the same. This
> > avoids biases and overflow, but it likely makes very little difference
> > in practice. I'm thus fine with the coefficients from imx-pxp.c.
> 
> Would it then make sense to update the coeffs in the pxp driver instead?
> 
> Either option works for me.

It could, but I won't be able to easily test it. As the hardware clamps
the calculated value, there's no risk of wraparound, and the difference
in the +/-1 error distribution will not be noticeable, so I'll just copy
the coefficients from imx-pxp.c to ensure coherency.

-- 
Regards,

Laurent Pinchart