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

[Marek Vasut]

On 9/28/22 02:40, Laurent Pinchart wrote:
> 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.

That works too, thanks !