Re: [PATCH v6 2/7] gpu: ipu-v3: ipu-ic: Fix BT.601 coefficients

[Philipp Zabel]

Hi Steve,

On Thu, 2019-03-07 at 15:33 -0800, Steve Longerbeam wrote:
> The ycbcr2rgb and inverse rgb2ycbcr tables define the BT.601 Y'CbCr
> encoding coefficients.
> 
> The rgb2ycbcr table specifically describes the BT.601 encoding from
> full range RGB to full range YUV. Add table comments to make this more
> clear.
> 
> The ycbcr2rgb inverse table describes encoding YUV limited range to RGB
> full range. To be consistent with the rgb2ycbcr table, convert this to
> YUV full range to RGB full range, and adjust/expand on the comments.
> 
> The ic_csc_rgb2rgb table is just an identity matrix, so rename to
> ic_encode_identity.
> 
> Fixes: 1aa8ea0d2bd5d ("gpu: ipu-v3: Add Image Converter unit")
> 
> Suggested-by: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
> Signed-off-by: Steve Longerbeam <slongerbeam@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  drivers/gpu/ipu-v3/ipu-ic.c | 61 ++++++++++++++++++++++---------------
>  1 file changed, 37 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/gpu/ipu-v3/ipu-ic.c b/drivers/gpu/ipu-v3/ipu-ic.c
> index 18816ccf600e..b63a2826b629 100644
> --- a/drivers/gpu/ipu-v3/ipu-ic.c
> +++ b/drivers/gpu/ipu-v3/ipu-ic.c
> @@ -175,7 +175,7 @@ static inline void ipu_ic_write(struct ipu_ic *ic, u32 value, unsigned offset)
>  	writel(value, ic->priv->base + offset);
>  }
>  
> -struct ic_csc_params {
> +struct ic_encode_coeff {

This less accurate. These are called IC (Task) Parameters in the
reference manual, the 64-bit aligned words are called CSC words. Beside
the coefficients, this structure also contains the coefficient scale,
the offsets, and the saturation mode flag.

>  	s16 coeff[3][3];	/* signed 9-bit integer coefficients */
>  	s16 offset[3];		/* signed 11+2-bit fixed point offset */
>  	u8 scale:2;		/* scale coefficients * 2^(scale-1) */
> @@ -183,13 +183,15 @@ struct ic_csc_params {
>  };
>  
>  /*
> - * Y = R *  .299 + G *  .587 + B *  .114;
> - * U = R * -.169 + G * -.332 + B *  .500 + 128.;
> - * V = R *  .500 + G * -.419 + B * -.0813 + 128.;
> + * BT.601 encoding from RGB full range to YUV full range:
> + *
> + * Y =  .2990 * R + .5870 * G + .1140 * B
> + * U = -.1687 * R - .3313 * G + .5000 * B + 128
> + * V =  .5000 * R - .4187 * G - .0813 * B + 128
>   */
> -static const struct ic_csc_params ic_csc_rgb2ycbcr = {
> +static const struct ic_encode_coeff ic_encode_rgb2ycbcr_601 = {
>  	.coeff = {
> -		{ 77, 150, 29 },
> +		{  77, 150,  29 },
>  		{ 469, 427, 128 },
>  		{ 128, 405, 491 },

We could subtract 512 from the negative values, to improve readability.

>  	},
> @@ -197,8 +199,11 @@ static const struct ic_csc_params ic_csc_rgb2ycbcr = {
>  	.scale = 1,
>  };
>  
> -/* transparent RGB->RGB matrix for graphics combining */
> -static const struct ic_csc_params ic_csc_rgb2rgb = {
> +/*
> + * identity matrix, used for transparent RGB->RGB graphics
> + * combining.
> + */
> +static const struct ic_encode_coeff ic_encode_identity = {
>  	.coeff = {
>  		{ 128, 0, 0 },
>  		{ 0, 128, 0 },
> @@ -208,17 +213,25 @@ static const struct ic_csc_params ic_csc_rgb2rgb = {
>  };
>  
>  /*
> - * R = (1.164 * (Y - 16)) + (1.596 * (Cr - 128));
> - * G = (1.164 * (Y - 16)) - (0.392 * (Cb - 128)) - (0.813 * (Cr - 128));
> - * B = (1.164 * (Y - 16)) + (2.017 * (Cb - 128);
> + * Inverse BT.601 encoding from YUV full range to RGB full range:
> + *
> + * R = 1. * Y +      0 * (Cb - 128) + 1.4020 * (Cr - 128)
> + * G = 1. * Y -  .3442 * (Cb - 128) - 0.7142 * (Cr - 128)

Should that be      ^^^^^ .3441   and     ^^^^^ .7141 ?
The coefficients and offsets after rounding should end up the same.

Also, let's consistently either add the leading zero, or leave it out.

> + * B = 1. * Y + 1.7720 * (Cb - 128) +      0 * (Cr - 128)
> + *
> + * equivalently (factoring out the offsets):
> + *
> + * R = 1. * Y  +      0 * Cb + 1.4020 * Cr - 179.456
> + * G = 1. * Y  -  .3442 * Cb - 0.7142 * Cr + 135.475
> + * B = 1. * Y  + 1.7720 * Cb +      0 * Cr - 226.816
>   */
> -static const struct ic_csc_params ic_csc_ycbcr2rgb = {
> +static const struct ic_encode_coeff ic_encode_ycbcr2rgb_601 = {
>  	.coeff = {
> -		{ 149, 0, 204 },
> -		{ 149, 462, 408 },
> -		{ 149, 255, 0 },
> +		{ 128,   0, 179 },
> +		{ 128, 468, 421 },
> +		{ 128, 227,   0 },
>  	},
> -	.offset = { -446, 266, -554 },
> +	.offset = { -359, 271, -454 },

These seem to be correct. Again, I think this would be easier to read if
the negative coefficients were written with a sign as well.

>  	.scale = 2,
>  };
>  
> @@ -228,7 +241,7 @@ static int init_csc(struct ipu_ic *ic,
>  		    int csc_index)
>  {
>  	struct ipu_ic_priv *priv = ic->priv;
> -	const struct ic_csc_params *params;
> +	const struct ic_encode_coeff *coeff;
>  	u32 __iomem *base;
>  	const u16 (*c)[3];
>  	const u16 *a;
> @@ -238,26 +251,26 @@ static int init_csc(struct ipu_ic *ic,
>  		(priv->tpmem_base + ic->reg->tpmem_csc[csc_index]);
>  
>  	if (inf == IPUV3_COLORSPACE_YUV && outf == IPUV3_COLORSPACE_RGB)
> -		params = &ic_csc_ycbcr2rgb;
> +		coeff = &ic_encode_ycbcr2rgb_601;
>  	else if (inf == IPUV3_COLORSPACE_RGB && outf == IPUV3_COLORSPACE_YUV)
> -		params = &ic_csc_rgb2ycbcr;
> +		coeff = &ic_encode_rgb2ycbcr_601;
>  	else if (inf == IPUV3_COLORSPACE_RGB && outf == IPUV3_COLORSPACE_RGB)
> -		params = &ic_csc_rgb2rgb;
> +		coeff = &ic_encode_identity;
>  	else {
>  		dev_err(priv->ipu->dev, "Unsupported color space conversion\n");
>  		return -EINVAL;
>  	}
>  
>  	/* Cast to unsigned */
> -	c = (const u16 (*)[3])params->coeff;
> -	a = (const u16 *)params->offset;
> +	c = (const u16 (*)[3])coeff->coeff;
> +	a = (const u16 *)coeff->offset;

This looks weird to me. I'd be in favor of not renaming the type.

regards
Philipp