Re: [PATCH v6 13/14] drm/mediatek: Support CRC in OVL

[CK Hu (胡俊光)]

Hi, Shawn:

On Thu, 2024-03-28 at 03:22 +0000, Shawn Sung (宋孝謙) wrote:
> Hi CK,
> 
> On Tue, 2024-03-26 at 06:11 +0000, CK Hu (胡俊光) wrote:
> > > @@ -488,6 +567,83 @@ void mtk_ovl_layer_config(struct device
> > > *dev,
> > > unsigned int idx,
> > >  	    (state->base.fb && !state->base.fb->format->has_alpha))
> > >  		ignore_pixel_alpha = OVL_CONST_BLEND;
> > >  
> > > +	/*
> > > +	 * OVL only supports 8 bits data in CRC calculation, transform
> > > 10-bit
> > > +	 * RGB to 8-bit RGB by leveraging the ability of the Y2R (YUV-
> > > to-RGB)
> > > +	 * hardware to multiply coefficients, although there is nothing
> > > to do
> > > +	 * with the YUV format.
> > > +	 */
> > > +	if (ovl->data->supports_clrfmt_ext) {
> > > +		u32 y2r_coef = 0, y2r_offset = 0, r2r_coef = 0, csc_en
> > > = 0;
> > > +
> > > +		if (is_10bit_rgb(fmt)) {
> > > +			con |= OVL_CON_MTX_AUTO_DIS | OVL_CON_MTX_EN |
> > > OVL_CON_MTX_PROGRAMMABLE;
> > > +
> > > +			/*
> > > +			 * Y2R coefficient setting
> > > +			 * bit 13 is 2^1, bit 12 is 2^0, bit 11 is 2^-
> > > 1,
> > > +			 * bit 10 is 2^-2 = 0.25
> > > +			 */
> > > +			y2r_coef = BIT(10);
> > > +
> > > +			/* -1 in 10bit */
> > > +			y2r_offset = GENMASK(10, 0) - 1;
> > 
> > I don't know why do this? If an input value is 0x100, then
> > 
> > 0x100 right shit 2 bit become 0x40.
> > 0x40 - 1 = 0x3f.
> > 0x3f left shift 2 bit become 0xfc.
> > 
> > So input 0x100 and output 0xfc. Why?
> > 
> 
> There is no input here, all the settings are direct bit assignment,
> and
> all the values are calculated by the designer. The main purpose of it
> is to configure the Y2R module to be able to transform 10bit RGB
> format
> into 8bit RGB, while this is not Y2R module is originally designed
> for.

The input I mean is the input of Y2R, the pixel value. If input a pixel
value 0x100 into Y2R, then Y2R output pixel value 0xfc, why do this? I
think it should not be minus 1. If remove the minus 1, then input 0x100
and output 0x100.

> 
> > > +
> > > +			/*
> > > +			 * R2R coefficient setting
> > > +			 * bit 19 is 2^1, bit 18 is 2^0, bit 17 is 2^-
> > > 1,
> > > +			 * bit 20 is 2^2 = 4
> > > +			 */
> > > +			r2r_coef = BIT(20);
> > > +
> > > +			/* CSC_EN is for R2R */
> > > +			csc_en = OVL_CLRFMT_EXT1_CSC_EN(idx);
> > > +
> > > +			/*
> > > +			 * 1. YUV input data - 1 and shift right for 2
> > > bits to remove it
> > > +			 * [R']   [0.25    0    0]   [Y in - 1]
> > > +			 * [G'] = [   0 0.25    0] * [U in - 1]
> > > +			 * [B']   [   0    0 0.25]   [V in - 1]
> > > +			 *
> > > +			 * 2. shift left for 2 bit letting the last 2
> > > bits become 0
> > 
> > You truncate the last two bit, so some quality lost. I think the
> > quality is main function and CRC is just for debug. So it's better
> > that
> > in normal case we keep quality and only for debug to lost the
> > quality.
> 
> Got it. Will modify in the next version to enter this section only if
> we need to calculate the CRC.
> 
> > I have another question. You just truncate the last two bit but it
> > is
> > still 10 bit value, so CRC could calculate this 10 bit value? I
> > don't
> > know why you say CRC just for 8 bit?
> > 
> 
> Yes, for RGB format, OVL can only handle 8bit per channel for CRC
> calculation, so I assume there may be similar issue handling 10bit
> YUV
> formats (P010) in the future.

I think this truncation is not necessary. No matter you truncate in
Y2R, OVL would finally truncate pixel value last 2 two bit when
calculate CRC. So this truncation is not necessary.

Regards,
CK


> 
> Thanks,
> Shawn
>