linux-kernel - Re: [PATCH v4 1/4] gpu: ipu-v3: ipu-ic: Rename yuv2rgb encoding matrices

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Date:   Tue, 12 Feb 2019 11:17:46 +0100
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Steve Longerbeam <slongerbeam@...il.com>,
        linux-media@...r.kernel.org
Cc:     Tim Harvey <tharvey@...eworks.com>,
        "open list:DRM DRIVERS FOR FREESCALE IMX" 
        <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/4] gpu: ipu-v3: ipu-ic: Rename yuv2rgb encoding
 matrices

Hi Steve,

On Mon, 2019-02-11 at 10:24 -0800, Steve Longerbeam wrote:
[...]
> Looking more closely at these coefficients now, I see you are right, 
> they are the BT.601 YUV full-range coefficients (Y range 0 to 1, U and V 
> range -0.5 to 0.5). Well, not even that -- the coefficients are not 
> being scaled to the limited ranges, but the 0.5 offset (128) _is_ being 
> added to U/V, but no offset for Y. So it is even more messed up.
>
> Your corrected coefficients and offsets look correct to me: Y 
> coefficients scaled to (235 - 16) / 255 and U/V coefficients scaled to 
> (240 - 16)  / 255, and add the offsets for both Y and U/V.
> 
> But what about this "SAT_MODE" field in the IC task parameter memory? 

That just controls the saturation. The result after the matrix
multiplication is either saturated to [0..255] or to [16..235]/[16..240]
when converting from the internal representation to the 8 bit output.

> According to the manual the hardware will automatically convert the 
> written coefficients to the correct limited ranges.

Where did you get that from? "The final calculation result is limited
according to the SAT_MODE parameter and rounded to 8 bits." I see no
mention of coefficients being modified.

> I see there is a "sat" field defined in the struct but is not being
> set in the tables.
> 
> So what should we do, define the full range coefficients, and make use 
> of SAT_MODE h/w feature, or scale/offset the coefficients ourselves and 
> not use SAT_MODE? I'm inclined to do the former.

SAT_MODE should be set for conversions to YUV limited range so that the
coefficients can be rounded to the closest value. Otherwise we'd have to
round towards zero, possibly with a larger error, to make sure the
results are inside the valid ranges.

regards
Philipp