Re: [PATCH 1/8] dt-bindings: display: ti,am65x-dss: Add port properties for DSS

[Tomi Valkeinen]

On 22/07/2022 19:16, Nishanth Menon wrote:
> On 17:28-20220720, Rob Herring wrote:
> > > On the bridge side R0->R2, G0->G1, B0->B2 would be tied to ground.
> > > The bridge sees 24bits of data,  but the lsb's are always zero.
> >
> > Unless the bridge ignores the LSBs, that's not the right way to do 16 to
> > 24 bit. The LSBs should be connected to the MSB of the color component
> > to get full color range.
>
> I unfortunately cannot point specifics without violating NDAs, so
> will just give a broad perspective.
>
> Correct, this is not ideal, but in certain scenarios with limited
> pins (due to iovoltage groups), we are indeed starting to see this
> kind of usage model starting to pop up. Tradeoff is in a limit on
> image quality, but that tends to be acceptable in certain lower cost
> solutions.

It doesn't require more pins. If the lowest bits are tied to ground the 
image is always a bit darker than it should, and you do not get the full 
brightness. But if you wire e.g. the red component:

SoC : Bridge
R2 ->   R0
R3 ->   R1
R4 ->   R2
R0 ->   R3
R1 ->   R4
R2 ->   R5
R3 ->   R6
R4 ->   R7

or

R4 ->   R0
R4 ->   R1
R4 ->   R2
R0 ->   R3
R1 ->   R4
R2 ->   R5
R3 ->   R6
R4 ->   R7

You'll get the full range.

 Tomi