Re: [PATCH 1/2] dt-bindings: clock: fixed-factor: Add TI AM62 SoC OLDI clock

[Tomi Valkeinen]

On 16/01/2023 11:51, Aradhya Bhatia wrote:
> Hi Stephen,
>
> Thanks for taking a look at the patch.
>
> On 12-Jan-23 01:14, Stephen Boyd wrote:
> > Quoting Aradhya Bhatia (2022-12-26 01:57:44)
> > > Add "ti,k3-am62-oldi-clk-div" to the fixed factor clock compatible enum
> > > list.
> > >
> > > "ti,k3-am62-oldi-clk-div" is a fixed-factor clock that helps the TI
> > > display subsystem request a pixel clock for itself and a corresponding
> > > serial clock for its OLDI Transmitters. The serial clock is 7 times the
> > > pixel clock. This clock needs the clock set rate request to be
> > > propagated to the parent clock provider.
> > >
> > > Signed-off-by: Aradhya Bhatia <a-bhatia1@xxxxxx>
> > > ---
> > >   Documentation/devicetree/bindings/clock/fixed-factor-clock.yaml | 1 +
> > >   1 file changed, 1 insertion(+)
> > >
> > > diff --git 
> > > a/Documentation/devicetree/bindings/clock/fixed-factor-clock.yaml 
> > > b/Documentation/devicetree/bindings/clock/fixed-factor-clock.yaml
> > > index 8f71ab300470..0696237530f7 100644
> > > --- a/Documentation/devicetree/bindings/clock/fixed-factor-clock.yaml
> > > +++ b/Documentation/devicetree/bindings/clock/fixed-factor-clock.yaml
> > > @@ -14,6 +14,7 @@ properties:
> > >     compatible:
> > >       enum:
> > >         - fixed-factor-clock
> > > +      - ti,k3-am62-oldi-clk-div
> >
> > I don't see this compatible anywhere in the kernel tree. Is there a
> > patch that adds a node using this? I wonder why the display subsystem
> > can't add this fixed factor clk directly in the driver. Does the OLDI
> > Transmitter send a clk to the display subsystem?
> >
> > I'm asking all these questions because we got rid of vendor compatibles
> > here in hopes of simplifying the logic. Maybe the problem can be
> > approached differently, but I don't know all the details.
>
>
> +--------+                       +------------------+
> |        |                       |                  |
> |  PLL   +---+----+------------->| OLDI Transmitter |
> |        |   |    |              |                  |
> +--------+   |    |              +------------------+
>               |    |
>               |    |              +------------------+
>               |    |              |                  |
>               |    +------------->| OLDI Transmitter |
>               |                   |                  |
>               |                   +------------------+
>               |
>               |                   +------------------+
>               |   +----------+    |                  |
>               |   |    /7    |    |      Display     |
>               +-->|   Clock  +--->| Sub-System (DSS) |
>                   |    Div   |    |                  |
>                   +----------+    +------------------+
>
> This is how the the clock architecture for DSS looks like.
>
> The clock divider is not a part of DSS, but outside it.
>
> The clock request flow is initiated by the DSS driver because it has the
> required timing parameter information. It requests a certain pixel
> frequency. But the frequency required by the OLDI TXes is 7 times
> that pixel frequency.
>
> (Just for clarification, in some cases, the OLDI TX does require only
> 3.5 times the pixel frequency, but in those situations there is another
> divider in-front of OLDI TX that gets activated with a signal and
> divides the incoming frequency by 2, thereby requiring the PLL to still
> generate a 7x frequency.)
>
> Hence, the idea is that the clock divider is able to propagate the set
> rate request back to PLL, asking for a frequency 7 times more than the
> DSS's asking rate.
>
> If this is something less than ideal and should not go up, then I can
> implement a new clock device with a separate but similar clock driver.
>
> Let me know what you think!

As a clarification I would also add to the above that on other TI SoCs 
with DSS, and also for the second video port on AM62, the clock 
framework provides DSS a clock using the pclk frequency.

 Tomi