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

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Message-ID: <367fba29-bc08-1f27-249c-09e406adfbbb@ideasonboard.com>
Date:   Tue, 17 Jan 2023 11:40:24 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To:     Aradhya Bhatia <a-bhatia1@...com>, Stephen Boyd <sboyd@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>
Cc:     Samuel Holland <samuel@...lland.org>,
        Maxime Ripard <mripard@...nel.org>,
        Linux Clock List <linux-clk@...r.kernel.org>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Devarsh Thakkar <devarsht@...com>, Jai Luthra <j-luthra@...com>
Subject: Re: [PATCH 1/2] dt-bindings: clock: fixed-factor: Add TI AM62 SoC
 OLDI clock

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@...com>
>>> ---
>>>   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