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

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Message-ID: <1856e963-4514-92f3-5d43-d5b711083193@ti.com>
Date:   Mon, 16 Jan 2023 15:21:30 +0530
From:   Aradhya Bhatia <a-bhatia1@...com>
To:     Stephen Boyd <sboyd@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>
CC:     Tomi Valkeinen <tomba@...nel.org>,
        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

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!


Regards
Aradhya