Date:   Mon, 6 Feb 2023 11:04:53 +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>,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
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

Hi Stephen,

On 26-Jan-23 05:36, Stephen Boyd wrote:
> Quoting Tomi Valkeinen (2023-01-17 01:40:24)
>> 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 divider is fixed as well? And presumably inside the SoC?
Yes, and yes!

> 
>>>
>>> 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.
> 
> Got it. Can the PLL driver provide a pll_div_7 clk that is used for the
> DSS pixel clk?
> 
The PLL driver can not map the clock divider and hence can't provide the
pll_div_7 clock directly to DSS.

>>>
>>> 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.
>>
> 
> Are you saying that adding a fixed div-7 clk in the DSS driver is wrong?
Yes. All variants of DSS accept a pixel clock and it would be wrong to
implement a fixed div-7 in the DSS driver.

All that said, I now understand that the new compatible shouldn't go
there. I will implement a new driver and post it. =)


Regards
Aradhya

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