linux-kernel - Re: [PATCH v3 3/3] clk: qcom: Add display clock controller driver for SDM845

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Date:   Thu, 12 Jul 2018 22:51:33 +0530
From:   Taniya Das <tdas@...eaurora.org>
To:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Sandeep Panda <spanda@...eaurora.org>,
        Abhinav Kumar <abhinavk@...eaurora.org>, ryadav@...eaurora.org
Cc:     Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Amit Nischal <anischal@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] clk: qcom: Add display clock controller driver for
 SDM845

++ Display driver team,

On 7/9/2018 8:36 PM, Stephen Boyd wrote:
> Quoting Taniya Das (2018-07-09 02:34:07)
>>
>>
>> On 7/9/2018 1:07 PM, Stephen Boyd wrote:
>>> Quoting Taniya Das (2018-07-09 00:07:21)
>>>>
>>>>
>>>> On 7/9/2018 11:46 AM, Stephen Boyd wrote:
>>>>>>
>>>>>>     > Why is the nocache flag needed? Applies to all clks in this file.
>>>>>>     >
>>>>>>
>>>>>> This flag is required for all RCGs whose PLLs are controlled outside the
>>>>>> clock controller. The display code would require the recalculated rate
>>>>>> always.
>>>>>
>>>>> Right. Why is the PLL controlled outside of the clock controller? The
>>>>> rate should propagate upward to the PLL from here, so who's going
>>>>> outside of that?
>>>>>
>>>> The DSI0/1 PLL are not part of the display clock controller, but in the
>>>> display subsystem which are managed by the DRM drivers. When DRM drivers
>>>> query for the rate clock driver should always return the non cached rates.
>>>
>>> Why? Is the DSI PLL changing rate all the time, randomly, without going
>>> through the clk APIs to do so?
>>>
>>
>> Hmm, I am afraid I do not have an answer for this, but this was the
>> requirement to always return the non cached rates from the clock driver.
>>
> 
> Ok. Who knows about this requirement? Can we add someone from the
> display driver to understand more?
> 
As per my discussions offline with the display teams,

There is a use-case where the clock framework is unaware of the PLL VCO 
frequency change and thus the drivers would query to get the actual HW 
frequency rather than the cached one.

Do you think keeping these flags would have any impact other than always 
getting the non-cached rates?

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