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

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Date:   Mon, 16 Jul 2018 11:27:48 +0530
From:   Taniya Das <tdas@...eaurora.org>
To:     spanda@...eaurora.org, Stephen Boyd <sboyd@...nel.org>
Cc:     Abhinav Kumar <abhinavk@...eaurora.org>,
        Michael Turquette <mturquette@...libre.com>,
        ryadav@...eaurora.org, 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

Hello Stephen,

On 7/13/2018 1:55 PM, spanda@...eaurora.org wrote:
> On 2018-07-13 01:11, Stephen Boyd wrote:
>> Quoting Taniya Das (2018-07-12 10:21:33)
>>> ++ 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?
>>>
>>
>> The flag will make it so clk_get_rate() works in spite of something
>> changing the frequency behind the framework's back, but I want to
>> understand what and why it's changing without framework involvement. We
>> shouldn't need the flag here, because this flag is typically for clks
>> that are controlled by some other entity that the kernel doesn't have
>> control over. In this case, it seems like we have full control of the
>> clk tree for the display PLL down to this clk, so it should be perfectly
>> fine to not have this flag. The presence of the flag means that the
>> display driver is doing something wrong.
> 
> These clocks are sourced from DSI PLL. In dsi command mode there is an 
> idle use case when there
> is no activity on display, we switch of the source DSI PLL to save power 
> by calling clk_disable_unprepare().
> In this scenario if some client queries the clk_get_rate(), then if 
> NO_CACHE flag is used the clk
> driver will return the cached rate which is some non-zero value set when 
> we last called clk_set_rate(),
> before enabling the clock, whereas actually in HW this clk is disabled. 
> So we used the NO_CACHE flag
> for the call to land in clk_recalc_rate and we return zero if the PLL is 
> disabled.
> I remember some customer had scripts that runs through all the clocks 
> during idle screen scenario
> and call clk_get_rate(), where they complained display clk_get_rate 
> returns none zero value.
> 
> 

Do you think the clock driver needs any update for flag for the next series?

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