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

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Message-ID: <153142449680.48062.13809855257701591509@swboyd.mtv.corp.google.com>
Date:   Thu, 12 Jul 2018 12:41:36 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Abhinav Kumar <abhinavk@...eaurora.org>,
        Michael Turquette <mturquette@...libre.com>,
        Sandeep Panda <spanda@...eaurora.org>,
        Taniya Das <tdas@...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

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.