linux-kernel - Re: [PATCH 2/4] clk: qcom: Add clk_rcg2_gfx3d

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Date:   Mon, 13 Aug 2018 12:00:38 +0530
From:   Amit Nischal <anischal@...eaurora.org>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Jordan Crouse <jcrouse@...eaurora.org>,
        Michael Turquette <mturquette@...libre.com>,
        Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Odelu Kukatla <okukatla@...eaurora.org>,
        Taniya Das <tdas@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-clk-owner@...r.kernel.org
Subject: Re: [PATCH 2/4] clk: qcom: Add clk_rcg2_gfx3d_ops for SDM845

On 2018-08-08 11:28, Stephen Boyd wrote:
> Quoting Jordan Crouse (2018-08-06 08:04:37)
>> On Mon, Aug 06, 2018 at 02:37:18PM +0530, Amit Nischal wrote:
>> > On 2018-08-03 04:14, Stephen Boyd wrote:
>> > >Quoting Amit Nischal (2018-07-30 04:28:56)
>> > >>On 2018-07-25 12:28, Stephen Boyd wrote:
>> > >>>
>> > >>> Ok. Sounds good! Is the rate range call really needed? It can't be
>> > >>> determined in the PLL code with some table or avoided by making sure
>> > >>> GPU
>> > >>> uses OPP table with only approved frequencies?
>> > >>>
>> > >>
>> > >>Currently fabia PLL code does not have any table to check this and
>> > >>intention
>> > >>was to avoid relying on the client to call set_rate with only approved
>> > >>frequencies so we have added the set_rate_range() call in the GPUCC
>> > >>driver
>> > >>in order to set the rate range.
>> > >>
>> > >
>> > >But GPU will use OPP so it doesn't seem like it really buys us anything
>> > >here. And it really doesn't matter when the clk driver implementation
>> > >doesn't use the min/max to clamp the values of the round_rate()
>> > >call. Is
>> > >that being done here? I need to double check. I would be more convinced
>> > >if the implementation was looking at min/max to constrain the rate
>> > >requested.
>> > >
>> >
>> > So our understanding is that GPU(client) driver will always call the
>> > set_rate with approved frequencies only and we can completely rely
>> > on the
>> > client. Is our understanding is correct?
>> 
>> 
>> First: on sdm845 the software doesn't set the GPU clocks - we rely on 
>> the GMU
>> firmware to do that on our behalf but for the GPU at least this is an 
>> academic
>> exercise.
> 
> So what is this GPU clk driver for then?
> 
>> 
>> But that said: traditionally we've expected that the clock driver 
>> correctly
>> clamp the requested rate to the correct values. In the past we have 
>> taken
>> advantage of this and we may in the future. I don't think it is 
>> reasonable
>> to require the leaf driver to only pass "approved" frequencies 
>> especially
>> since we depend on our own OPP table that may or may not be similar to 
>> the
>> one used by the clock driver.
>> 
> 
> Ok. Sounds like things can't be kept in sync between the clk driver and
> the OPP tables. Why is that hard to do?
> 
> Either way, I'd be fine if the code actually used the frequency limits
> to round the rate to something within range, but I don't recall seeing
> that being done here. So if the min/max limits stay, the clk driver
> should round to within that range.
> 

Thanks Stephen for your suggestion. I have modified the existing
determine_rate() op to use the min/max limits and round the requested
rate so that it stays withing the set_rate range. I will submit the
same in the next patch series.


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