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Subject: Re: [PATCH v4 12/37] PM / devfreq: tegra20: Use MC timings for
 building OPP table
To:     Chanwoo Choi <cw00.choi@samsung.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Georgi Djakov <georgi.djakov@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Peter De Schrijver <pdeschrijver@nvidia.com>,
        MyungJoo Ham <myungjoo.ham@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Mikko Perttunen <cyndis@kapsi.fi>
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        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        devicetree@vger.kernel.org
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From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <7988b6cf-e60c-7e5c-ffc3-8075c20af3d3@gmail.com>
Date:   Thu, 2 Jul 2020 08:43:30 +0300
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02.07.2020 08:30, Chanwoo Choi пишет:
> On 7/2/20 2:07 PM, Dmitry Osipenko wrote:
>> 02.07.2020 07:18, Chanwoo Choi пишет:
>>> Hi Dmitry,
>>>
>>> On 6/9/20 10:13 PM, Dmitry Osipenko wrote:
>>>> The clk_round_rate() won't be usable for building OPP table once
>>>> interconnect support will be added to the EMC driver because that CLK API
>>>> function limits the rounded rate based on the clk rate that is imposed by
>>>> active clk-users, and thus, the rounding won't work as expected if
>>>> interconnect will set the minimum EMC clock rate before devfreq driver is
>>>> loaded. The struct tegra_mc contains memory timings which could be used by
>>>> the devfreq driver for building up OPP table instead of rounding clock
>>>> rate, this patch implements this idea.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>>>> ---
>>>>  drivers/devfreq/tegra20-devfreq.c | 18 +++++++++++-------
>>>>  1 file changed, 11 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/devfreq/tegra20-devfreq.c b/drivers/devfreq/tegra20-devfreq.c
>>>> index 6469dc69c5e0..bf504ca4dea2 100644
>>>> --- a/drivers/devfreq/tegra20-devfreq.c
>>>> +++ b/drivers/devfreq/tegra20-devfreq.c
>>>> @@ -123,8 +123,7 @@ static int tegra_devfreq_probe(struct platform_device *pdev)
>>>>  {
>>>>  	struct tegra_devfreq *tegra;
>>>>  	struct tegra_mc *mc;
>>>> -	unsigned long max_rate;
>>>> -	unsigned long rate;
>>>> +	unsigned int i;
>>>>  	int err;
>>>>  
>>>>  	mc = tegra_get_memory_controller();
>>>> @@ -151,12 +150,17 @@ static int tegra_devfreq_probe(struct platform_device *pdev)
>>>>  
>>>>  	tegra->regs = mc->regs;
>>>>  
>>>> -	max_rate = clk_round_rate(tegra->emc_clock, ULONG_MAX);
>>>> -
>>>> -	for (rate = 0; rate <= max_rate; rate++) {
>>>> -		rate = clk_round_rate(tegra->emc_clock, rate);
>>>> +	if (!mc->num_timings) {
>>>
>>> Could you explain what is meaning of 'num_timing?
>>
>> The num_timings is the number of memory timings defined in a
>> device-tree. One timing configuration per memory clock rate.
> 
> OK. I understand.
> 
>>
>>> Also, why add the opp entry in case of mc->num_timings is zero?
>>
>> Timings may be not defined in some device-trees at all and in this case
>> memory always running on a fixed clock rate.
> 
> You mean that 'timings' information is optional?

Yes

Actually, looks like I missed to properly test this case where timings
are missing in DT and it shouldn't work with the current code. I'll fix
it in the next version.

>>
>> The devfreq driver won't be practically useful if mc->num_timings is
>> zero since memory frequency can't be changed, but anyways we'd want to
>> load the devfreq driver in order to prevent confusion about why it's not
>> loaded.
>>
>> For example, you may ask somebody to show contents of
>> /sys/class/devfreq/tegra20-devfreq/trans_stat and the person says to you
>> that this file doesn't exist, now you'll have to figure out what
>> happened to the devfreq driver.
> 
> I understand why add OPP entry point when timing is not defined on DT.
> But, actually, I think that you better to change 'timings' info is mandatory
> instead of optional. Because the devfreq driver is for DVFS
> and the driver supporting DVFS have to have the frequency information
> like OPP.
> 
> Or, 
> If you want to keep 'timing' is optional on DT,
> I recommend that you add one timing data to tegra mc driver
> when DT doesn't include the any timing information
> I think that is it more clear.

Okay, I'll move it into the MC driver in the next version.

Thank you for the review!