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From: Rajendra Nayak <rnayak@codeaurora.org>
To: Viresh Kumar <viresh.kumar@linaro.org>, swboyd@chromium.org
Cc: linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org,
	linux-pm@vger.kernel.org, linux-serial@vger.kernel.org,
	linux-spi@vger.kernel.org, dri-devel@lists.freedesktop.org,
	linux-scsi@vger.kernel.org, ulf.hansson@linaro.org,
	dianders@chromium.org, rafael@kernel.org
Subject: Re: [RFC v2 01/11] OPP: Don't overwrite rounded clk rate
Date: Wed, 12 Jun 2019 13:12:51 +0530
Message-ID: <c173a57d-a4de-99f7-e8d8-28a7612f4ca3@codeaurora.org> (raw)
In-Reply-To: <20190611105432.x3nzqiib35t6mvyg@vireshk-i7>


On 6/11/2019 4:24 PM, Viresh Kumar wrote:
> On 20-03-19, 15:19, Rajendra Nayak wrote:
>> From: Stephen Boyd <swboyd@chromium.org>
>>
>> Doing this allows us to call this API with any rate requested and have
>> it not need to match in the OPP table. Instead, we'll round the rate up
>> to the nearest OPP that we see so that we can get the voltage or level
>> that's required for that OPP. This supports users of OPP that want to
>> specify the 'fmax' tables of a device instead of every single frequency
>> that they need. And for devices that required the exact frequency, we
>> can rely on the clk framework to round the rate to the nearest supported
>> frequency instead of the OPP framework to do so.
>>
>> Note that this may affect drivers that don't want the clk framework to
>> do rounding, but instead want the OPP table to do the rounding for them.
>> Do we have that case? Should we add some flag to the OPP table to
>> indicate this and then not have that flag set when there isn't an OPP
>> table for the device and also introduce a property like 'opp-use-clk' to
>> tell the table that it should use the clk APIs to round rates instead of
>> OPP?
>>
>> Signed-off-by: Stephen Boyd <swboyd@chromium.org>
>> Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
>> ---

[]...

> 
> I see a logical problem with this patch.
> 
> Suppose the clock driver supports following frequencies: 500M, 800M,
> 1G, 1.2G and the OPP table contains following list: 500M, 1G, 1.2G
> (i.e. missing 800M).
> 
> Now 800M should never get programmed as it isn't part of the OPP
> table. But if you pass 600M to opp-set-rate, then it will end up
> selecting 800M as clock driver will round up to the closest value.

correct

> 
> Even if no one is doing this right now, it is a sensible usecase,
> specially during testing of patches and I don't think we should avoid
> it.
> 
> What exactly is the use case for which we need this patch ? 
Like the changelog says 'This supports users of OPP that want to
specify the 'fmax' tables of a device instead of every single frequency
that they need'

so the 'fmax' tables basically say what the max frequency the device can
operate at for a given performance state/voltage level.

so in your example it would be for instance

500M, Perf state = 2
1G, Perf state = 3
1.2G, Perf state = 4

Now when the device wants to operate at say 800Mhz, you need to set the
Perf state to 3, so this patch basically avoids you having to put those additional
OPPs in the table which would otherwise look something like this

500M, Perf state = 2
800M, Perf state = 3 <-- redundant OPP
1G, Perf state = 3
1.2G, Perf state = 4

Your example had just 1 missing entry in the 'fmax' tables in reality its a lot more,
atleast on all qualcomm platforms.


-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
of Code Aurora Forum, hosted by The Linux Foundation

  reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-20  9:49 [RFC v2 00/11] DVFS in the OPP core Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 01/11] OPP: Don't overwrite rounded clk rate Rajendra Nayak
2019-06-11 10:54   ` Viresh Kumar
2019-06-12  7:42     ` Rajendra Nayak [this message]
2019-06-12  8:25       ` Viresh Kumar
2019-06-13  9:54         ` Viresh Kumar
2019-06-14  5:27           ` Viresh Kumar
2019-06-17  3:50             ` Viresh Kumar
2019-06-17  4:07               ` Rajendra Nayak
2019-06-17  4:17                 ` Viresh Kumar
2019-06-17  4:25                   ` Rajendra Nayak
2019-06-14  5:54           ` Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 02/11] OPP: Make dev_pm_opp_set_rate() with freq=0 as valid Rajendra Nayak
2019-06-14  6:32   ` Viresh Kumar
2019-06-17  4:04     ` Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 03/11] tty: serial: qcom_geni_serial: Use OPP API to set clk/perf state Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 04/11] spi: spi-geni-qcom: " Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 05/11] arm64: dts: sdm845: Add OPP table for all qup devices Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 06/11] scsi: ufs: Add support to manage multiple power domains in ufshcd-pltfrm Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 07/11] scsi: ufs: Add support for specifying OPP tables in DT Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 08/11] arm64: dts: sdm845: Add ufs opps and power-domains Rajendra Nayak
2019-05-14  7:53   ` Ulf Hansson
2019-03-20  9:49 ` [RFC v2 09/11] drm/msm/dpu: Use OPP API to set clk/perf state Rajendra Nayak
2019-04-10  3:49   ` Viresh Kumar
2019-04-10  3:49     ` Viresh Kumar
2019-03-20  9:49 ` [RFC v2 10/11] drm/msm: dsi: " Rajendra Nayak
2019-03-20  9:49 ` [RFC v2 11/11] arm64: dts: sdm845: Add DSI and MDP OPP tables and power-domains Rajendra Nayak
2019-04-10  3:51 ` [RFC v2 00/11] DVFS in the OPP core Viresh Kumar
2019-04-10  3:51   ` Viresh Kumar
2019-05-21  6:22 ` Viresh Kumar
2019-05-24  6:03   ` Rajendra Nayak
2019-06-17  4:26 ` Viresh Kumar

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