linux-kernel - Re: [PATCH 03/13] opp: Keep track of currently programmed OPP

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Date:   Wed, 27 Jan 2021 22:01:03 +0530
From:   Akhil P Oommen <akhilpo@...eaurora.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Dmitry Osipenko <digetx@...il.com>
Cc:     Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Sibi Sankar <sibis@...eaurora.org>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 03/13] opp: Keep track of currently programmed OPP

On 1/22/2021 10:15 AM, Viresh Kumar wrote:
> On 22-01-21, 00:41, Dmitry Osipenko wrote:
>> 21.01.2021 14:17, Viresh Kumar пишет:
>>> @@ -1074,15 +1091,18 @@ int dev_pm_opp_set_rate(struct device *dev, unsigned long target_freq)
>>>   
>>>   	if (!ret) {
>>>   		ret = _set_opp_bw(opp_table, opp, dev, false);
>>> -		if (!ret)
>>> +		if (!ret) {
>>>   			opp_table->enabled = true;
>>> +			dev_pm_opp_put(old_opp);
>>> +
>>> +			/* Make sure current_opp doesn't get freed */
>>> +			dev_pm_opp_get(opp);
>>> +			opp_table->current_opp = opp;
>>> +		}
>>>   	}
>>
>> I'm a bit surprised that _set_opp_bw() isn't used similarly to
>> _set_opp_voltage() in _generic_set_opp_regulator().
>>
>> I'd expect the BW requirement to be raised before the clock rate goes UP.
> 
> I remember discussing that earlier when this stuff came in, and this I
> believe is the reason for that.
> 
> We need to scale regulators before/after frequency because when we
> increase the frequency a regulator may _not_ be providing enough power
> to sustain that (even for a short while) and this may have undesired
> effects on the hardware and so it is important to prevent that
> malfunction.
> 
> In case of bandwidth such issues will not happen (AFAIK) and doing it
> just once is normally enough. It is just about allowing more data to
> be transmitted, and won't make the hardware behave badly.
> 
I agree with Dmitry. BW is a shared resource in a lot of architectures. 
Raising clk before increasing the bw can lead to a scenario where this 
client saturate the entire BW for whatever small duration it may be. 
This will impact the latency requirements of other clients.

-Akhil.