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

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Message-ID: <20210122044532.pc7cpcgy3kjbqmls@vireshk-i7>
Date:   Fri, 22 Jan 2021 10:15:32 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     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 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.

-- 
viresh