linux-kernel - Re: [PATCH v4.9] PM / OPP: Update voltage in case freq == old

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Date:   Tue, 10 Jul 2018 15:59:12 +0200
From:   Greg KH <greg@...ah.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...eaurora.org>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Waldemar Rymarkiewicz <waldemar.rymarkiewicz@...il.com>,
        "4 . 6+" <stable@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4.9] PM / OPP: Update voltage in case freq == old_freq

On Mon, Jul 02, 2018 at 02:19:47PM +0530, Viresh Kumar wrote:
> From: Waldemar Rymarkiewicz <waldemar.rymarkiewicz@...il.com>
> 
> Original commit c5c2a97b3ac7 ("PM / OPP: Update voltage in case freq ==
> old_freq").
> 
> This commit fixes a rare but possible case when the clk rate is updated
> without update of the regulator voltage.
> 
> At boot up, CPUfreq checks if the system is running at the right freq. This
> is a sanity check in case a bootloader set clk rate that is outside of freq
> table present with cpufreq core. In such cases system can be unstable so
> better to change it to a freq that is preset in freq-table.
> 
> The CPUfreq takes next freq that is >= policy->cur and this is our
> target_freq that needs to be set now.
> 
> dev_pm_opp_set_rate(dev, target_freq) checks the target_freq and the
> old_freq (a current rate). If these are equal it returns early. If not,
> it searches for OPP (old_opp) that fits best to old_freq (not listed in
> the table) and updates old_freq (!).
> 
> Here, we can end up with old_freq = old_opp.rate = target_freq, which
> is not handled in _generic_set_opp_regulator(). It's supposed to update
> voltage only when freq > old_freq  || freq > old_freq.
> 
> if (freq > old_freq) {
> 		ret = _set_opp_voltage(dev, reg, new_supply);
> [...]
> if (freq < old_freq) {
> 		ret = _set_opp_voltage(dev, reg, new_supply);
> 		if (ret)
> 
> It results in, no voltage update while clk rate is updated.
> 
> Example:
> freq-table = {
> 	1000MHz   1.15V
> 	 666MHZ   1.10V
> 	 333MHz   1.05V
> }
> boot-up-freq        = 800MHz   # not listed in freq-table
> freq = target_freq  = 1GHz
> old_freq            = 800Mhz
> old_opp = _find_freq_ceil(opp_table, &old_freq);  #(old_freq is modified!)
> old_freq            = 1GHz
> 
> Fixes: 6a0712f6f199 ("PM / OPP: Add dev_pm_opp_set_rate()")
> Cc: 4.6+ <stable@...r.kernel.org> # v4.6+
> Signed-off-by: Waldemar Rymarkiewicz <waldemar.rymarkiewicz@...il.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> Sending it for stable kernels from 4.6 until 4.9.

Always give me a hint as to what the original commit is, otherwise I
have to go dig for it :(

greg k-h