linux-kernel - Re: [PATCH -next] arm64: Export __cpu_logical

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Date:   Mon, 27 Jul 2020 17:05:15 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Sumit Gupta <sumitg@...dia.com>
Cc:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Hulk Robot <hulkci@...wei.com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Bibek Basu <bbasu@...dia.com>,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH -next] arm64: Export __cpu_logical_map

On Sun, Jul 26, 2020 at 05:16:27PM +0530, Sumit Gupta wrote:
> > > > ERROR: modpost: "__cpu_logical_map" [drivers/cpufreq/tegra194-cpufreq.ko] undefined!
> > > >
> > > > ARM64 tegra194-cpufreq driver use cpu_logical_map, export
> > > > __cpu_logical_map to fix build issue.
> > > >
> >
> > I wonder why like other instances in the drivers, the mpidr is not get
> > directly from the cpu. The cpufreq_driver->init call happens when the cpu
> > is being brought online and is executed on the required cpu IIUC.
> >
> Yes, this occurs during hotplug case.
> But in the case of system boot, 'cpufreq_driver->init' is called later
> during cpufreq platform driver's probe. The value of CPU in 'policy->cpu'
> can be different from the current CPU. That's why read_cpuid_mpidr() can't
> be used.
>

Fair enough, why not do cross call like in set_target ? Since it is one-off
in init, I don't see any issue when you are doing it runtime for set_target.

> > read_cpuid_mpidr() is inline and avoids having to export the logical_cpu_map.
> > Though we may not add physical hotplug anytime soon, less dependency
> > on this cpu_logical_map is better given that we can resolve this without
> > the need to access the map.
> >

To be honest, we have tried to remove all the dependency on cluster id
in generic code as it is not well defined. This one is tegra specific
driver so should be fine. But I am still bit nervous to export
cpu_logical_map as we have no clue what that would mean for physical
hotplug.


--
Regards,
Sudeep