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

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Date:   Sun, 26 Jul 2020 17:16:27 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Sudeep Holla <sudeep.holla@....com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
CC:     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>,
        Sumit Gupta <sumitg@...dia.com>, Bibek Basu <bbasu@...dia.com>
Subject: Re: [PATCH -next] arm64: Export __cpu_logical_map

>>> 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.

> 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.
> 
> --
> Regards,
> Sudeep
>