Kernel warning in cpufreq_add_dev()

Russell King - ARM Linux linux at
Fri Aug 19 04:00:32 PDT 2016

While checking the kernel on SA1110 Assabet, CPUFREQ issues a warning:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 1 at /home/rmk/git/linux-rmk/drivers/cpufreq/cpufreq.c:1080 cpufreq_add_dev+0x140/0x62c
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 4.8.0-rc2+ #883
Hardware name: Intel-Assabet
[<c0212190>] (dump_backtrace) from [<c021249c>] (show_stack+0x18/0x1c)
 r6:00000000 r5:c05e87c3 r4:00000000
[<c0212484>] (show_stack) from [<c037260c>] (dump_stack+0x20/0x28)
[<c03725ec>] (dump_stack) from [<c021f4cc>] (__warn+0xd0/0xfc)
[<c021f3fc>] (__warn) from [<c021f520>] (warn_slowpath_null+0x28/0x30)
 r10:00000000 r8:00000000 r7:00000000 r6:c064525c r5:00000000 r4:00000000
[<c021f4f8>] (warn_slowpath_null) from [<c04343a8>] (cpufreq_add_dev+0x140/0x62c)
[<c0434268>] (cpufreq_add_dev) from [<c03d83f4>] (bus_probe_device+0x5c/0x84)
 r10:00000000 r8:00000000 r7:00000000 r6:c064525c r5:c0657d60 r4:c065a9f8
[<c03d8398>] (bus_probe_device) from [<c03d677c>] (device_add+0x390/0x520)
 r6:c0645264 r5:00000000 r4:c064525c
[<c03d63ec>] (device_add) from [<c03d6a90>] (device_register+0x1c/0x20)
 r10:c0639848 r8:c061e524 r7:00000001 r6:00000000 r5:c064525c r4:c064525c
[<c03d6a74>] (device_register) from [<c03db5a0>] (register_cpu+0x88/0xac)
[<c03db518>] (register_cpu) from [<c061e544>] (topology_init+0x20/0x2c)
 r7:c0660b20 r6:c063f4a0 r5:c0639834 r4:00000000
[<c061e524>] (topology_init) from [<c020974c>] (do_one_initcall+0xc0/0x178)
[<c020968c>] (do_one_initcall) from [<c061be70>] (kernel_init_freeable+0xfc/0x1c4)
 r10:c0639848 r9:00000000 r8:00000088 r7:c0660b20 r6:c063f4a0 r5:c0639834
[<c061bd74>] (kernel_init_freeable) from [<c050d730>] (kernel_init+0x10/0xf4)
 r10:00000000 r8:00000000 r7:00000000 r6:00000000 r5:c050d720 r4:00000000
[<c050d720>] (kernel_init) from [<c020fcf0>] (ret_from_fork+0x14/0x24)
---[ end trace df94656649275917 ]---

This is because of an incompatibility between the expectations of cpufreq
and how register_cpu() works:

int register_cpu(struct cpu *cpu, int num)
        error = device_register(&cpu->dev);
        if (!error)
                per_cpu(cpu_sys_devices, num) = &cpu->dev;

When the device is registered via device_register(), any subsystems
registered for the cpu_subsys will have their "add_dev" method called.

The cpufreq add_dev, via cpufreq_online() and cpufreq_policy_alloc(),
tries to get the CPU device:

static struct cpufreq_policy *cpufreq_policy_alloc(unsigned int cpu)
        struct device *dev = get_cpu_device(cpu);
        if (WARN_ON(!dev))
                return NULL;

but this fails:

struct device *get_cpu_device(unsigned cpu)
        if (cpu < nr_cpu_ids && cpu_possible(cpu))
                return per_cpu(cpu_sys_devices, cpu);

because the percpu data has not yet been written - it'll be written
after a successful device registration.  So, using get_cpu_device()
from within cpufreq_add_dev() is broken, and results in the above
kernel warning.

RMK's Patch system:
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to

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