That's unclear why lockdep shows the following warning but adding a
lockdep class to struct pmac_i2c_bus solves it

[   20.507795] ======================================================
[   20.507796] [ INFO: possible circular locking dependency detected ]
[   20.507800] 4.8.0-rc7-00037-gd2ffb010 #21 Not tainted
[   20.507801] -------------------------------------------------------
[   20.507803] swapper/0/1 is trying to acquire lock:
[   20.507818]  (&bus->mutex){+.+.+.}, at: [<c000000000052830>] .pmac_i2c_open+0x30/0x100
[   20.507819]
[   20.507819] but task is already holding lock:
[   20.507829]  (&policy->rwsem){+.+.+.}, at: [<c00000000068adcc>] .cpufreq_online+0x1ac/0x9d0
[   20.507830]
[   20.507830] which lock already depends on the new lock.
[   20.507830]
[   20.507832]
[   20.507832] the existing dependency chain (in reverse order) is:
[   20.507837]
[   20.507837] -> #4 (&policy->rwsem){+.+.+.}:
[   20.507844]        [<c00000000082385c>] .down_write+0x6c/0x110
[   20.507849]        [<c00000000068adcc>] .cpufreq_online+0x1ac/0x9d0
[   20.507855]        [<c0000000004d76d8>] .subsys_interface_register+0xb8/0x110
[   20.507860]        [<c000000000689bb0>] .cpufreq_register_driver+0x1d0/0x250
[   20.507866]        [<c000000000b4f8f4>] .g5_cpufreq_init+0x9cc/0xa28
[   20.507872]        [<c00000000000a98c>] .do_one_initcall+0x5c/0x1d0
[   20.507878]        [<c000000000b0f86c>] .kernel_init_freeable+0x1ac/0x28c
[   20.507883]        [<c00000000000b3bc>] .kernel_init+0x1c/0x140
[   20.507887]        [<c0000000000098f4>] .ret_from_kernel_thread+0x58/0x64
[   20.507894]
[   20.507894] -> #3 (subsys mutex#2){+.+.+.}:
[   20.507899]        [<c000000000820448>] .mutex_lock_nested+0xa8/0x590
[   20.507903]        [<c0000000004d7f24>] .bus_probe_device+0x44/0xe0
[   20.507907]        [<c0000000004d5208>] .device_add+0x508/0x730
[   20.507911]        [<c0000000004dd528>] .register_cpu+0x118/0x190
[   20.507916]        [<c000000000b14450>] .topology_init+0x148/0x248
[   20.507921]        [<c00000000000a98c>] .do_one_initcall+0x5c/0x1d0
[   20.507925]        [<c000000000b0f86c>] .kernel_init_freeable+0x1ac/0x28c
[   20.507929]        [<c00000000000b3bc>] .kernel_init+0x1c/0x140
[   20.507934]        [<c0000000000098f4>] .ret_from_kernel_thread+0x58/0x64
[   20.507939]
[   20.507939] -> #2 (cpu_add_remove_lock){+.+.+.}:
[   20.507944]        [<c000000000820448>] .mutex_lock_nested+0xa8/0x590
[   20.507950]        [<c000000000087a9c>] .register_cpu_notifier+0x2c/0x70
[   20.507955]        [<c000000000b267e0>] .spawn_ksoftirqd+0x18/0x4c
[   20.507959]        [<c00000000000a98c>] .do_one_initcall+0x5c/0x1d0
[   20.507964]        [<c000000000b0f770>] .kernel_init_freeable+0xb0/0x28c
[   20.507968]        [<c00000000000b3bc>] .kernel_init+0x1c/0x140
[   20.507972]        [<c0000000000098f4>] .ret_from_kernel_thread+0x58/0x64
[   20.507978]
[   20.507978] -> #1 (&host->mutex){+.+.+.}:
[   20.507982]        [<c000000000820448>] .mutex_lock_nested+0xa8/0x590
[   20.507987]        [<c0000000000527e8>] .kw_i2c_open+0x18/0x30
[   20.507991]        [<c000000000052894>] .pmac_i2c_open+0x94/0x100
[   20.507995]        [<c000000000b220a0>] .smp_core99_probe+0x260/0x410
[   20.507999]        [<c000000000b185bc>] .smp_prepare_cpus+0x280/0x2ac
[   20.508003]        [<c000000000b0f748>] .kernel_init_freeable+0x88/0x28c
[   20.508008]        [<c00000000000b3bc>] .kernel_init+0x1c/0x140
[   20.508012]        [<c0000000000098f4>] .ret_from_kernel_thread+0x58/0x64
[   20.508018]
[   20.508018] -> #0 (&bus->mutex){+.+.+.}:
[   20.508023]        [<c0000000000ed5b4>] .lock_acquire+0x84/0x100
[   20.508027]        [<c000000000820448>] .mutex_lock_nested+0xa8/0x590
[   20.508032]        [<c000000000052830>] .pmac_i2c_open+0x30/0x100
[   20.508037]        [<c000000000052e14>] .pmac_i2c_do_begin+0x34/0x120
[   20.508040]        [<c000000000056bc0>] .pmf_call_one+0x50/0xd0
[   20.508045]        [<c00000000068ff1c>] .g5_pfunc_switch_volt+0x2c/0xc0
[   20.508050]        [<c00000000068fecc>] .g5_pfunc_switch_freq+0x1cc/0x1f0
[   20.508054]        [<c00000000068fc2c>] .g5_cpufreq_target+0x2c/0x40
[   20.508058]        [<c0000000006873ec>] .__cpufreq_driver_target+0x23c/0x840
[   20.508062]        [<c00000000068c798>] .cpufreq_gov_performance_limits+0x18/0x30
[   20.508067]        [<c00000000068915c>] .cpufreq_start_governor+0xac/0x100
[   20.508071]        [<c00000000068a788>] .cpufreq_set_policy+0x208/0x260
[   20.508076]        [<c00000000068abdc>] .cpufreq_init_policy+0x6c/0xb0
[   20.508081]        [<c00000000068ae70>] .cpufreq_online+0x250/0x9d0
[   20.508085]        [<c0000000004d76d8>] .subsys_interface_register+0xb8/0x110
[   20.508090]        [<c000000000689bb0>] .cpufreq_register_driver+0x1d0/0x250
[   20.508094]        [<c000000000b4f8f4>] .g5_cpufreq_init+0x9cc/0xa28
[   20.508099]        [<c00000000000a98c>] .do_one_initcall+0x5c/0x1d0
[   20.508103]        [<c000000000b0f86c>] .kernel_init_freeable+0x1ac/0x28c
[   20.508107]        [<c00000000000b3bc>] .kernel_init+0x1c/0x140
[   20.508112]        [<c0000000000098f4>] .ret_from_kernel_thread+0x58/0x64
[   20.508113]
[   20.508113] other info that might help us debug this:
[   20.508113]
[   20.508121] Chain exists of:
[   20.508121]   &bus->mutex --> subsys mutex#2 --> &policy->rwsem
[   20.508121]
[   20.508123]  Possible unsafe locking scenario:
[   20.508123]
[   20.508124]        CPU0                    CPU1
[   20.508125]        ----                    ----
[   20.508128]   lock(&policy->rwsem);
[   20.508132]                                lock(subsys mutex#2);
[   20.508135]                                lock(&policy->rwsem);
[   20.508138]   lock(&bus->mutex);
[   20.508139]
[   20.508139]  *** DEADLOCK ***
[   20.508139]
[   20.508141] 3 locks held by swapper/0/1:
[   20.508150]  #0:  (cpu_hotplug.lock){++++++}, at: [<c000000000087838>] .get_online_cpus+0x48/0xc0
[   20.508159]  #1:  (subsys mutex#2){+.+.+.}, at: [<c0000000004d7670>] .subsys_interface_register+0x50/0x110
[   20.508168]  #2:  (&policy->rwsem){+.+.+.}, at: [<c00000000068adcc>] .cpufreq_online+0x1ac/0x9d0
[   20.508169]
[   20.508169] stack backtrace:
[   20.508173] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.8.0-rc7-00037-gd2ffb010 #21
[   20.508175] Call Trace:
[   20.508180] [c0000000790c2b90] [c00000000082cc70] .dump_stack+0xe0/0x14c (unreliable)
[   20.508184] [c0000000790c2c20] [c000000000828c88] .print_circular_bug+0x350/0x388
[   20.508188] [c0000000790c2cd0] [c0000000000ecb0c] .__lock_acquire+0x196c/0x1d30
[   20.508192] [c0000000790c2e50] [c0000000000ed5b4] .lock_acquire+0x84/0x100
[   20.508196] [c0000000790c2f20] [c000000000820448] .mutex_lock_nested+0xa8/0x590
[   20.508201] [c0000000790c3030] [c000000000052830] .pmac_i2c_open+0x30/0x100
[   20.508206] [c0000000790c30c0] [c000000000052e14] .pmac_i2c_do_begin+0x34/0x120
[   20.508209] [c0000000790c3150] [c000000000056bc0] .pmf_call_one+0x50/0xd0
[   20.508213] [c0000000790c31e0] [c00000000068ff1c] .g5_pfunc_switch_volt+0x2c/0xc0
[   20.508217] [c0000000790c3250] [c00000000068fecc] .g5_pfunc_switch_freq+0x1cc/0x1f0
[   20.508221] [c0000000790c3320] [c00000000068fc2c] .g5_cpufreq_target+0x2c/0x40
[   20.508226] [c0000000790c3390] [c0000000006873ec] .__cpufreq_driver_target+0x23c/0x840
[   20.508230] [c0000000790c3440] [c00000000068c798] .cpufreq_gov_performance_limits+0x18/0x30
[   20.508235] [c0000000790c34b0] [c00000000068915c] .cpufreq_start_governor+0xac/0x100
[   20.508239] [c0000000790c3530] [c00000000068a788] .cpufreq_set_policy+0x208/0x260
[   20.508244] [c0000000790c35d0] [c00000000068abdc] .cpufreq_init_policy+0x6c/0xb0
[   20.508249] [c0000000790c3940] [c00000000068ae70] .cpufreq_online+0x250/0x9d0
[   20.508253] [c0000000790c3a30] [c0000000004d76d8] .subsys_interface_register+0xb8/0x110
[   20.508258] [c0000000790c3ad0] [c000000000689bb0] .cpufreq_register_driver+0x1d0/0x250
[   20.508262] [c0000000790c3b60] [c000000000b4f8f4] .g5_cpufreq_init+0x9cc/0xa28
[   20.508267] [c0000000790c3c20] [c00000000000a98c] .do_one_initcall+0x5c/0x1d0
[   20.508271] [c0000000790c3d00] [c000000000b0f86c] .kernel_init_freeable+0x1ac/0x28c
[   20.508276] [c0000000790c3db0] [c00000000000b3bc] .kernel_init+0x1c/0x140
[   20.508280] [c0000000790c3e30] [c0000000000098f4] .ret_from_kernel_thread+0x58/0x64
Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>