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Date:   Fri, 8 Feb 2019 09:12:48 +0100
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Nishanth Menon <nm@...com>, Stephen Boyd <sboyd@...nel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Dave Gerlach <d-gerlach@...com>,
        Wolfram Sang <wsa@...-dreams.de>
Subject: Re: [PATCH 0/2] cpufreq/opp: rework regulator initialization

Hi Viresh,

On 2019-02-08 07:49, Viresh Kumar wrote:
> On 07-02-19, 13:22, Marek Szyprowski wrote:
>> Recent commit 9ac6cb5fbb17 ("i2c: add suspended flag and accessors for
>> i2c adapters") added a visible warning for an attempt to do i2c transfer
>> over a suspended i2c bus. This revealed a long standing issue in the
>> cpufreq-dt driver, which gives a following warning during system
>> suspend/resume cycle:
>>
>> --->8---
>> Enabling non-boot CPUs ...
>> CPU1 is up
>> CPU2 is up
>> CPU3 is up
>> ------------[ cut here ]------------
>> WARNING: CPU: 4 PID: 29 at drivers/i2c/i2c-core-base.c:1869 __i2c_transfer+0x6f8/0xa50
>> Modules linked in:
>> CPU: 4 PID: 29 Comm: cpuhp/4 Tainted: G        W         5.0.0-rc4-next-20190131-00024-g54b06b29cc65 #5324
>> Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
>> [<c01110e8>] (unwind_backtrace) from [<c010d11c>] (show_stack+0x10/0x14)
>> [<c010d11c>] (show_stack) from [<c09a2584>] (dump_stack+0x90/0xc8)
>> [<c09a2584>] (dump_stack) from [<c0120bd0>] (__warn+0xf8/0x124)
>> [<c0120bd0>] (__warn) from [<c0120c3c>] (warn_slowpath_null+0x40/0x48)
>> [<c0120c3c>] (warn_slowpath_null) from [<c065cda0>] (__i2c_transfer+0x6f8/0xa50)
>> [<c065cda0>] (__i2c_transfer) from [<c065d168>] (i2c_transfer+0x70/0xe4)
>> [<c065d168>] (i2c_transfer) from [<c053ce4c>] (regmap_i2c_read+0x48/0x64)
>> [<c053ce4c>] (regmap_i2c_read) from [<c0536f1c>] (_regmap_raw_read+0xf8/0x450)
>> [<c0536f1c>] (_regmap_raw_read) from [<c053772c>] (_regmap_bus_read+0x38/0x68)
>> [<c053772c>] (_regmap_bus_read) from [<c05365a0>] (_regmap_read+0x60/0x250)
>> [<c05365a0>] (_regmap_read) from [<c05367cc>] (regmap_read+0x3c/0x5c)
>> [<c05367cc>] (regmap_read) from [<c047cfc0>] (regulator_is_enabled_regmap+0x20/0x90)
>> [<c047cfc0>] (regulator_is_enabled_regmap) from [<c0477660>] (_regulator_is_enabled+0x34/0x40)
>> [<c0477660>] (_regulator_is_enabled) from [<c0478674>] (create_regulator+0x1a4/0x25c)
>> [<c0478674>] (create_regulator) from [<c047c818>] (_regulator_get+0xe4/0x278)
>> [<c047c818>] (_regulator_get) from [<c068f1dc>] (dev_pm_opp_set_regulators+0xa0/0x1c0)
>> [<c068f1dc>] (dev_pm_opp_set_regulators) from [<c0698cc8>] (cpufreq_init+0x98/0x2d0)
>> [<c0698cc8>] (cpufreq_init) from [<c06959e4>] (cpufreq_online+0xc8/0x71c)
>> [<c06959e4>] (cpufreq_online) from [<c06960fc>] (cpuhp_cpufreq_online+0x8/0x10)
>> [<c06960fc>] (cpuhp_cpufreq_online) from [<c01213d4>] (cpuhp_invoke_callback+0xf4/0xebc)
>> [<c01213d4>] (cpuhp_invoke_callback) from [<c0122e4c>] (cpuhp_thread_fun+0x1d8/0x320)
>> [<c0122e4c>] (cpuhp_thread_fun) from [<c0149858>] (smpboot_thread_fn+0x194/0x340)
>> [<c0149858>] (smpboot_thread_fn) from [<c014573c>] (kthread+0x124/0x160)
>> [<c014573c>] (kthread) from [<c01010b4>] (ret_from_fork+0x14/0x20)
>> Exception stack(0xe897dfb0 to 0xe897dff8)
>> dfa0:                                     00000000 00000000 00000000 00000000
>> dfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>> dfe0: 00000000 00000000 00000000 00000000 00000013 00000000
>> irq event stamp: 3865
>> hardirqs last  enabled at (3873): [<c0186dec>] vprintk_emit+0x228/0x2a4
>> hardirqs last disabled at (3880): [<c0186cf0>] vprintk_emit+0x12c/0x2a4
>> softirqs last  enabled at (3052): [<c0102564>] __do_softirq+0x3a4/0x66c
>> softirqs last disabled at (3043): [<c0128464>] irq_exit+0x140/0x168
>> ---[ end trace db48b455d924fec2 ]---
>> CPU4 is up
>> CPU5 is up
>> CPU6 is up
>> CPU7 is up
>> --->8---
>>
>> This is a scenario that triggers the above issue:
>>
>> 1. system disables non-boot cpu's at the end of system suspend procedure,
>> 2. this in turn deinitializes cpufreq drivers for the disabled cpus,
>> 3. early in the system resume procedure all cpus are got back to online
>>    state,
>> 4. this in turn causes cpufreq to be initialized for the newly onlined
>>    cpus,
>> 5. cpufreq-dt acquires all its resources (clocks, regulators) during
>>    ->init() callback,
>> 6. getting regulator require to check its state, what in turn requires
>>    i2c transfer,
>> 7. during system early resume stage this is not really possible.
>>
>> The issue is caused by cpufreq-dt driver not keeping its resources for
>> the whole driver lifetime and relying that they can be always acquired
>> at any system context. This problem has been observed on Samsung
>> Exynos based Odroid XU3/XU4 boards, but it happens on all boards, which
>> have separate regulators for different CPU clusters.
> Why don't you get similar problem during suspend? I think you can get
> it when the CPUs are offlined as I2C would have gone by then. The
> cpufreq or OPP core can try and run some regulator or genpd or clk
> calls to disable resources, etc. Even if doesn't happen, it certainly
> can.

CPUfreq is suspended very early during system suspend and thus it does
nothing when CPUs are being offlined.

> Also at resume the cpufreq core may try to change the frequency right
> from ->init() on certain cases, though not everytime and so the
> problem can come despite of this series.

cpufreq is still in suspended state (it is being 'resume' very late in
the system resume procedure), so if driver doesn't explicitly change any
opp in ->init(), then cpufreq core waits until everything is resumed. To
sum up, this seems to be fine, beside the issue with regulator
initialization I've addressed in this patchset.

> We guarantee that the resources are available during probe but not
> during resume, that's where the problem is.

Yes, so I've changed cpufreq-dt to the common approach, in which the
driver keeps all needed resources for the whole lifetime of the device.

> @Rafael any suggestions on how to fix this ?

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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