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Date:   Fri, 8 Feb 2019 11:22:02 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Samsung SoC <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

On Fri, Feb 8, 2019 at 7:50 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 07-02-19, 13:22, Marek Szyprowski wrote:
> > Dear All,
> >
> > 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.
>
> 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.
>
> We guarantee that the resources are available during probe but not
> during resume, that's where the problem is.
>
> @Rafael any suggestions on how to fix this ?

There are cpufreq driver suspend and resume callbacks, maybe use them?

The driver could do the I2C transactions in its suspend/resume
callbacks and do nothing in online/offline if those are part of
system-wide suspend/resume.

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