Date:   Wed, 13 Sep 2017 19:33:07 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>, marc_gonzalez@...madesigns.com,
        Mason <slash.tmp@...e.fr>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "David S . Miller" <davem@...emloft.net>,
        Steve Glendinning <steve.glendinning@...well.net>,
        Lukas Wunner <lukas@...ner.de>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] net: Fix crashes due to activity during suspend

Hi Florian,

On Thu, Sep 7, 2017 at 3:09 PM, Florian Fainelli <f.fainelli@...il.com> wrote:
> On 08/23/2017 10:13 AM, Florian Fainelli wrote:
>> On 08/23/2017 04:45 AM, Geert Uytterhoeven wrote:
>>> On Tue, Aug 22, 2017 at 8:49 PM, Florian Fainelli <f.fainelli@...il.com> wrote:
>>>> On 08/22/2017 11:37 AM, Geert Uytterhoeven wrote:
>>>>> If an Ethernet device is used while the device is suspended, the system may
>>>>> crash.
>>>>>
>>>>> E.g. on sh73a0/kzm9g and r8a73a4/ape6evm, the external Ethernet chip is
>>>>> driven by a PM controlled clock.  If the Ethernet registers are accessed
>>>>> while the clock is not running, the system will crash with an imprecise
>>>>> external abort.
>>>>>
>>>>> This patch series fixes two of such crashes:
>>>>>   1. The first patch prevents the PHY polling state machine from accessing
>>>>>      PHY registers while a device is suspended,
>>>>>   2. The second patch prevents the net core from trying to transmit packets
>>>>>      when an smsc911x device is suspended.
>>>>>
>>>>> Both crashes can be reproduced on sh73a0/kzm9g and r8a73a4/ape6evm during
>>>>> s2ram (rarely), or by using pm_test (more likely to trigger):
>>>>>
>>>>>     # echo 0 > /sys/module/printk/parameters/console_suspend
>>>>>     # echo platform > /sys/power/pm_test
>>>>>     # echo mem > /sys/power/state
>>>>>
>>>>> With this series applied, my test systems survive a loop of 100 test
>>>>> suspends.
>>>>
>>>> It seems to me like part, if not the entire problem is that smsc91xx's
>>>> suspend and resume functions are way too simplistic and absolutely do
>>>> not manage the PHY during suspend/resume, the PHY state machine is not
>>>> even stopped, so of course, this will cause bus errors if you access
>>>> those registers.
>>>>
>>>> You are addressing this as part of patch 2, but this seems to me like
>>>> this is still a bit incomplete and you'd need at least phy_stop() and/or
>>>> phy_suspend() (does a power down of the PHY) and phy_start() and/or
>>>> phy_resume() calls to complete the PHY state machine shutdown during
>>>> suspend.
>>>>
>>>> Have you tried that?
>>>
>>> Unfortunately that doesn't help.
>>> In state PHY_HALTED, the PHY state machine still calls the .adjust_link()
>>> callback while the device is suspended.
>>
>> Humm that is correct yes.
>>
>>> Do you have a clue? This is too far beyond my phy-foo...
>>
>> I was initially contemplating a revert of
>> 7ad813f208533cebfcc32d3d7474dc1677d1b09a ("net: phy: Correctly process
>> PHY_HALTED in phy_stop_machine()") but this is not the root of the
>> problem. The problem really is that phy_stop() does not wait for the PHY
>> state machine to be stopped so you cannot rely on that and past the
>> function return be offered any guarantees that adjust_link is not called.
>>
>> We seem to be getting away with that in most drivers because when we see
>> phydev->link = 0, we either do nothing or actually turn of the HW block.
>>
>> How about we export phy_stop_machine() to drivers which would provide a
>> synchronization point that would ensure that no HW accesses are done
>> past this point?
>>
>> I am absolutely not clear on the implications of using a freezable
>> workqueue with respect to the PHY state machine and how devices are
>> going to wind-up being powered down or not...
>
> Geert, as you may have notice a revert of the change was sent so 4.13
> should be fine, but ultimately I would like to put the non-reverted code
> back in after we add a few safeguards:

With the revert, I no longer need "[PATCH 1/2] net: phy: Freeze PHY polling
before suspending devices".
I just did more than 50 successful suspend/resume cycles to verify that.

I still need "[PATCH 2/2] net: smsc911x: Quiten netif during suspend", so
I'll submit a v2 for that.

> - and you reported the bus errors on smsc911x when we call adjust_link
> during suspend, and due to a lack of hard synchronization so phy_stop()
> here does not give you enough guarantees to let you turn off power to
> the smsc911x block
>
> If that seems accurate then we can work on something that should be
> working again (famous last words).

Sounds accurate to me.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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