Re: [PATCH 0/2] net: Fix crashes due to activity during suspend

[Florian Fainelli <f.fainelli@gmail.com>]

On 08/23/2017 04:45 AM, Geert Uytterhoeven wrote:
> Hi Florian,
> 
> On Tue, Aug 22, 2017 at 8:49 PM, Florian Fainelli <f.fainelli@gmail.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...

Thanks!
-- 
Florian