Re: [PATCH] arm64: defconfig: Enable cros-ec and battery driver

[Krzysztof Kozlowski <k.kozlowski-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>]

On 05/27/2016 12:28 PM, Jon Hunter wrote:
> Hi Krzysztof,
> 
> On 27/05/16 09:37, Krzysztof Kozlowski wrote:
> 
> ...
> 
>> Indeed I was struggling with similar issue in bq27x00_battery. The issue
>> was introduced by... me :(  when moving the ownership of power supply
>> structure from driver to the core. However IMHO my change exposed the
>> fundamental problem with power supply.
>>
>> Anyway a fix for this issue was:
>> 7f1a57fdd6cb6e7b (power_supply: Fix possible NULL pointer dereference on
>> early uevent)
>> AFAIU, this fix no longer fixes all the issues, right?
>>
>> As for the fundamental problem, the power supply core should not call
>> back the driver (get_property()) until the probe ends. Even if the
>> di->bat was initialized, some other fields of driver could not be set
>> yet. In general, the probe did not end so we should avoid calling driver
>> internal functions.
> 
> For my understanding, can you elaborate why the power-supply core should
> not call back to the drivers ->get_property() before the probe ends? I
> assume that registering the power-supply should be the last thing done
> in the probe and so the power-supply should be configured at that point.

It is not only about power supply but other resources allocated by the
driver. If the power_supply_register() is a last call, then no problem.
But if not, then these resources won't be available.

Actually I exaggerated a little bit as a fundamental problem as this is
quite common pattern. When driver provides something (like power supply)
then after registration it should be ready for calls coming from the
core or user space. It does not have to be power supply. It might be
exposing sysfs entries or file operations (exposed before calling
power_supply_register()).

> The problems with the bq27xxx seem to stem from the periodic update of
> the bq27xxx status and so it is not clear to me that this is a generic
> problem for all power-supply devices.

Initially, the generic problem was that the core would call back the
driver from power_supply_register() in a synchronous way through
power_supply_changed(). The commit 7f1a57fdd6c changed it to an
asynchronous call. Here it looks like the same problem - the
power_supply_register() calls thermal which calls
thermal_zone_device_update() and we are back at the driver... before
finishing power_supply_register() call.

Best regards,
Krzysztof