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

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Date:	Wed, 25 May 2016 15:44:15 -0400
From:	Rhyland Klein <rklein@...dia.com>
To:	Jon Hunter <jonathanh@...dia.com>,
	Thierry Reding <treding@...dia.com>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Sebastian Reichel <sre@...nel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"Dmitry Eremin-Solenikov" <dbaryshkov@...il.com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Alexandre Courbot <gnurou@...il.com>,
	<linux-kernel@...r.kernel.org>, <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] arm64: defconfig: Enable cros-ec and battery driver

On 5/25/2016 1:26 PM, Jon Hunter wrote:
> 
> On 25/05/16 17:36, Rhyland Klein wrote:
> 
> ...
> 
>> I can see that getting the temperature could work. I would point out
>> that I don't see any recent changes to bq27xxx or the power_supply_core
>> that would imply this is a regression. My guess is that up until now,
>> for devices that support the TEMP property, CONFIG_THERMAL isn't been
>> enabled.
>>
>> So here are my thoughts.... we can do 2 things here:
>>
>> 1) patch bq27xxx in some manner that will allow the bq27xxx driver to
>> work report the temp during register (such as above patch).
>> 2) Patch the core to avoid using get_property callback during registration.
> 
> I wonder if #2 will cause other problems for other devices as this
> really changes the functionality.
> 
>> I think for our immediate concern and crash, #1 is fine. It will work
>> and is fine. I however think this is just a symptom of the larger issue
>> (#2). In this case, the problem we see is that di->bat is used before it
>> is set, and we have a way around it. However, for EVERY device that
>> registers and has TEMP prop (and CONFIG_THERMAL enabled) it is going to
>> receive a call with its relative di->bat uninitialized too.
> 
> I don't think we are understanding each other and I still think that
> this could be specific to the bq27xxx. And here is why ...
> 
> The power supply driver is going to receive a call to it's
> ->get_property() function with a *VALID* pointer to the power_supply
> structure, *psy. When initialised, di->bat == psy (assuming bq27xxx
> naming) and so in other words, they point to the same thing. Therefore,
> in the normal case, you should not need to access di->bat from within
> ->get_property() because you already have a valid pointer to the
> power_supply structure, *psy.
> 
> So the ONLY problem would be IF the ->get_property function calls
> power_supply_get_drvdata() to get a pointer to the drvdata, *di, and
> then dereferences and uses the pointer, di->bat, which may NOT be
> initialised yet. I am wondering how likely this is as it seems a bit
> daft to do this, unless you are doing something like the bq27xxx driver
> is attempting to do.
> 
> Again IMO the problem is related to how the bq27xxx driver has
> implemented the status update.
> 

And you might be completely correct, that is something that can only
happen specifically with the bq27xxx driver. In which case, making the
fix there should be the fix. I just know from the commit log (and some
previous work with power supply drivers) that the case of get_property
being called during registration has caused problems before. That's why
I am trying to make sure we cover the generic case if it exists. Using
scheduled work is common for power_supplies to regularly update their
status.

As for your proposed patches for bq27xxx, I think the latest one you
suggested (@12:36PM EST) with the change for
battery_update->battery_poll as well makes the most sense for bq27xxx. I
would like to point out though that if we patch this, the cache won't be
populated for the first TEMP request, which has the same end effect as
the patch I proposed to power_supply_read_temp. I believe both will
return 0 for the temp.

I think that patch would work just fine in place of what I suggested for
this specific crash.

-rhyland


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