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

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Date:	Wed, 25 May 2016 18:26:09 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	Rhyland Klein <rklein@...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 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.

Cheers
Jon

-- 
nvpublic