linux-kernel - Re: [PATCH] ACPI: full battery charge cannot exceed current one

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Message-ID: <52A6434B.9040100@nic.cz>
Date:	Mon, 09 Dec 2013 23:25:15 +0100
From:	Jan Včelák <jan.vcelak@....cz>
To:	Matthew Garrett <mjg59@...f.ucam.org>
CC:	lenb@...nel.org, rjw@...ysocki.net, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI: full battery charge cannot exceed current one

>> Some degraded batteries report maximal capacity to be smaller than
>> current charge of the battery. This can confuse some user space
>> applications (like upower).
> 
> Can't we just fix upower?

It looks like upower already resolved that:

http://cgit.freedesktop.org/upower/tree/src/linux/up-device-supply.c#n652

If the purpose of sysfs is to provide raw values retrieved from battery,
then I agree that this should not be handled in kernel.
Otherwise I would expect semantically correct data there.

>> +	/* Some degraded batteries report lower full charge than current one. */
>> +	if (battery->capacity_now > battery->full_charge_capacity)
>> +		battery->full_charge_capacity = battery->capacity_now;
>> +
> 
> Does upower read *_full on every read of the value? How do we guarantee 
> that it won't read energy_now without updating energy_full and still 
> come up with an incorrect calculation?

That's a good point. I'm not sure about other applications, but upower
always refreshes all values. Function up_device_supply_refresh_battery
seems to be the only place where 'charge_now' is being read:

http://cgit.freedesktop.org/upower/tree/src/linux/up-device-supply.c#n485
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