linux-kernel - Re: [PATCH] pcf50633: use a dedicated workqueue for irq processing

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Message-ID: <20090731092313.GA21633@home.pavel.comp>
Date:	Fri, 31 Jul 2009 13:23:13 +0400
From:	Paul Fertser <fercerpav@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	sameo@...ux.intel.com, linux-kernel@...r.kernel.org,
	Anton Vorontsov <cbou@...l.ru>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] pcf50633: use a dedicated workqueue for irq processing

On Thu, Jul 30, 2009 at 03:34:25PM -0700, Andrew Morton wrote:
> On Tue, 28 Jul 2009 00:41:15 +0400
> Paul Fertser <fercerpav@...il.com> wrote:
> 
> > Using the default kernel "events" workqueue causes problems with
> > synchronous adc readings if initiated from some task on the same
> > workqueue.
> > 
> > I had a deadlock trying to use pcf50633_adc_sync_read from a
> > power_supply class driver because the reading was initiated from the
> > workqueue and it waited for the irq processing to complete (to get the
> > result) and that was put on the same workqueue.
> 
> I don't get it.
> 
> Do you meant that pcf50633_adc_sync_read() was called via a
> schedule_work() handler?  If so, wasn't that a bug?

Andrew, i think it is called this way:

power_supply_changed -> schedule_work(&psy->changed_work) ->
power_supply_changed_work -> kobject_uevent(&psy->dev->kobj, KOBJ_CHANGE)
-> power_supply_uevent -> power_supply_show_property -> get_property ->
bat->pdata->get_voltage -> pcf50633_adc_sync_read

I assumed that since power_supply class is well-established and widely used
that's an intended behaviour. CCing relevant maintainers to get their
opinions.

A bit of background: we sometimes want to use dumb batteries with our
openmoko devices (for gta01 aka Neo1973 there's no alternative even), so
power_class driver needs to be able to obtain battery voltage measured by
PMU.

-- 
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