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

[Paul Fertser <fercerpav@gmail.com>]

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@gmail.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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