Date:	Wed, 31 Jul 2013 12:28:53 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Tejun Heo <htejun@...il.com>
Cc:	Mark Brown <broonie@...nel.org>,
	Peter Chen <peter.chen@...escale.com>,
	Fabio Estevam <festevam@...il.com>,
	alexander.shishkin@...ux.intel.com, kernel@...gutronix.de,
	linux-usb@...r.kernel.org,
	Fabio Estevam <fabio.estevam@...escale.com>,
	Jeff Garzik <jeff@...zik.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	Mike Turquette <mturquette@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] chipidea: Use devm_request_irq()

On Wed, Jul 31, 2013 at 05:54:11AM -0400, Tejun Heo wrote:
> Hello,
> 
> On Wed, Jul 31, 2013 at 11:44:34AM +0200, Uwe Kleine-König wrote:
> > > > OK, so the possible problem is that remove is called while the irq is
> > > > still active. That means you have to assert that all resources the irq
> 
> If your driver destruction path is running while your irq handler is
> still running, it's a crappy / broken driver.  You need a deactivation
Well, you cannot avoid assuming that the irq is still active when your
driver's remove callback is called. But I agree about crappyness at the
end of the destruction path. The problem is that crap is as easy as:

	probe(..)
	{
		clk = devm_get_clk(...);
		clk_prepare_enable(clk);
		writel(1, base + IRQENABLE);
		devm_request_irq(...);
	}

	remove(..)
	{
		writel(0, base + IRQENABLE);
		clk_disable_unprepare(clk);
	}

and I think there are more and more drivers doing that.

> step whether you're using devm or not.  IRQs can be shared and the
> device should be in a quiesced state before the driver detaches
> itself.  Note that you can queue deactivation routine using devm.  For
> an example, please take a look at
> drivers/ata/libata-core.c::ata_host_start().
> 
> > > > handler is using (e.g. ioremap, clk_prepare_enable) are only freed
> > > > *after* the irq is done. For ioremap that means it must be done using
> > > > devm_ioremap_resource. For a clock it's not that easy because the irq
> > > > handler has to assert that a used clk is kept prepared which can only be
> > > > done using clk_prepare which in turn is not allowed in an irq handler.
> > > 
> > > > Hmm. So the only possible fixes are
> > > > 	- devm* can be told to also care about clk_disable_unprepare
> > > > 	- after disabling irqs in the remove callback wait for all
> > > > 	  active irqs to be done. (i.e. call synchronize_irq(irq))
> > > > 	- don't use devm_request_irq
> 
> Again, the right thing to do is having a proper deactivation step.
> This is nothing devm can do automatically.  There's no way for it to
> find out that the device is actually quiesced.  Let's say it waits for
> the current instance of irq handler to finish.  How would it know that
> it won't start again between the flushing of the current instance and
> irq deregistration.  Add an explicit deactivation step using
> devres_alloc().
> 
> > > I'm not sure that devm_ guarantees any ordering in the cleanups it does
> > > so I'd not like to rely on the first option either, if there were some
> > > guarantee of that it'd help.  The nice thing about explicitly freeing
> > > the IRQ is that you can tell that all this stuff is safe by inspection.
> > devm_* at least uses list_for_each_entry_reverse
> > (drivers/base/devres.c:release_nodes()). Without this guarantee devm_
> > would not make much sense IMHO.
> 
> devm guarantees that the destruction callbacks are called in the
> reverse order of registration.
That's fine.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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