Date:	Thu, 11 Oct 2007 18:23:50 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Maxim Levitsky <maximlevitsky@...il.com>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [linux-pm] Re: [QUESTION] I need advice for writing .suspend/.resume
 functions

On Fri, 12 Oct 2007, Rafael J. Wysocki wrote:

> On Thursday, 11 October 2007 05:13, Maxim Levitsky wrote:
> > Hi,
> > 
> > I have few questions about .suspend()/.resume() driver functions and how best to write them.
> > 
> > I have written a support for suspend/resume for saa7134 v4l driver.
> > Now looking at code again and again, I found few problems, and I am seeking your advice how to fix them.
> > 
> > First of all the .suspend() function:
> > 
> > Looking at various drivers (including v4l ones) it seems that in general the function:
> > 
> > 1) tells upper layers that it is suspended
> > 2) saves the state of device
> > (generally there is nothing to save, since the driver maintains a copy of device state in memory)
> > 
> > 3) disables the device (including DMA)
> > 4) does usual pci_save_state+pci_set_power_state(pci_dev, pci_choose_state(pci_dev, state))
> > (I am talking about pci devices of course)
> > 
> > But there is one problem that my .suspend() function have together with quite a lot of drivers:
> > It can race with IRQ handler. Suppose the handler is called just before .suspend(), and thus .suspend() literally 
> > pulls the hadware from that handler.
> >
> > I was told that I should use synchronize_irq(), and it looks exactly like the solution.
> 
> Yes, that seems to be the right solution.
> 
> > But I was surprised to see that very few drivers use it in their .suspend() routines.
> 
> Frankly, I'm not sure why that is so.

USB calls it.

> > Another issue, even bigger happens during the resume:
> > Suppose the IRQ line is shared with some other device and it gets resumed first.
> > And my IRQ handler is called because of the other device.
> > Now my IRQ handler can't determine whenever the IRQ for the device, since the hardware is still powered off.
> > 
> > Probably I can fix the following issues doing this:
> > 
> > 1) do free_irq() in .suspend(), and request_irq() in .resume
> > this solves both problems since free_irq()  calls synchronize_irq()
> > Few drivers do that this way. I would like to know if this is the right way.
> 
> AFAICS, it is not and these drivers should probably be modified not to do so.
> 
> The problem, as I see it, is that we don't know what state the device will be
> in during the resume (this may be a resume from disk and the device may have
> been initialized by the BIOS and we get it actually generating interrupts).
>  
> > 2) Disable the card's IRQ register , then call synchronize_irq(), at that
> > point I can be sure that no IRQ handler is running and will run then set some
> > per device flag say dev->insuspend 
> > 
> > let IRQ handler check this flag, and bail out if set, so false interrupts are caught on resume
> > Probably not a good idea, since I didn't find such implementation in kernel
> 
> I'm not sure of that.  Sounds better than freeing the IRQs in .suspend() to me.

USB uses 2).  Actually we set the "insuspend" flag after disabling IRQ
generation but before calling synchronize_irq() -- presumably this
doesn't lead to any problems.

The problem of devices initialized by the BIOS and generating unwanted 
interrupts is handled by a PCI quirk routine.  The device's IRQ-enable 
flag is turned off early on in the boot kernel.

Alan Stern

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