linux-kernel - Re: [PATCH] synchronize

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Message-Id: <200710200510.01409.maximlevitsky@gmail.com>
Date:	Sat, 20 Oct 2007 05:10:01 +0200
From:	Maxim Levitsky <maximlevitsky@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	benh@...nel.crashing.org, akpm <akpm@...ux-foundation.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linuxppc-dev list <linuxppc-dev@...abs.org>
Subject: Re: [PATCH] synchronize_irq needs a barrier

On Saturday 20 October 2007 04:25:34 Linus Torvalds wrote:
> 
> On Sat, 20 Oct 2007, Maxim Levitsky wrote:
> > 
> > and the interrupt handler:
> > 
> > 	smp_rmb();
> > 	if (dev->insuspend)
> > 		goto out;
> 
> Something like that can work, yes. However, you need to make sure that:
> 
>  - even when you ignore the interrupt (because the driver doesn't care, 
>    it's suspending), you need to make sure the hardware gets shut up by 
>    reading (or writing) the proper interrupt status register.
I agree, but while device is powered off, its registers can't be accessed
Thus, if I ack the IRQ every time the handler is called, I will access the 
powered off device (this is probably won't hurt a lot, but a bit incorrectly)

> 
>    Otherwise, with a level interrupt, the interrupt will continue to be 
>    held active ("screaming") and the CPU will get interrupted over and 
>    over again, until the irq subsystem will just turn the irq off 
>    entirely.
> 
>  - when you resume, make sure that you get the engine going again, with 
>    the understanding that some interrupts may have gotten ignored.
Here it isn't a problem, this is a video capture card, and I suspend it by just stopping dma
on all active buffers even if in the middle of capture, and I send those buffers to card again
to fill them from the beginning during the resume.
> 
> Also, in general, these kinds of things shouldn't always even be 
> neicessary. If you use the suspend_late()/resume_early() hooks, those will 
> be called with interrupts off, and while they can be harder to debug (and 
> may be worth avoiding for non-critical drivers), they do allow for simpler 
> models partly exactly because they don't need to worry about interrupts 
> etc.
Exactly, I am aware of suspend_late , but I don't want to abuse it.
> 
> 		Linus
> 


Best regards,
	Maxim Levitsky

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