linux-kernel - Re: [PATCH] Disable Bus Master on PCI device shutdown

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Date:	Thu, 07 Jun 2012 08:21:00 -0600
From:	Khalid Aziz <khalid.aziz@...com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	linux-kernel@...r.kernel.org, bhelgaas@...gle.com,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH] Disable Bus Master on PCI device shutdown

On Wed, 2012-06-06 at 12:42 -0700, Eric W. Biederman wrote:
> Absent anyone even knowing if there are devices that exist that can not
> tolerate their bus master bit being flipped when DMA is not ongoing I
> think the current state of the code is good.  When we find the broken
> hardware that can not tolerate a standard PCI bit being used in a
> standard way we can add a flag in the core to avoid doing that.
> 
> pci_device_shutdown calls drv->shutdown before calling
> pci_device_disable.  Which means that only devices that have trouble
> with this bit being flipped while DMA is ongoing and don't bother
> to stop their own DMA will have a problem.
> 
> As for shifting problems I do think we have shifted the problem in a
> very positive way.  Now instead of having a random failure at a random
> location caused by DMA happing at a random moment for no expected reason
> we have failures happening when we disable or enable a device, which
> should be much more debugable.

That is a very good point. Failure at a predictable point is much better
than random failures with the root cause being elsewhere from the point
of failure in time and code. This at least gives us a good shot at being
able to debug buggy hardware and drivers.

-- 
====================================================================
Khalid Aziz                                         Unix Systems Lab
(970)898-9214                                        Hewlett-Packard
khalid.aziz@...com                                  Fort Collins, CO

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