Subject: Re: [RFC PATCH 2/2] igb/ixgbe: add code to trigger function reset if reset_devices is set

From: David Woodhouse <[email-masked]>
Date: Thu, 05 Aug 2010 17:27:43 +0100

On Sun, 2010-08-01 at 01:15 -0700, David Miller wrote:
> From: Jeff Kirsher <[email-masked]>
> Date: Fri, 30 Jul 2010 17:59:12 -0700
> 
> > From: Alexander Duyck <[email-masked]>
> > 
> > This change makes it so that both igb and ixgbe can trigger a full pcie
> > function reset if the reset_devices kernel parameter is defined.  The main
> > reason for adding this is that kdump can cause serious issues when the
> > kdump kernel resets the IOMMU while DMA transactions are still occurring.
> > 
> > Signed-off-by: Alexander Duyck <[email-masked]>
> > Signed-off-by: Jeff Kirsher <[email-masked]>
> 
> I tend to disagree with the essence of this change.
> 
> Which is that we should add workaround after workaround for things
> that aren't functioning properly in kdump and kexec.
> 
> They should have a pass that shuts devices down properly, so that this
> kind of stuff doesn't need to happen in the kernel we then boot into.

For a normal kexec, arguably true.

But in the kdump case, the original kernel has *crashed* and we really
don't have that option -- we need to jump *straight* to the new kernel
and have it reset the hardware.

The device driver really *ought* to be able to reset the hardware from
whatever state it's in when the new kernel starts up. Anything less is
broken, and reminds me of those crappy drivers that only work after a
soft-reboot from Windows.

Most drivers *do* quite happily initialise their device and reliably get
it into a known state; it's just that this particular hardware goes into
a *particularly* stroppy fit when it gets a DMA master abort (which is
what happens when the IOMMU stops it from scribbling into memory after
the new kernel has taken over).

> What happens on non-PCIE systems then?  Do they just lose when this
> happens?

If they have a device that's this broken, and the driver can't get it
into a working state any other way, then yes -- I don't see any way to
*avoid* them losing.

I don't like the reset_devices thing though -- the device driver ought
to cope (and reset the device with a full PCIe reset if that's the only
way to make it stop sulking) *regardless* of that option, if it's
necessary.

-- 
David Woodhouse                            Open Source Technology Centre
[email-masked]                              Intel Corporation