Re: [RFC/PATCH] Kdump: Disabling PCI interrupts in capture kernel

[Grant Grundler <grundler@parisc-linux.org>]

On Tue, Jun 07, 2005 at 03:59:18AM -0600, Eric W. Biederman wrote:
> > *lots* of PCI devices predate PCI2.3. Possibly even the majority.
> 
> In general generic hardware bits for disabling DMA, disabling interrupts
> and the like are all advisory.  With the current architecture things
> will work properly even if you don't manage to disable DMA (assuming
> you don't reassign IOMMU entries at least).

ISTR, pSeries (IBM), some alpha, some sparc64, and parisc (64-bit) require
use of the IOMMU for *any* DMA. ie IOMMU entries need to be programmed.
Probably want to make a choice to ignore those arches for now
or sort out how to deal with an IOMMU.

> Shared interrupts are an interesting case.  The simplest solution I can
> think of for a crash dump capture kernel is to periodically poll
> the hardware, as if all interrupts are shared.  At that level
> I think we could get away with ignoring all hardware interrupt sources.

Yes, that's perfectly ok. We are no longer in a multitasking env.

> Does anyone know of a anything that would break by always polling
> the hardware?  I guess there could be a problem with drivers
> that don't understand shared interrupts, are there enough of those
> to be an issue.

PCI requires drivers support Shared IRQs.
A few oddballs might be broken but I expect networking/mass storage
drivers get this right.

grant