linux-kernel - Re: [RFC] [Patch] calgary iommu: Use the first kernel's tce tables in kdump

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Date:	Wed, 12 Mar 2008 14:08:06 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Muli Ben-Yehuda <muli@...ibm.com>
Cc:	Chandru <chandru@...ibm.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, jdmason@...zu.us
Subject: Re: [RFC] [Patch] calgary iommu: Use the first kernel's tce tables
	in kdump

On Wed, Mar 12, 2008 at 11:58:20AM +0200, Muli Ben-Yehuda wrote:
> On Wed, Mar 12, 2008 at 10:38:49AM +0530, Chandru wrote:
> 
> > The system booted! , :) with the following change. Muli acceptable??
> >
> > static void calgary_watchdog(unsigned long data)
> > {
> > ...
> > ...
> > /* Disable bus that caused the error and ignore if it's kdump kernel */
> > + if ( !is_kdump_kernel()) {
> > target = calgary_reg(bbar, phb_offset(tbl->it_busno) |
> > PHB_CONFIG_RW_OFFSET);
> > val32 = be32_to_cpu(readl(target));
> > val32 |= PHB_SLOT_DISABLE;
> > writel(cpu_to_be32(val32), target);
> > + }
> > readl(target); /* flush */
> 
> Yikes, not really :-)
> 
> You're basically saying "if we're in a kdump kernel, let's ignore all
> DMA errors and hope for the best". This is not really acceptable. What
> we could do is limit the scope of ignorance - only ignore errors from
> the point the kdump kernel starts booting until we have reinitialized
> the device and then go back to handling DMA errors correctly.
> 

Agree. Ignoring all the DMA errors in kdump kernel does not sound very
good.

On a side note, typically how much time does it take to for DMA operations
to finish. Can we wait for a random amount of time in second kernel,
hoping all the DMA operations are complete, and then setup a new table?

This can be atleast a stop gap solution till we come up with a good
solution?

Thanks
Vivek
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