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 10:38:49 +0530
From:	Chandru <chandru@...ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>, Muli Ben-Yehuda <muli@...ibm.com>
CC:	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


> Hi Chandru,
>
> - How do we make sure that previous kernel's TCE tables are not
>   overwritten
>   by new kernel (In case previous kernel allocated TCE tables in first
>   640 KB?)
>   
TCE tables are allocated using alloc_bootmem_low() with goal set to 0. 
Don't know if this will be sufficient. Investigating...
> - How do we make sure that when new kernel tries to setup an entry in
>   TCE table, then it does not try to clear up an existing entry which is
>   still in use?
>   
A bitmap is created once the first kernel's TCE table is found. This 
bitmap is populated by reading the entries in the tce table. Non-zero 
entries in the table are marked as used/reserved in the bitmap.
> Did you try the Muli suggestion of ignoring DMA error in exception
> handler?
With what I tried, I was not successful
>  What happens if I setup new table and try to switch to new
> table?
This is the root cause of the problem.
>  Some sort of error will occur.
The pci bus on that PHB goes into an undefined state ( returning 
0xffffffff for all reads on that bus ).
>  Can't we modify the handler and
> ignore it for kdump case and move on?
>   
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 */
..
..
}
> Thanks
> Vivek
>   
-Chandru
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