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SubjectRe: [BUG] PDC20268 crashing during DMA setup on stock Debian 2.6.12-1-powerpc
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On Oct 19, 2005, at 18:13:44, Benjamin Herrenschmidt wrote:
> On Wed, 2005-10-19 at 13:48 -0400, Kyle Moffett wrote:
>
>> Do you have any other ideas WRT this bug? I've been browsing
>> around in the code a bit, and I plan to try diffing my 2.6.8.1
>> version of the files against the latest Debian to see what
>> changed, although I suspect it will be a relatively fat hunk of
>> changes. Thanks for your
>> help!
>>
>
> Nope. The lspci output looks perfectly normal. I looks like a
> mixture of issues with BM DMA being disabled for a reason I haven't
> figured out and then the code crashing because it doesn't like BM-
> DMA being disabled ...

BusMaster-DMA definitely should be enabled on that card. After a lot
of looking through icky IDE code, I've determined that the reason for
the crash is that if there is a "mate", or another IDE bus on the
same card, then hwif->dma_master is set to hwif->mate->dma_base on
the secondary channel. Since DMA explicitly wasn't enabled on the
primary channel, hwif->dma_master on the secondary is 0 even though
dma is enabled, and therefore we hit that BUG().

Therefore it seems that the only issue is ide_get_or_set_dma_base is
returning 0 when it should return a valid DMA base. In that
function, the only ways that function can even theoretically return 0
without printing any weird error messages is "if (hwif->mmio)" or "if
(hwif->mate && hwif->mate->dma_base)". The latter can't happen
before hwif->mate is set up (since the problem is while initializing
the primary). Could hwif->mmio be nonzero somehow? The only drivers
that seem to set it are pci/sgiioc4, pci/siimage, ppc/pmac, and a
couple misc arch drivers.

I see a couple theoretical possibilities:
* hwif->mate and hwif->mate->dma_base are set for the primary
while still initializing it (before any secondary is set up.
* hwif->mmio is set somehow even though it shouldn't be, is the
value ever pre-initialized to 0?

> Again, best is you pour printk's all over setup-pci.c and ide-dma.c
> to figure out what's going on...

I wish I could but the machine is remote and in-production, so it's
hard to have time to do much with it. I'm trying as best I can to
walk through the sources by hand, specifically with regards to
changes between the two. I'm hoping I can come up with a good enough
guess by Thanksgiving, so that when I have a week near the server to
test things out I can make significant progress.

Cheers,
Kyle Moffett

--
I have yet to see any problem, however complicated, which, when you
looked at it in the right way, did not become still more complicated.
-- Poul Anderson



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