Re: [microblaze-uclinux] System hangs with I-cache enabled

[John Williams <jwilliams@xxxxxxxxxxxxxx>]

Hi Alex,

Alex Krasnov wrote:

> I have built the uclinux kernel from the latest source and am running it
> on the XUP board using the reference design with caches enabled.
> Whenever I enable the I-cache in the kernel, the system hangs after a
> short period (at most a few minutes). In particular, an almost certain
> way to hang it is to do an NFS transfer, though the problem occurs even
> with NFS disabled. XMD reports that the pipeline is stalled at a random
> PC. The problem occurs consistently regardless of other hardware or
> kernel features.

Are you using the mch_opb_ddr memory controller, by any chance?

And what version  / service pack of EDK?

There was a bug in the mch_opb_ddr core, in EDK7.1, and I am not sure if
it has been completely resolved.  It caused pipeline stalls like that
you describe.

Also, there was a bug in the MicroBlaze v4.00.a core, around the same
time, whereby partial cache invalidate operations would cause problems
when the MCH (CacheLink) channels were used.

You can see the legacy of this bug in
linux-2.4.x/include/asm-microblaze/pgalloc.h, around line 116 - if the
FSL caches are enabled, all cache flush operations do the entire cache,
not just the necessary range.

> Before I go on a lengthy debugging journey, I would like to know whether
> this is a known problem or whether there is a simple explanation. The
> only possible explanation I see is that the I-cache is inconsistent with
> main memory; however, this seems unlikely since the kernel flushes the
> I-cache when necessary. Also, I added full flushes in several places,
> including in the scheduler. Adding printfs to the scheduler seemed to
> help, so perhaps there is a race condition. I can provide additional
> information or debug output if necessary.

Cache inconsistency in single processor systems is really only an issue
if you have some peripheral doing DMA - but, in currently that's just
the ethernet MAC, and that driver is stable and well tested / used.

John
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