Cache coherency question

Liu Dave-r63238 DaveLiu at
Fri Aug 25 13:26:16 EST 2006

Tim> I'm using an MVME6100 with Linux 2.6.14 and experiencing what 
Tim> I think is a cache coherency problem.  An external PCIX 
Tim> master performs a DMA transfer of a well known data pattern 
Tim> into SDRAM, but when the device driver goes to look at the 
Tim> data it is mostly okay, but occasionally has garbage.

If the hardware is reliable, I think you are right. The cache coherency

Tim> I'm allocating the memory using alloc_skb, then giving a PCI 
Tim> translated pointer to the PCIX master.  Since I think it is a 
Tim> cache coherency problem, I tried to use the dma_cache_inv 
Tim> macro to invalidate the cache before looking at the data, but 
Tim> realized that for this board, that function does nothing 
Tim> since CONFIG_NOT_COHERENT_CACHE is undefined.  So
Tim> questions:

You are assuming and using the hardware cache coherency mechanism.

Tim> 1) Should the processor bus cache snooping actually work on 
Tim> an MVME6100?
Tim> E.g. is it correct that CONFIG_NOT_COHERENT_CACHE is undefined?

Yes, you can undefine the CONFIG_NOT_COHERENT_CACHE, but you must
make sure the host bridge did the snooping setup for PCI inbound
If you setup the snooping window, I think the host bridge will assert
/GLB signal
to processor. The processor will snoop the 60x/MPX bus to keep cache

Also, you can define the CONFIG_NOT_COHERENT_CACHE, then you are
The system has not hardware coherency. You need use the software to keep
the cache

Tim> 1a) Does this type of bus snooping work when a bus master 
Tim> physically external to the PowerPC chip is doing the transfer?

As above, you must tell the bridge what address need snoop. If you did,
The proceesor will do snooping.

Tim> 2) If this really is a cache coherency problem, are there 
Tim> other cache management functions available in ppc linux 
Tim> besides the dma_cache_inv, dma_cache_wback, 
Tim> dma_cache_wback_inv macros I should be looking at?

I think it is a cache coherency problem,
You can get these functions, dma_map_single


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