I don’t see how it can work otherwise. For example, take a common-buffer DMA
where I’m writing a stream of commands to a graphics chip on the PCI bus. If
that buffer is writeback-cached, the processor is depositing the data on its
cache, and the cache lines are marked “modified” - however, there’s no data
actually written to physical memory, and it would take another front-side
bus master to access that memory to force the system to dump the modified
cache lines onto main memory. Now, the moment I kick the DMA transfer alive,
the DMA controller is going to go to memory to fetch the stream of data, the
only problem is, data isn’t there because the buffer hasn’t been flushed
from its cache lines. In fact, some of that data may not even be in the
cache yet, it may be lying in the processor’s write buffers.
But processor caches stay coherent through the MESI protocol, which operates
at hardware level and is independent of the OS. That protocol operates
through every processor snooping every transaction on the front-side bus.
But DMA controllers are masters on the PCI bus, not on the front-side bus !
Hence, I see no way a DMA controller can be aware of the comings and goings
of the MESI protocol on the front-side bus.
For example, take a look at the DDK documentation for function
VideoPortStartDma(), it specifically says that the function flushes the
memory region in the host processor’s caches, then it builds a
scatter-gather list and calls HwVidExecuteDma().
Alberto.
-----Original Message-----
From: xxxxx@lists.osr.com
[mailto:xxxxx@lists.osr.com]On Behalf Of Doug
Sent: Friday, November 14, 2003 11:05 AM
To: Windows System Software Devs Interest List
Subject: [ntdev] Re: Flushing DMA Buffer Allocated with
AllocateCommonBuffer
The documentation does require it. While the page in the DDK help labeled
“Using Common-Buffer Bus-Master DMA” makes no mention of allocating an MDL
and passing it to KeFlushIoBuffers, the page “Flushing Cached Data during
DMA Operations” infers it if you mark the buffers as CacheEnabled.
Problem in the DDK docs?
“Mark Roddy” wrote in message news:xxxxx@ntdev…
> 3) common buffers are ‘special’. They are platform specific designed to
> be used for dma. The documentation in the DDK does not require that you
> flush common buffers.
>
> Note that on x86 platforms KeFlushIoBuffers is a NOP. Draw your own
> conclusions.
>
—
Questions? First check the Kernel Driver FAQ at
http://www.osronline.com/article.cfm?id=256
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