Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 9 Jan 2009 11:19:36 +0000 | From | Ralf Baechle <> | Subject | Re: [PATCH] dmatest: flush and invalidate destination buffer before DMA |
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On Thu, Jan 08, 2009 at 09:36:03AM +0100, Haavard Skinnemoen wrote:
> In the general case, however, I think MIPS has a bug: I've seen drivers > DMA to/from tiny buffers stored inside another struct. This is legal > because the driver can guarantee that the other fields in the struct > aren't accessed in the mean time, but any fields sharing a cacheline > with the buffer must be written back before the lines are invalidated.
Depending on the implementation details, the use of such a struct might be relying on implementation-specific behaviour. This is what Documentation/DMA-API.txt has to say:
[...] int dma_get_cache_alignment(void)
Returns the processor cache alignment. This is the absolute minimum alignment *and* width that you must observe when either mapping memory or doing partial flushes.
Notes: This API may return a number *larger* than the actual cache line, but it will guarantee that one or more cache lines fit exactly into the width returned by this call. It will also always be a power of two for easy alignment. [...]
Since dma_get_cache_alignment() is a function not a constant its result can't be used in the definition of a struct unless possibly excessive padding is used.
The debate has shown that we problably need BUG_ON() assertions in the DMA API implementations to catch this sort of dangerous use.
Ralf
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