Re: [PATCH] dmatest: flush and invalidate destination buffer before DMA

[Ralf Baechle <ralf@linux-mips.org>]

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