dma_alloc_coherent versus streaming DMA, neither works satisfactory

Russell King - ARM Linux
2015-05-07T14:30:11Z

On Thu, May 07, 2015 at 04:08:54PM +0200, Mike Looijmans wrote:
> I read the rest of the thread, apparently it was never integrated.
>
> The patch for "non-consistent" is a BUG FIX, not some feature request or so.
> I was already wondering why my driver had to kalloc pages to get proper
> caching on it.

I disagree.

> From https://www.kernel.org/doc/Documentation/DMA-attributes.txt:
> """
> DMA_ATTR_NON_CONSISTENT ... lets the platform to choose to return either
> consistent or non-consistent memory as it sees fit. By using this API,
> you are guaranteeing to the platform that you have all the correct and
> necessary sync points for this memory in the driver.
> """

DMA attributes are something that came in _after_ the DMA API had been
around for many years. It's a "new feature" that was added to an
existing subsystem, and because there have been no need for it to be
implemented on ARM, the new feature was never implemented.

More than that, the vast majority of ARM hardware can't provide this
kind of memory, and there are _no_ kernel APIs to ensure that if
cacheable memory were to be returned, they could issue the necessary
cache flushes to ensure that the device could see the data.

So it's _not_ a bug fix, and there have been very good reasons not to
implement it.

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