[PATCH] ARM: dma-mapping: support non-consistent DMA attribute

[Daniel Drake]

On Wed, Feb 25, 2015 at 8:42 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> When userspace wants to access the buffer later, it needs to ask DRM
> whether the buffer is safe to access - this causes DRM to check whether
> the buffer is still being used for a render operation, and can then
> flip the buffer back to CPU ownership.
>
> The idea that a buffer needs to be constantly mapped and unmapped in
> userspace would create its own problems: there is a cost to setting up
> and tearing down the mappings.

OK, so lets say the userspace mapping is created just once, and there
is a DRM-based access control mechanism as you describe, mandating
when it can/can't be accessed. To ask the question a different way
then: How is the one-time userspace mapping created?

As far as I can see, the if the dma_mmap_* API is used to do that, the
mapping will always be uncached or writecombine. There's no way that
it could currently be mapped with CPU caches enabled, unless
dma_mmap_* is avoided.

Daniel