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SubjectRe: [RFC] generic device DMA implementation
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david@gibson.dropbear.id.au said:
> The point is, there has to be an advantage to using consistent memory
> if it is available AND the possibility of it not being available.

I'm really thinking of this from the driver writer's point of view. The
advantage of consistent memory is that you don't have to think about where to
place all the sync points (sync points can be really subtle and nasty and an
absolute pain---I shudder to recall all of the problems I ran into writing a
driver on a fully inconsistent platform).

The advantage here is that you can code the driver only to use consistent
memory and not bother with the sync points (whatever the cost of this is).
Most platforms support reasonably cheap consistent memory, so most people
simply don't want to bother with inconsistent memory if they can avoid it.

If you do the sync points, you can specify the DMA_CONFORMANCE_NON_CONSISTENT
level and have the platform choose what type of memory you get. For a
platform which makes memory consistent by turning off CPU caching at the page
level, it's probably better to return non-consistent memory if the driver can
cope with it.

James


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