From: Baoquan He
> Sent: 24 February 2022 14:11
...
> With my understanding, there are two kinds of DMA mapping, coherent
> mapping (which is also persistent mapping), and streaming mapping. The
> coherent mapping will be handled during driver init, and released during
> driver de-init. While streaming mapping will be done when needed at any
> time, and released after usage.
The lifetime has absolutely nothing to do with it.
It is all about how the DMA cycles (from the device) interact with
(or more don't interact with) the cpu memory cache.
For coherent mapping the cpu and device can write to (different)
words in the same cache line at the same time, and both will see
both updates.
On some systems this can only be achieved by making the memory
uncached - which significantly slows down cpu access.
For non-coherent (streaming) mapping the cpu writes back and/or
invalidates the data cache so that the dma read cycles from memory
read the correct data and the cpu re-reads the cache line after
the dma has completed.
They are only really suitable for data buffers.
David
-
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On 02/24/22 at 02:27pm, David Laight wrote:
> From: Baoquan He
> > Sent: 24 February 2022 14:11
> ...
> > With my understanding, there are two kinds of DMA mapping, coherent
> > mapping (which is also persistent mapping), and streaming mapping. The
> > coherent mapping will be handled during driver init, and released during
> > driver de-init. While streaming mapping will be done when needed at any
> > time, and released after usage.
>
> The lifetime has absolutely nothing to do with it.
>
> It is all about how the DMA cycles (from the device) interact with
> (or more don't interact with) the cpu memory cache.
>
> For coherent mapping the cpu and device can write to (different)
> words in the same cache line at the same time, and both will see
> both updates.
> On some systems this can only be achieved by making the memory
> uncached - which significantly slows down cpu access.
>
> For non-coherent (streaming) mapping the cpu writes back and/or
> invalidates the data cache so that the dma read cycles from memory
> read the correct data and the cpu re-reads the cache line after
> the dma has completed.
> They are only really suitable for data buffers.
Thanks for valuable input, I agree the lifetime is not stuff we can rely
on to judge. But how do we explain dma_alloc_noncoherent() is not streaming
mapping? Then which kind of dma mapping is it?
I could miss something important to understand this which is obvious to
other people, I will make time to check.