linux-kernel - RE: [PATCH 1/2] dma-mapping: check dma

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Date:   Thu, 24 Feb 2022 14:27:02 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Baoquan He' <bhe@...hat.com>, Christoph Hellwig <hch@....de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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Subject: RE: [PATCH 1/2] dma-mapping: check dma_mask for streaming mapping
 allocs

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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