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.
