| Message ID | YhToFzlSufrliUsi@MiWiFi-R3L-srv |
|---|---|
| State | New |
| Headers | show |
| Series | [1/2] dma-mapping: check dma_mask for streaming mapping allocs | expand |
On 02/23/22 at 03:25pm, Christoph Hellwig wrote: > On Wed, Feb 23, 2022 at 08:28:13AM +0800, Baoquan He wrote: > > Could you tell more why this is wrong? According to > > Documentation/core-api/dma-api.rst and DMA code, __dma_alloc_pages() is > > the core function of dma_alloc_pages()/dma_alloc_noncoherent() which are > > obviously streaming mapping, > > Why are they "obviously" streaming mappings? Because they are obviously not coherent mapping? 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. Are we going to add another kind of mapping? It's not streaming mapping, but use dev->coherent_dma_mask, just because it uses dma_alloc_xxx() api. > > > why do we need to check > > dev->coherent_dma_mask here? Because dev->coherent_dma_mask is the subset > > of dev->dma_mask, it's safer to use dev->coherent_dma_mask in these > > places? This is confusing, I talked to Hyeonggon in private mail, he has > > the same feeling. > > Think of th coherent_dma_mask as dma_alloc_mask. It is the mask for the > DMA memory allocator. dma_mask is the mask for the dma_map_* routines. I will check code further. While this may need be noted in doc, e.g dma_api.rst or dma-api-howto.rst. If you have guide, I can try to add some words to make clear this. Or leave this to people who knows this clearly. I believe it will be very helpful to understand DMA api.
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.
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c index 9478eccd1c8e..e66847aeac67 100644 --- a/kernel/dma/mapping.c +++ b/kernel/dma/mapping.c @@ -543,10 +543,11 @@ static struct page *__dma_alloc_pages(struct device *dev, size_t size, { const struct dma_map_ops *ops = get_dma_ops(dev); - if (WARN_ON_ONCE(!dev->coherent_dma_mask)) - return NULL; - if (WARN_ON_ONCE(gfp & (__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM))) - return NULL; + if (WARN_ON_ONCE(!dev->dma_mask)) + return NULL; + + /* let the implementation decide on the zone to allocate from: */ + gfp &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM); size = PAGE_ALIGN(size); if (dma_alloc_direct(dev, ops))
For newly added streaming mapping APIs, the internal core function __dma_alloc_pages() should check dev->dma_mask, but not ev->coherent_dma_mask which is for coherent mapping. Meanwhile, just filter out gfp flags if they are any of __GFP_DMA, __GFP_DMA32 and __GFP_HIGHMEM, but not fail it. This change makes it consistent with coherent mapping allocs. Signed-off-by: Baoquan He <bhe@redhat.com> --- kernel/dma/mapping.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)