Re: [PATCH v2 3/3] swiotlb: Honour dma_alloc_coherent() alignment in swiotlb_alloc()
From: Robin Murphy
Date: Wed Jan 31 2024 - 11:03:53 EST
On 31/01/2024 12:25 pm, Will Deacon wrote:
core-api/dma-api-howto.rst states the following properties of
dma_alloc_coherent():
| The CPU virtual address and the DMA address are both guaranteed to
| be aligned to the smallest PAGE_SIZE order which is greater than or
| equal to the requested size.
However, swiotlb_alloc() passes zero for the 'alloc_align_mask'
parameter of swiotlb_find_slots() and so this property is not upheld.
Instead, allocations larger than a page are aligned to PAGE_SIZE,
Calculate the mask corresponding to the page order suitable for holding
the allocation and pass that to swiotlb_find_slots().
I guess this goes back to at least e81e99bacc9f ("swiotlb: Support
aligned swiotlb buffers") when the explicit argument was added - not
sure what we do about 5.15 LTS though (unless the answer is to not care...)
As before, though, how much of patch #1 is needed if this comes first?
Cheers,
Robin.
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
kernel/dma/swiotlb.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 4485f216e620..8ec37006ac70 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1632,12 +1632,14 @@ struct page *swiotlb_alloc(struct device *dev, size_t size)
struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
struct io_tlb_pool *pool;
phys_addr_t tlb_addr;
+ unsigned int align;
int index;
if (!mem)
return NULL;
- index = swiotlb_find_slots(dev, 0, size, 0, &pool);
+ align = (1 << (get_order(size) + PAGE_SHIFT)) - 1;
+ index = swiotlb_find_slots(dev, 0, size, align, &pool);
if (index == -1)
return NULL;