Re: MT76x2U crashes XHCI driver on AMD Ryzen system

From: Robin Murphy
Date: Mon Feb 18 2019 - 12:37:38 EST

On 18/02/2019 14:37, Stanislaw Gruszka wrote:
Another issue is that dma_map_sg() & dma_map_page() may require some
constraints. I'm not sure about that and I want to clarify that with
CCed mm maintainers. I think DMA drivers may expect sg->offset < PAGE_SIZE
for both dma_map_sg() and dma_map_page(). Additionally dma_map_page()
maight expect that offset & length specify buffer within one page.

Luckily, this came up a while back[1] and we seemed to reach a consensus that sg->offset >= PAGE_SIZE for dma_map_sg() was weird but valid. IIRC it was only the Intel IOMMU code which failed to handle that case appropriately (and which I fixed) - the AMD IOMMU code always looked like it should be OK, but I'm not sure I've ever seen definitive test results (and I don't have hardware to do so myself).

For dma_map_page(), length >= PAGE_SIZE should be perfectly valid and handled correctly. The offset >= PAGE_SIZE case is a bit harder to justify, but at the same time has less scope for the DMA API backend to get it wrong, so either way is likely to be OK in practice (in particular the AMD IOMMU code looks like it won't have a problem, since its map_page() implementation converts page and offset to a plain physical address before doing anything else).