Re: MT76x2U crashes XHCI driver on AMD Ryzen system

[Stanislaw Gruszka]

On Tue, Feb 26, 2019 at 12:24:08PM +0100, Stanislaw Gruszka wrote:
> On Tue, Feb 26, 2019 at 11:44:13AM +0100, Joerg Roedel wrote:
> > On Tue, Feb 26, 2019 at 11:34:51AM +0100, Stanislaw Gruszka wrote:
> > > On Tue, Feb 26, 2019 at 11:05:36AM +0100, Joerg Roedel wrote:
> > > If sg->offset > PAGE_SIZE is fine then most likely we have problem with
> > > alignment.
> > 
> > The map_sg implementation in the AMD IOMMU driver uses sg_phys() which
> > handles the sg->page + sg->offset calculation fine.
> > 
> > > Note hat issue is with dma_map_sg(), switching to dma_map_single()
> > > by using urb->transfer_buffer instead of urb->sg make things work
> > > on AMD IOMMU.
> > 
> > On the other hand this points to a bug in the driver, I'll look further
> > if I can spot something there.
> 
> I think so too. And I have done some changes that avoid strange allocation
> scheme and use usb synchronous messages instead of allocating buffers
> with unaligned sizes. However things work ok on Intel IOMMU and 
> there is no documentation what are dma_map_sg() requirement versus
> dma_map_single() which works. I think there are some unwritten
> requirements and things can work on some platforms and fails on others
> (different IOMMUs, no-IOMMU on some ARCHes)  

For the record: we have another bug report with this issue:
https://bugzilla.kernel.org/show_bug.cgi?id=202673

I provided there patch that change alignment for page_frag_alloc() and
it did not fixed the problem. So this is not alignment issue.
Now I think it could be page->refcount issue ...

Stanislaw