Re: MT76x2U crashes XHCI driver on AMD Ryzen system

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Tue, Feb 26, 2019 at 11:05:36AM +0100, Joerg Roedel wrote:
> On Mon, Feb 18, 2019 at 03:37:48PM +0100, Stanislaw Gruszka wrote:
> > 0001-mt76x02u-use-usb_bulk_msg-to-upload-firmware.patch
> > 0002-mt76usb-do-not-use-compound-head-page-for-SG-I-O.patch
> > 
> > Or problem can be solved by just one of it (either first or second).
> > 
> > Additionally I'm not 100% sure if
> > 
> > 0002-mt76usb-do-not-use-compound-head-page-for-SG-I-O.patch
> > 
> > is correct. So perhaps some IOMMU maintainer could look at it.
> 
> The patch looks good, but I don't understand why it is needed. The AMD
> IOMMU driver should handle sg->offset > PAGE_SIZE just fine. Can you
> verify that this is the problem? I will look into that again if it turns
> out there is bug in the IOMMU driver.

I'm try to get that information from bug reporter, but I can't get it so
far.

If sg->offset > PAGE_SIZE is fine then most likely we have problem with
alignment. We use page_frag_alloc() in mt76usb for buffer allocation
in scheme like this

page_frag_alloc(max_payload);	// something like 14434
page_frag_alloc(1024);
page_frag_alloc(2048)
page_frag_alloc(2048)
page_frag_alloc(2048)
...

page_frag_alloc works smart and fast way internally by allocating
fragments just but changing internal offset:

        offset = nc->offset - fragsz;
        if (unlikely(offset < 0)) {
                page = virt_to_page(nc->va);
	.
	.
	.

        }

        nc->offset = offset;
	return nc->va + offset;

but unlike other allocators like kmalloc that make effort to provide
ARCH_DMA_MINALIGN buffers, it does not care about alignment. Above
scheme of allocation in mt76usb breaks it. 

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.

Stanislaw