linux-kernel - Re: MT76x2U crashes XHCI driver on AMD Ryzen system

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Date:   Tue, 12 Mar 2019 08:13:14 +0100
From:   Stanislaw Gruszka <sgruszka@...hat.com>
To:     Rosen Penev <rosenp@...il.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Samuel Sieb <samuel@...b.net>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, jan.viktorin@...il.com
Subject: Re: MT76x2U crashes XHCI driver on AMD Ryzen system

On Mon, Mar 11, 2019 at 09:43:19AM +0100, Stanislaw Gruszka wrote:
> > > However after fixing that mt76usb still did not work. To make things
> > > work we had to change rx frag size from 2048 to PAGE_SIZE and change
> > > virt_to_head_page() to virt_to_page() when setting SG's.
> >
> > > I think I understand why first change was needed. If we do 2 separate
> > > dma maps of 2 different buffers in single page i.e (PAGE + off=0
> > > and PAGE + off=2048) it causes problem. So either map_sg() return
> > > error which mt76usb does not handle correctly or there is issue
> > > in AMD IOMMU because two dma maps use the same page.
> 
> Any comment on that? Is fine or not to do 2 or more dma mappings
> within the same single page on AMD IOMMU? If not, is there any
> mechanism for drivers to find out about this limitation to prevent
> to prepare wrong SG buffers?

FTR: it was confirmed by Jan (bug reporter) the 2 or more dma mappings
within single page works with AMD IOMMU. Most likely it was needed
previously to workaround this sg->offset problem until proper fix to
AMD IOMMU was applied.

Stanislaw