linux-kernel - Re: 5.10 regression caused by: "uas: fix sdev->host->dma_dev": many XHCI swiotlb buffer is full / DMAR: Device bounce map failed errors on thunderbolt connected XHCI controller

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Message-ID: <fded04e2-f2e9-de92-ab1f-5aa088904e90@redhat.com>
Date:   Fri, 27 Nov 2020 19:12:13 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Tom Yan <tom.ty89@...il.com>,
        Mathias Nyman <mathias.nyman@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb <linux-usb@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org, Lu Baolu <baolu.lu@...ux.intel.com>
Subject: Re: 5.10 regression caused by: "uas: fix sdev->host->dma_dev": many
 XHCI swiotlb buffer is full / DMAR: Device bounce map failed errors on
 thunderbolt connected XHCI controller

Hi,

On 11/27/20 5:19 PM, Christoph Hellwig wrote:
> On Fri, Nov 27, 2020 at 01:32:16PM +0100, Hans de Goede wrote:
>> I ran some more tests, I can confirm that reverting:
>>
>> 5df7ef7d32fe "uas: bump hw_max_sectors to 2048 blocks for SS or faster drives"
>> 558033c2828f "uas: fix sdev->host->dma_dev"
>>
>> Makes the problem go away while running a 5.10 kernel. I also tried doubling
>> the swiotlb size by adding: swiotlb=65536 to the kernel commandline but that
>> does not help.
>>
>> Some more observations:
>>
>> 1. The usb-storage driver does not cause this issue, even though it has a
>> very similar change.
>>
>> 2. The problem does not happen until I plug an UAS decvice into the dock.
>>
>> 3. The problem continues to happen even after I unplug the UAS device and
>> rmmod the uas module
>>
>> 3. made me take a bit closer look to the troublesome commit, it passes:
>> udev->bus->sysdev, which I assume is the XHCI controller itself as device
>> to scsi_add_host_with_dma, which in turn seems to cause permanent changes
>> to the dma settings for the XHCI controller. I'm not all that familiar with
>> the DMA APIs but I'm getting the feeling that passing the actual XHCI-controller's
>> device as dma-device to scsi_add_host_with_dma is simply the wrong thing to
>> do; and that the intended effects (honor XHCI dma limits, but do not cause
>> any changes the XHCI dma settings) should be achieved differently.
>>
>> Note that if this is indeed wrong, the matching usb-storage change should
>> likely also be dropped.
> 
> One problem in this area is that the clamping of the DMA size through
> dma_max_mapping_size mentioned in the commit log doesn't work when
> swiotlb is called from intel-iommu. I think we need to wire up those
> calls there as well.

Ok, but that does not sound like a quick last minute fix for 5.10, so maybe
for 5.10 we should just revert the uas and usb-storage changes which trigger
this problem and then retry those for 5.11 ?

Regards,

Hans