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

[Christoph Hellwig <hch@lst.de>]

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.