RE: [PATCH] xhci: Cope with VIA VL805 readahead

[David Laight]

From: Robin Murphy
> Sent: 10 October 2017 19:09
> 
> The VIA VL805 host controller is well-known for causing problems on
> systems with IOMMUs enabled, ranging from triggering endless streams of
> fault messages to locking itself up completely. It appears that the root
> of the problem might be an over-aggressive prefetching of TRBs, wherein
> consuming commands near the end of a queue segment causes it to read off
> the end of the segment, even across a page boundary. This blows up when
> DMA mapping ops are backed by an IOMMU, since there is no guarantee that
> addresses outside the allocated segment are accessible at all.
> 
> Some trial-and-error investigation reveals that we can avoid such
> cross-page reads by not using the last few TRBs in a segment; to that
> end, factor out the implicit index of the end-of-segemnt link TRB, and
> implement a quirk to move it slightly further forward when necessary.

Does this fix all of your problems?
Or is there a second issue when the iommu is disabled?
...
> +unsigned int xhci_segment_link_idx(struct xhci_hcd *xhci)
> +{
> +	if (xhci->quirks & XHCI_READAHEAD_QUIRK)
> +		return TRBS_PER_SEGMENT - 4;
> +
> +	return TRBS_PER_SEGMENT - 1;
> +}

There is no point calculating this every time it is needed.
Save the value in the xhci structure.

I wonder whether it is actually worth just setting TRBS_PER_SEGMENT
to 252 but allocating a full page (256 TRBs)
(ie doing it unconditionally for all devices).
I suspect the performance drop will be immeasurable small.

	David