[Bug 215740] kernel warning: DMA-API: xhci_hcd: cacheline tracking EEXIST, overlapping mappings aren't supported

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=215740

--- Comment #47 from Guenter Roeck (linux@roeck-us.net) ---
I (and Google, for that matter) long since went beyond that after the above
feedback ("Equally important, however, is the question of why the people
experiencing this problem turned on CONFIG_DMA_API_DEBUG in the first place.
That setting is meant only for kernel testing and it drastically reduces
performance.").

For context, we don't enable CONFIG_DMA_API_DEBUG in production images. We
enable it in test images to catch problems which are only caught in debug
images. Getting false positives in such images doesn't help and hides real
problems.

So we gave up trying to fix the problem in the upstream kernel and implemented
a (downstream) workaround which excludes the "hub" and "xhcd_hcd" drivers from
DMA cacheline tracking. That may not be perfect, but it does the trick for us.

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