[PATCH] nvme/pci: Poll CQ on timeout

[keith.busch@intel.com (Keith Busch)]

On Tue, Feb 28, 2017@03:10:17PM +0100, Christoph Hellwig wrote:
> On Fri, Feb 24, 2017@05:59:28PM -0500, Keith Busch wrote:
> > If an IO timeout occurs, it's helpful to know if the controller did not
> > post a completion or the driver missed an interrupt. While we never expect
> > the latter, this patch will make it possible to tell the difference so
> > we don't have to guess.
> 
> Do you have any good real use case for it?  I mostly don't like it
> becuase it ties us to polling for a specific tag, something I'd like
> to change in the ->poll API.

I don't expect this to often catch anything, but this is a cheap way of
constraining the problem just by having this in place: the "Timeout"
message definitely means the device did not post an entry on that
command's completion queue, so either the device is broken or we messed
up the queue mapping.

In the event it does trigger (I've seen this only a handlful of times
on new platforms and devices, as well as legacy IRQs), we know a whole
lot more about the problem, compared to just seeing "Timeout".

If you want to remove the tag specific polling, I can look into an
alternative.