On 16/03/2013 09:44 PM, Greg KH wrote:
I'm guessing here that you aren't talking about Linux.
My question is not related to Linux. It's related to xhci, and I asked on this mailing list because it's a public list with people experienced in xhci.
The question is: what possible scenario could make the no-snoop option in transfer TRBs useful? I explained in detail why I believe it's useless.
The "old" usb storage spec is extreemly slow, that is the issue. If you use the "new" spec, using streams, then you can start to actually use the hardware well, and then you might see some speedups. If you are using the old spec, then the delays in the driver and device itself will swamp any perceived speedups you might do here in the xhci driver for this. Look at a USB trace to verify this, it should be pretty obvious there.
Right, but streams have nothing to do with the no-snoop option. It's an unrelated feature. (Other than no-snoop bit also being present on transfer TRBs posted to a stream transfer ring... - which again, doesn't explain why no-snoop might give any benefit)
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