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Date: Thu, 30 Jan 2014 17:07:11 -0500 (EST) From: Alan Stern <stern@...land.harvard.edu> To: Sarah Sharp <sarah.a.sharp@...ux.intel.com> cc: David Laight <David.Laight@...LAB.COM>, "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, David Miller <davem@...emloft.net>, Dan Williams <dan.j.williams@...el.com>, "Nyman, Mathias" <mathias.nyman@...el.com>, Mark Lord <mlord@...ox.com>, Freddy Xin <freddy@...x.com.tw> Subject: Re: [PATCH RFC 1/1] usb: Tell xhci when usb data might be misaligned On Thu, 30 Jan 2014, Sarah Sharp wrote: > > That's a good plan. However _some_ restriction will turn out to be > > necessary. > > > > For example, what will you do if a driver submits an SG list containing > > 300 elements, each 3 bytes long? That's too many to fit in a single > > ring segment, but it's smaller than a TD fragment -- it's even smaller > > than maxpacket -- so there's no place to split it. (Not that I think > > drivers _will_ submit requests like this; this is just to demonstrate > > the point.) > > > > It ought to be acceptable to require, for example, that an SG URB > > contain no more than (say) 100 elements that are smaller than 512 > > bytes. > > At that point, the xHCI driver or USB core should probably use a bounce > buffer. It feels like we should attempt to push down scatter-gather > lists as far down in the stack as possible, so the upper layers don't > have to care what alignment, length, or random 64KB boundary splits we > need. Okay. That should be doable, if awkward. > > ehci-hcd gets along okay with the restriction that each SG element > > except the last has to be a multiple of the maxpacket size. xhci-hcd > > can relax this quite a lot, but not all the way. > > What does the EHCI driver do when it receives a SG list from the USB > networking layer that violates this restriction? It never receives such lists. usb_submit_urb() returns -EINVAL before the request gets sent to ehci-hcd. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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