netdev - Re: [PATCH RFC 1/1] usb: Tell xhci when usb data might be misaligned

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Message-ID: <20140130161721.25560.qmail@stuge.se>
Date:	Thu, 30 Jan 2014 17:17:21 +0100
From:	Peter Stuge <peter@...ge.se>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH RFC 1/1] usb: Tell xhci when usb data might be
	misaligned

David Laight wrote:
> Some xhci (USB3) controllers have a constraint on the offset within a
> bulk transfer of the end of the transfer ring.

Mhm.

> code using libusb can generate arbitrarily long transfers that usually
> get split into 8k fragments.

libusb splits transfers into 16k urbs, or doesn't with newer code
when both kernel and libusb support scatter-gather.

> In fact libusb always uses 8k fragments.

Hm? Worst-case libusb-1.0 submits 16k urbs. libusb-0.1 I'm unsure
about, but could check.

When both sides support it, scatter-gather is used and a single urb
is submitted.

IIRC usbfs doesn't mess with urb buffers at all.

Where's the 8k coming from?

> This all means that the xhci driver needs to accept unlimited numbers
> of 'aligned' fragments and only restrict the number of misaligned ones.

libusb applications have so far never made efforts to align their
buffers to anything. That seems to become relevant for zero-copy?

//Peter
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