netdev - RE: [PATCH] usb: xhci: Link TRB must not occur with a USB payload burst.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 11 Nov 2013 15:34:53 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	David Laight <David.Laight@...LAB.COM>
cc:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	<netdev@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: RE: [PATCH] usb: xhci: Link TRB must not occur with a USB payload
 burst.

On Mon, 11 Nov 2013, David Laight wrote:

> > Suppose, for example, the MBP is 1024.  If you have a TD with length
> > 1500, and if it had only one fragment, the last (and only) fragment's
> > length would not less than the MBP and it would not be an exact
> > multiple of the MBP.
> 
> That doesn't matter - eg example 2 in figure 25

You're right.  I do wish the spec had been written more clearly.

> Reading it all again makes me think that a LINK trb is only
> allowed on the burst boundary (which might be 16k bytes).
> The only real way to implement that is to ensure that TD never
> contain LINK TRB.

That's one way to do it.  Or you could allow a Link TRB at an 
intermediate MBP boundary.

It comes down to a question of how often you want the controller to
issue an interrupt.  If a ring segment is 4 KB (one page), then it can
hold 256 TRBs.  With scatter-gather transfers, each SG element
typically refers to something like a 2-page buffer (depends on how
fragmented the memory is).  Therefore a ring segment will describe
somewhere around 512 pages of data, i.e., something like 2 MB.  Since
SuperSpeed is 500 MB/s, you'd end up getting in the vicinity of 250
interrupts every second just because of ring segment crossings.

Using larger ring segments would help.

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