Date:	Fri, 8 Nov 2013 10:47:49 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Sarah Sharp" <sarah.a.sharp@...ux.intel.com>
Cc:	<netdev@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: RE: [PATCH] usb: xhci: Link TRB must not occur with a USB payload burst.

> From: Sarah Sharp [mailto:sarah.a.sharp@...ux.intel.com]
> On Thu, Nov 07, 2013 at 05:20:49PM -0000, David Laight wrote:
> >
> > Section 4.11.7.1 of rev 1.0 of the xhci specification states that a link TRB
> > can only occur at a boundary between underlying USB frames (512 bytes for 480M).
> 
> Which version of the spec are you looking at?  I'm looking at the
> updated version from 08/2012 and I don't see any such requirement.

The copy I downloaded last week is dated 5/21/10, seems the copy on the
web site hasn't been updated.

> I do see that section says "A TD Fragment shall not span Transfer Ring
> Segments" where a TD fragment is defined as exact multiples of Max Burst
> Size * Max Packet Size bytes for the endpoint.  Is that what you mean
> about USB frames?

That is the clause, it needs some understanding.
Figure 24 shows a correctly aligned link TRB - one where the TRB boundary
is aligned with a packet boundary (512 bytes at 480M, 1k at 5G etc).

> > If this isn't done the USB frames aren't formatted correctly and, for example,
> > the USB3 ethernet ax88179_178a card will stop sending (while still receiving)
> > when running a netperf tcp transmit test with (say) and 8k buffer.
> 
> Which driver does that use?  Is it using the scatter gather list
> mechanism for URBs (i.e. urb->sg)?  Or is it submitting multiple URBs
> for fragmented buffers?  Or is it submitting isochronous URBs with
> multiple frames?  Or is it submitting bulk URBs with one transfer buffer
> that crosses the 64KB boundary?
> 
> I'm trying to understand what the USB device driver is doing in order to
> create multi-TRB TDs that would violate the TD fragment rules.

The usb setup is done by net/usbnet.c, the ax88179_178a driver just adds
a header to the ethernet frame. Since the hardware support TCP segment
offload the urb->sg list is used, the first fragment seems to be 1578
bytes and is followed by two or three fragments (split on an 0x8000
boundary) making up the rest of the 65234 bytes.
(I haven't yet seen a buffer that crossed a 64k boundary).

I'm running netperf - which is doing a burst of 8k sends into
a TCP socket.

> > While this change improves things a lot (it runs for 20 minutes rather than
> > a few seconds), there is still something else wrong.
> >
> > This should be a candidate for stable, the ax88179_178a driver defaults to
> > gso and tso enabled so it passes a lot of fragmented skb to the USB stack.
> 
> I want to understand what the larger issue is here before pushing
> bandaid fixes.
> 
> I don't think this is the right approach, because prepare_ring() can be
> called for isochronous URBs that get mapped to multiple TDs.  The TD
> fragment rules only apply to one TD, so failing URB submission because
> many isochronous TDs span more than one ring segment doesn't seem like
> the right approach.  Inserting no-op TRBs will also result in odd
> isochronous behavior, since a no-op TRB means the xHC should not send or
> request an isochronous packet for that service interval.

I wasn't aware of that effect of NOP TRB on isoc data.
Three obvious options:
1) Pass an extra parameter indicating that the caller will handle an
   TRB data alignment issues at the link TRB.
2) Add a test for bulk endpoints (ok until the isoc code gets modified
   to support SG data).
3) Copy the LINK TRB instead of adding NOPs (would require the rest of
   the code use the ring length instead of reading the TRB type).

The second is probably the easiest way to isolate the change.
However there could be problems on an isoc endpoint if the data
crosses a 64k boundary.

> The patch also doesn't address the underlying issue of constructing
> multi-TRB TDs so that it fits the TD fragment rules.  Your patch ensures
> there are no link TRBs in the middle of a TD, but it doesn't ensure that
> the TRBs in the TD are exact multiples of the Max Burst Size * Max
> Packet Size bytes for the endpoint.

That doesn't matter, look at figure 25. A TD can be any number of USB
packets and any number or TRB. The constraint is that a TD cannot
contain a LINK TRB.

> Instead, new code in count_sg_trbs_needed() should break the TD into
> proper TD fragments.  The queueing code for all endpoints would also
> have to follow those rules.  This would have to happen while still
> respecting the 64KB boundary rule.
> 
> The driver would also have to make sure that TD fragments didn't have
> link TRBs in them.  That's an issue, since the spec decidedly unclear
> about what exactly comprises a TD fragment.  Is the xHC host greedy, and
> will lump all multiples into one TD fragment?  Or will it assume the TD
> fragment has ended once it consumes one multiple of Max Burst Size * Max
> Packet Size bytes?

It only matters at a LINK TRB (unless you want to request an interrupt).
The constraint seems to be that the data TRB before a link TRB must end
on a MBP boundary.

> This ambiguity means it's basically impossible to figure out whether a
> TD with a link TRB in the middle is constructed properly.  The xHCI spec
> architect didn't have a good solution to this problem, so I punted and
> never implemented TD fragments.

The problem is that the hardware does implement them:-)

> If this is really an issue, it's going to be pretty complex to solve.

I thought about solutions and then decided it was easiest to skip to
the next ring segment. The data could be scanned for an earlier
boundary that was aligned, but it really didn't seem worth the effort.

If all of the TRB reference buffers that are larger than MBP then,
when the LINK is encountered, the previous TRB can be split on its
last MBP boundary, if not a bounce buffer would have to be used
(and the code doesn't really know if this a tx or rx transaction).
I've a tx that is split 1576/328/32768/30560 where the 328 could
not be split - so this does happen.

> 
> > Signed-off-by: David Laight <david.laight@...lab.com>
> > ---
> >
> > Although I've got a USB2 analyser its trigger and trace stuff is almost
> > unusable! In any case this fails much faster on USB3 (Intel i7 cpu).
> 
> I thought you were using a big endian system?

No - I was just reading the code and spotted the endianness bug.

	David

> > diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> > index 5480215..23abc9b 100644
> > --- a/drivers/usb/host/xhci-ring.c
> > +++ b/drivers/usb/host/xhci-ring.c
> > @@ -2927,8 +2932,43 @@ static int prepare_ring(struct xhci_hcd *xhci, struct xhci_ring *ep_ring,
> >  	}
> >
> >  	while (1) {
> > -		if (room_on_ring(xhci, ep_ring, num_trbs))
> > -			break;
> > +		if (room_on_ring(xhci, ep_ring, num_trbs)) {
> > +			union xhci_trb *trb = ep_ring->enqueue;
> > +			unsigned int usable = ep_ring->enq_seg->trbs +
> > +					TRBS_PER_SEGMENT - 1 - trb;
> > +			u32 nop_cmd;
> > +
> > +			/*
> > +			 * Section 4.11.7.1 TD Fragments states that a link
> > +			 * TRB must only occur at the boundary between
> > +			 * data bursts (eg 512 bytes for 480M).
> > +			 * While it is possible to split a large fragment
> > +			 * we don't know the size yet.
> > +			 * Simplest solution is to fill the trb before the
> > +			 * LINK with nop commands.
> > +			 */
> > +			if (num_trbs == 1 || num_trbs <= usable || usable == 0)
> > +				break;
> > +
> > +			if (num_trbs >= TRBS_PER_SEGMENT) {
> > +				xhci_err(xhci, "Too many fragments %d, max %d\n",
> > +						num_trbs, TRBS_PER_SEGMENT - 1);
> > +				return -ENOMEM;
> > +			}
> > +
> > +			nop_cmd = cpu_to_le32(TRB_TYPE(TRB_TR_NOOP) |
> > +					ep_ring->cycle_state);
> > +			for (; !TRB_TYPE_LINK_LE32(trb->link.control); trb++) {
> > +				trb->generic.field[0] = 0;
> > +				trb->generic.field[1] = 0;
> > +				trb->generic.field[2] = 0;
> > +				trb->generic.field[3] = nop_cmd;
> > +				ep_ring->num_trbs_free--;
> > +			}
> > +			ep_ring->enqueue = trb;
> > +			if (room_on_ring(xhci, ep_ring, num_trbs))
> > +				break;
> > +		}
> >
> >  		if (ep_ring == xhci->cmd_ring) {
> >  			xhci_err(xhci, "Do not support expand command ring\n");
> 
> Sarah Sharp


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