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

On Wed, 13 Nov 2013, David Laight wrote:

quoted

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that doesn't matter; we don't get an interrupt when a ring segment is
crossed.  Instead we set the interrupt-on-completion flag on the last
TRB of the TD, not on any earlier fragment or link TRB.

That's because you don't worry about handling URBs which require too
many TRBs (i.e., more than are available).  You just add more ring
segments.  Instead, you could re-use segments on the fly.

For example, suppose you have only two ring segments and you get an URB
which requires enough TRBs to fill up four segments.  You could fill in
the first two segments worth, and get an interrupt when the controller
traverses the Link TRB between them.  At that point you store the third
set of TRBs in the first segment, which is now vacant.  Similarly, when
the second Link TRB is traversed, you fill in the fourth set of TRBs.

That isn't going to work.

Queuing URBs and processing them in pieces was _your_ suggestion.  I
merely repeated it to Sarah and filled in some detail.

It might work for very long TD, but for very fragmented ones the interrupt
rate would be stupid.

Not if the individual SG elements are reasonably large -- say at least 
512 bytes.  They have to be that big if they are to work with ehci-hcd, 
so why not also for xhci-hcd?

 In any case you'd definitely need enough ring
segments for the TRB that describe a single 'max packet size' block.

That would be two TRBs (for maxpacket = 1024), meaning you'd need at
least one ring segment.  That's not much of a restriction.

quoted

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Finally, it's interesting to note that the USB mass storage driver is
using scatter gather lists just fine without the driver following the TD
fragment rules.  Or at least no one has reported any issues.  I wonder
why it works?

I'd guess this is because the hardware is actually a lot more flexible
than the "No Link TRBs in the middle of a TD fragment" rule.

With the hardware I have (Intel i7 Sandy bridge) Link TRB cannot
be placed at arbitrary boundaries.
I don't know whether the actual restriction is only to packet boundaries.

That's what I had in mind by "more flexible" above.

I don't have a USB3 monitor and it would also require a more contrived
test than I've been doing.

You wouldn't need a monitor to check it, but you would need some 
careful tests.

quoted

The whole idea of TD fragments makes no sense to begin with.  What
point is there in grouping packets into MaxBurst-sized collections?

It probably saved a few logic gates somewhere, either that or it is
a bug in some hardware implementation that got documented in the spec
instead of being fixed :-)

I can believe the guess about it reflecting a bug.  Particularly since 
it is so badly written.

Alan Stern

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