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

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Message-ID: <20140131190035.GB18262@xanatos>
Date:	Fri, 31 Jan 2014 11:00:35 -0800
From:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Bjørn Mork <bjorn@...k.no>,
	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>,
	Alan Stern <stern@...land.harvard.edu>,
	Freddy Xin <freddy@...x.com.tw>
Subject: Re: [PATCH RFC 1/1] usb: Tell xhci when usb data might be misaligned

On Fri, Jan 31, 2014 at 08:17:58AM +0800, Ming Lei wrote:
> On Fri, Jan 31, 2014 at 6:15 AM, Sarah Sharp
> <sarah.a.sharp@...ux.intel.com> wrote:
> > On Thu, Jan 30, 2014 at 10:50:21PM +0100, Bjørn Mork wrote:
> >> FWIW, the plan looks fine to me.  Just adding a couple of hints to
> >> simplify the implementation.
> >>
> >> Sarah Sharp <sarah.a.sharp@...ux.intel.com> writes:
> >>
> >> > Let's do this fix the right way, instead of wall papering over the
> >> > issue.  Here's what we should do:
> >> >
> >> > 1. Disable scatter-gather for the ax88179_178a driver when it's under an
> >> >    xHCI host.
> >>
> >> No need to make this conditional.  SG is only enabled in the
> >> ax88179_178a driver if udev->bus->no_sg_constraint is true, so it
> >> applies only to xHCI hosts in the first place.
> >
> > Ah, so you're suggesting just reverting commit
> > 3804fad45411b48233b48003e33a78f290d227c8 "USBNET: ax88179_178a: enable
> > tso if usb host supports sg dma"?
> 
> If I understand the problem correctly, the current issue is that xhci driver
> doesn't support the arbitrary dma length not well, but per XHCI spec, it
> should be supported, right?
> 
> If the above is correct, reverting the commit isn't correct since there isn't
> any issue about the commit, so I suggest to disable the flag in xhci
> for the buggy devices, and it may be enabled again if the problem is fixed.

Ok, I like that plan, since it means I don't have to touch any
networking code to fix this. :)

I believe that means we'll have to disable the flag for all 1.0 xHCI
hosts, since those are the ones that need TD fragments.

> >> > 2. Revert the following commits:
> >> >    f2d9b991c549 xhci: Set scatter-gather limit to avoid failed block writes.
> >> >    d6c9ea9069af xhci: Avoid infinite loop when sg urb requires too many trbs
> >> >    35773dac5f86 usb: xhci: Link TRB must not occur within a USB payload burst
> >> >
> >> > 3. Dan and Mathias can work together to come up with an overall plan to
> >> >    change the xHCI driver architecture to be fully compliant with the TD
> >> >    fragment rules.  That can be done over the next few kernel releases.
> >> >
> >> > The end result is that we don't destabilize storage or break userspace
> >> > USB drivers, we don't break people's xHCI host controllers,
> >> > the ax88179_178a USB ethernet devices still work under xHCI (a bit with
> >> > worse performance), and other USB ethernet devices still get the
> >> > performance improvement introduced in 3.12.
> >>
> >> No other usbnet drivers has enabled SG...  Which is why you have only
> >> seen this problem with the ax88179_178a devices.  So there is no
> >> performance improvement to keep.
> 
> In my test environment, the patch does improve both throughput and
> cpu utilization, if you search the previous email for the patch, you can
> see the data.

Right, I did see the performance improvement note in that commit.  Do
you know if the ARM A15 dual core board was using a 0.96 xHCI host, or a
1.0 host?  You can find out by reloading the xHCI driver with dynamic
debugging turned on:

# sudo modprobe xhci_hcd dyndbg

and then look for lines like:

[25296.765767] xhci_hcd 0000:00:14.0: HCIVERSION: 0x100

Sarah Sharp
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