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

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Date:	Fri, 31 Jan 2014 08:17:58 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Sarah Sharp <sarah.a.sharp@...ux.intel.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 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.

>
>> > 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.

Thanks,
--
Ming Lei
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