Re: [PATCH 1/1] TX throttling bug-fixing patch of AX88179_178A

From: Ming Lei
Date: Thu Jul 25 2013 - 10:52:20 EST

On Thu, Jul 25, 2013 at 7:01 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> On Thu, 2013-07-25 at 13:25 +0800, Ming Lei wrote:
>> On Thu, Jul 25, 2013 at 1:10 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>> > On Thu, 2013-07-25 at 10:28 +0800, Ming Lei wrote:
>> >
>> >>
>> >> It depends if size of sg buffer(except for last one) in the sg list can be
>> >> divided by usb endpoint's max packet size(512 or 1024), at least there
>> >> is the constraint:
>> >>
>> >>
>> >>
>> >> I am wondering if network stack can meet that. If not, it might be a
>> >> bit difficult
>> >> because lots of USB host controller don't support that, and driver may have
>> >> to support SG and non-SG at the same time for working well on all HCs.
>> >
>> > I do not see the problem.
>> >
>> > If one skb has 2 fragments of 32KB, couldn't they be split into 64 1K
>> > segments by the device driver ?
>> OK, if length of fragments of all SKBs from network stack can always guarantee
>> to be divided by 1024, that is fine, seems I worry about too much, :-)
> Unfortunately, there is no such guarantee. TSO permits sendfile() zero
> copy operation, so the frags can be of any size, any offset...

USB protocol doesn't care offset or buffer start address, but has requirement
on sg buffer size(except for last one) in sg list.

> In this mode, the first element (skb->head) will typically contains the
> headers, and there are way below 512 bytes.
> So even with lowering netdev->gso_max_size under PAGE_SIZE, most of the
> packets will need to be copied into a single segment.

Maybe need to try it with TSO enabled, in my test on ax88179_178a NIC after
applying your disabling TSO patch, tx throughput is less than 600Mbps, but rx
is close to 900Mbps.

On Thu, Jul 25, 2013 at 9:34 PM, Ben Hutchings
<bhutchings@xxxxxxxxxxxxxx> wrote:
> Not that I have any experience with USB drivers, but perhaps
> usb_sg_init()?

USB SG library doesn't support submitting SG URB asynchronously, but that isn't
a big deal. The problem is that many USB host controllers can't build
one single
packet from two buffers, what is why USB stack requires size of all
for last one) in sg list can be divided by max endpoint packet
size.(1024 for usbnet)

Ming Lei
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