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

Eric Dumazet
2013-07-25T11:01:41Z

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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> 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:
> >>
> >> http://git.kernel.org/cgit/linux/kernel/git/gregkh/usb.git/commit/?h=usb-next&id=10e232c597ac757e7f8600649f7e872e86de190f
> >>
> >> 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...

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.

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