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

From: Eric Dumazet <eric.dumazet-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
To: Ming Lei <tom.leiming-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: "Ben Hutchings"
	<bhutchings-s/n/eUQHGBpZroRs9YW3xA@public.gmane.org>,
	"Oliver Neukum" <oliver-GvhC2dPhHPQdnm+yROfE0A@public.gmane.org>,
	"Grant Grundler"
	<grundler-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
	"Freddy Xin" <freddy-knRN6Y/kmf1NUHwG+Fw1Kw@public.gmane.org>,
	"David Miller" <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>,
	netdev <netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	linux-usb-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"ASIX Louis [蘇威陸]"
	<louis-knRN6Y/kmf1NUHwG+Fw1Kw@public.gmane.org>,
	"Allan Chou" <allan-knRN6Y/kmf1NUHwG+Fw1Kw@public.gmane.org>
Subject: Re: [PATCH 1/1] TX throttling bug-fixing patch of AX88179_178A
Date: Thu, 25 Jul 2013 04:01:41 -0700	[thread overview]
Message-ID: <1374750101.23313.16.camel@edumazet-glaptop> (raw)
In-Reply-To: <CACVXFVNyFk0Zpi4tC0ENiq8RcrtrjYW-Dhs+A56+Pw4BKm8y8A-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

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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