On Thu, 2013-07-25 at 13:25 +0800, Ming Lei wrote: > On Thu, Jul 25, 2013 at 1:10 PM, Eric Dumazet <eric.duma...@gmail.com> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
