Dear Jakub, Thanks for your comments~ On Mon, 2022-03-14 at 08:57 -0700, Jakub Kicinski wrote: > On Mon, 14 Mar 2022 15:01:23 +0800 Biao Huang wrote: > > > Drivers are expected to stop their queues at the end of xmit > > > routine > > > if > > > the ring can't accommodate another frame. It's more efficient to > > > stop > > > the queues early than have to put skbs already dequeued from the > > > qdisc > > > layer back into the qdiscs. > > > > Yes, if descriptors ring is full, it's meaningful to stop the > > queue > > at the end of xmit; > > But driver seems hard to know how many descriptors the next skb > > will > > request, e.g. 3 descriptors are available for next round send, but > > the > > next skb may need 4 descriptors, in this case, we still need judge > > whether descriptors are enough for skb transmission, then decide > > stop > > the queue or not, at the beginning of xmit routine. > > > > Maybe we should judge ring is full or not at the beginning and the > > end > > of xmit routine(seems a little redundancy). > > Assume the worst case scenario. You set the default ring size to 512, > skb can have at most MAX_SKB_FRAGS fragments (usually 17) so the max > number of descriptors should not be very high, hard to pre-compute, > or problematic compared to the total ring size. Yes, we'll check the available descriptor number at the end of xmit routine, and ensure it will larger than (MAX_SKB_FRAGS + 1) in next send. (refer to stmmac_main.c) Regards!
