Lino Sanfilippo <linosanfili...@gmx.de> : > On 21.05.2016 21:47, Francois Romieu wrote: > > Shuyu Wei <wsy2...@gmail.com> : > > [...] > >> diff --git a/drivers/net/ethernet/arc/emac_main.c > >> b/drivers/net/ethernet/arc/emac_main.c > >> index a3a9392..c2447b0 100644 > >> --- a/drivers/net/ethernet/arc/emac_main.c > >> +++ b/drivers/net/ethernet/arc/emac_main.c > >> @@ -686,6 +686,9 @@ static int arc_emac_tx(struct sk_buff *skb, struct > >> net_device *ndev) > >> > >> skb_tx_timestamp(skb); > >> > >> + /* Make sure timestamp is set */ > >> + smp_wmb(); > > > > Should be dma_wmb() (see davem's message). > > > > It's completely unrelated to SMP. > > > > Its also completely unrelated to dma so I doubt that this is what davem meant.
It's related to dma: nobody wants the device to perform dma from memory while the CPU is writing timestamp. The device must enforce a commit of any network buffer memory write before releasing control, i.e. before writing *info. See 'git log -p --grep=dma_[rw]mb': it appears several times. > As far as I understood he was referring to the dma descriptor. If the wmb() between *info = ... and *txbd_curr = ... is replaced by a dma_wmb(), the device will see a consistent descriptor when if performs a DMA read after the CPU wrote into the mailbox (arc_reg_set(..., TXPL_MASK)). Ok, I agree on this one. However, it doesn't help with the (SMP) requirement that no CPU sees the txbd_curr write before *info is written by the local CPU. Afaiui dma_wmb() is too weak for this part. If we don't want wmb() here, it will have to be dma_wmb() + smp_wmb(). -- Ueimor
