Hi, On Fri, Sep 2, 2011 at 9:58 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Fri, 2 Sep 2011, Ming Lei wrote: > >> On Wed, Aug 31, 2011 at 11:05 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: >> > >> > You might want to remove also the first wmb() in qh_append_tds(). >> >> I think the first wmb() is used to order the two writing: >> >> qtd->hw_token = HALT_BIT(ehci); >> wmb () >> *dummy = *qtd; >> >> Since dummy has been in hw queue already and must see halt bit of hw_token >> first, the barrier should be kept, otherwise the transaction represented by qtd >> may be executed too early and qtd->hw_next is still not ready. > > The transaction represented by qtd can't be executed before the > transaction represented by dummy, since qtd gets spliced at the end of > the transaction list (it becomes the new dummy). The transaction > represented by dummy won't be executed until after dummy->hw_token is I understand dummy is already linked in ehci hw queue, so if ' *dummy = *qtd;' is executed before 'qtd->hw_token = HALT_BIT(ehci);', dummy will be executed by EHCI. That is the reason why I think the 1st wmb() shouldn't be removed, right? > set to token, and there's already a memory barrier before that > instruction. This memory barrier will insure that qtd->hw_token is > ready in time. thanks, -- Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
