On Sat, Aug 27, 2011 at 10:48:35PM +0800, ming.lei@xxxxxxxxxxxxx wrote: > From: Ming Lei <ming.lei@xxxxxxxxxxxxx> > > This patch fixs one performance bug on ARM Cortex A9 dual core platform, > which has been reported on quite a few ARM machines(OMAP4, Tegra 2, snowball...), > see details from link of https://bugs.launchpad.net/bugs/709245. > > In fact, one mb() on ARM is enough to flush L2 cache, but > 'dummy->hw_token = token;' after mb() is added just for obeying > correct mb() usage. Really? A mb() should not be flushing any caches, it's just a memory barrier. Or is ARM somehow "special" in this way? > The patch has been tested ok on OMAP4 panda A1 board, the performance > of 'dd' over usb mass storage can be increased from 4~5MB/sec to > 14~16MB/sec after applying this patch. That's impressive, but I don't think this is really the proper way to do this... > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx> > --- > drivers/usb/host/ehci-q.c | 14 ++++++++++++++ > 1 files changed, 14 insertions(+), 0 deletions(-) > > diff --git a/drivers/usb/host/ehci-q.c b/drivers/usb/host/ehci-q.c > index 0917e3a..65b5021 100644 > --- a/drivers/usb/host/ehci-q.c > +++ b/drivers/usb/host/ehci-q.c > @@ -1082,6 +1082,20 @@ static struct ehci_qh *qh_append_tds ( > wmb (); > dummy->hw_token = token; > > + /* The mb() below is added to make sure that > + * 'token' can be writen into qtd, so that ehci > + * HC can see the up-to-date qtd descriptor. On > + * some archs(at least on ARM Cortex A9 dual core), > + * writing into coherenet memory doesn't mean the > + * value written can reach physical memory > + * immediately, and the value may be buffered > + * inside L2 cache. 'dummy->hw_token = token;' > + * after mb() is added for obeying correct mb() > + * usage. > + * */ > + mb(); > + token = dummy->hw_token; Your comment does not match the code, so something is wrong here. greg k-h -- 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
