Hi, On Sat, Aug 27, 2011 at 11:13 PM, Greg KH <greg@xxxxxxxxx> wrote: > 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? As Santosh pointed out, mb on ARM will flush L2 write buffer. The description here is wrong. I think the below should make the writing reach into memory on all ARCH after ' token = dummy->hw_token;' is executed. dummy->hw_token = token; mb() token = dummy->hw_token; The above is the idea introduced to fix the problem. > >> 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. If you mean "L2 cache flush", I confess to the mistaken description, and will update it later. If you mean others, could you help to point it out? 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
