Hi, On Saturday 27 August 2011 08:18 PM, 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.
Who said "one mb() on ARM is enough to flush L2 cache" ? It's just a memory barrier and it doesn't flush any cache. What it cleans is the CPU write buffers and the L2 cache write buffers.
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.
Though number looks great, how is the below patch helping to get better numbers.
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; +
This patch at max fix some corruption if the memory buffer used is buffer-able. Infact I see there is already a write memory barrier above. So just pushing that down by one line should be enough. > dummy->hw_token = token; > wmb (); Is there another patch along with this which removes, some cache clean on this buffer ? Regards Santosh -- 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
