On 19. 4. 16. 오후 10:57, Dmitry Osipenko wrote: > 16.04.2019 11:00, Chanwoo Choi пишет: >> Hi, >> >> On 19. 4. 15. 오후 11:54, Dmitry Osipenko wrote: >>> The write memory barrier isn't needed because the BUS buffer is flushed >>> by read after write that happens after the removed wmb(), we will also >>> use readl() instead of the relaxed version to ensure that read is indeed >>> completed. >>> >>> Signed-off-by: Dmitry Osipenko <[email protected]> >>> --- >>> drivers/devfreq/tegra-devfreq.c | 3 +-- >>> 1 file changed, 1 insertion(+), 2 deletions(-) >>> >>> diff --git a/drivers/devfreq/tegra-devfreq.c >>> b/drivers/devfreq/tegra-devfreq.c >>> index d62fb1b0d9bb..f0f0d78f6cbf 100644 >>> --- a/drivers/devfreq/tegra-devfreq.c >>> +++ b/drivers/devfreq/tegra-devfreq.c >>> @@ -243,8 +243,7 @@ static void tegra_devfreq_update_wmark(struct >>> tegra_devfreq *tegra, >>> static void actmon_write_barrier(struct tegra_devfreq *tegra) >>> { >>> /* ensure the update has reached the ACTMON */ >>> - wmb(); >>> - actmon_readl(tegra, ACTMON_GLB_STATUS); >>> + readl(tegra->regs + ACTMON_GLB_STATUS); >> >> I think that this meaning of actmon_write_barrier() keeps >> the order of 'store' assembly command without the execution change >> from compiler optimization by using the wmb(). > > The IO mapped memory is strongly-ordered on ARM, hence all readl/writel > accesses are guaranteed to be ordered by default. I think wmb() here is just > a cut-n-pasted relic from old downstream driver.
OK. > >> But, this patch edits it as following: >> The result of the following two cases are same? >> >> [original code] >> wmb() >> read_relaxed() >> >> [new code by this patch] >> readl_relaxed() >> rmb() > > Yes, the result is the same. The wmb() is not just about IO accesses, but > about all kind of memory accesses and at least on Tegra30 it is quite > expensive operation because it translates into L2XO cache syncing > (arm_heavy_mb) + dsb(). It should be more efficient to flush out writes with > a read-back and then wait for that read operation to be completed. > > OK. Thanks for explanation. Reviewed-by: Chanwoo Choi <[email protected]> -- Best Regards, Chanwoo Choi Samsung Electronics
