On 4/3/2018 1:47 PM, Alexander Duyck wrote: > On Mon, Apr 2, 2018 at 7:59 PM, Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote: >> Alex, >> >> On 4/2/2018 3:06 PM, Sinan Kaya wrote: >>> Code includes wmb() followed by writel() in multiple places. writel() >>> already has a barrier on some architectures like arm64. >>> >>> This ends up CPU observing two barriers back to back before executing the >>> register write. >>> >>> Since code already has an explicit barrier call, changing writel() to >>> writel_relaxed(). >>> >>> I did a regex search for wmb() followed by writel() in each drivers >>> directory. I scrubbed the ones I care about in this series. >>> >>> I considered "ease of change", "popular usage" and "performance critical >>> path" as the determining criteria for my filtering. >>> >>> We used relaxed API heavily on ARM for a long time but >>> it did not exist on other architectures. For this reason, relaxed >>> architectures have been paying double penalty in order to use the common >>> drivers. >>> >>> Now that relaxed API is present on all architectures, we can go and scrub >>> all drivers to see what needs to change and what can remain. >>> >>> We start with mostly used ones and hope to increase the coverage over time. >>> It will take a while to cover all drivers. >>> >>> Feel free to apply patches individually. >>> >>> Change since 7: >>> >>> * API clarification with Linus and several architecture folks regarding >>> writel() >>> >>> https://www.mail-archive.com/netdev@xxxxxxxxxxxxxxx/msg225806.html >>> >>> * removed wmb() in front of writel() at several places. >>> * remove old IA64 comments regarding ordering. >>> >> >> What do you think about this version? Did I miss any SMP barriers? > > I would say we should probably just drop the whole set and start over. > If we don't need the wmb() we are going to need to go through and > clean up all of these paths and consider the barriers when updating > the layout of the code. > > For example I have been thinking about it and in the case of x86 we > are probably better off not bothering with the wmb() and > writel_relaxed() and instead switch over to the smp_wmb() and writel() > since in the case of a strongly ordered system like x86 or sparc this > ends up translating out to a couple of compile barriers. > > I will also need time to reevaluate the Rx paths since dropping the > wmb() may have other effects which I need to verify. Sounds good, I'll let you work on it. @Jeff Kirsher: can you drop this version from your development branch until Alex posts the next version? > > Thanks. > > - Alex > -- Sinan Kaya Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project. -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
