[PATCH v7 0/7] netdev: intel: Eliminate duplicate barriers on weakly-ordered archs

On 3/27/2018 12:54 PM, Jeff Kirsher wrote:

On Tue, 2018-03-27 at 08:42 -0400, Sinan Kaya wrote:

quoted

On 3/23/2018 10:34 PM, okaya at codeaurora.org wrote:

quoted

On 2018-03-23 19:58, Jeff Kirsher wrote:

quoted

On Fri, 2018-03-23 at 14:53 -0700, Alexander Duyck wrote:

quoted

On Fri, Mar 23, 2018 at 11:52 AM, Sinan Kaya <okaya@codeaurora.
org>
wrote:

quoted

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.

I looked over the set and they seem good.

Reviewed-by: Alexander Duyck <redacted>

Grrr, patch 1 does not apply cleanly to my next-queue tree (dev-
queue
branch).  I will deal with this series in a day or two, after I
have dealt
with my driver pull requests.

Sorry, you will have to replace the ones you took from me.

Double sorry now.

I don't know if you have been following "RFC on writel and
writel_relaxed" thread
or not but there are some new developments about wmb() requirement. 

Basically, wmb() should never be used before writel() as writel()
seem to
provide coherency and observability guarantee.

wmb()+writel_relaxed() is slower on some architectures than plain
writel()

I'll have to rework these patches to have writel() only. 

Are you able to drop the applied ones so that I can post V8 or is it
too late?

Currently I do not have any of your patches applied to my next-queue
tree (dev-queue branch).  So feel free to do any revisions you need to
do and to re-submit to intel-wired-lan at lists.osuosl.org (IWL) mailing
list.

Thanks, Good to know. 

I'm waiting for the discussion to settle. I'll update as soon as I get
a clear direction.

-- 
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.