From: Sinan Kaya <okaya@xxxxxxxxxxxxxx> Date: Mon, 19 Mar 2018 22:42:15 -0400 > 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. I agree that for performance sensitive operations, specifically writing doorbell registers in the hot paths or RX and TX packet processing, this is a good change. However, in configuration paths and whatnot, it is much less urgent and useful. Therefore I think it would work better if you concentrated solely on hot code path cases. You can, on a driver by driver basis, submit the other transformations in the slow paths, and let the driver maintainers decide whether to take those on or not. Also, please stick exactly to the case where we have: wmb/mb/etc. writel() Because I see some changes where we have: writel() barrier() writel() for exmaple, and you are turning that second writel() into a relaxed on as well. The above is using a compile barrier, not a memory barrier, so effectively it is two writel()'s in sequence which is not what this patch set is about. If anything, that compile barrier() is superfluous and could be removed. But that is also a separate change from what this patch series is doing here. Finally, it makes it that much easier if we can see the preceeding memory barrier in the context of the patch that adjusts the writel into a writel_relaxed. In one case, a macro DOORBELL() is changed to use writel(). This makes it so that the patch reviewer has to scan over the entire driver in question to see exactly how DOORBELL() is used and whether it fits the criteria for the writel_relaxed() transformation. I would suggest that you adjust the name of the macro in a situation like this, f.e. to DOORBELL_RELAXED(). Thank you. -- 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
