On 3/16/2018 7:05 PM, Steve Wise wrote: >> >> On 3/16/2018 5:05 PM, Steve Wise wrote: >>>> Code includes wmb() followed by writel(). 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(). >>>> >>>> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx> >>> >>> NAK - This isn't correct for PowerPC. For PowerPC, writeX_relaxed() is just >>> writeX(). >>> >>> I was just looking at this with Chelsio developers, and they said the >>> writeX() should be replaced with __raw_writeX(), not writeX_relaxed(), to >>> get rid of the extra barrier for all architectures. >> >> OK. I can do that but isn't the problem at PowerPC adaptation? >> >> /* >> * We don't do relaxed operations yet, at least not with this semantic >> */ >> #define readb_relaxed(addr) readb(addr) >> #define readw_relaxed(addr) readw(addr) >> #define readl_relaxed(addr) readl(addr) >> #define readq_relaxed(addr) readq(addr) >> #define writeb_relaxed(v, addr) writeb(v, addr) >> #define writew_relaxed(v, addr) writew(v, addr) >> #define writel_relaxed(v, addr) writel(v, addr) >> #define writeq_relaxed(v, addr) writeq(v, addr) >> >> Why don't we fix the PowerPC's relaxed operators? Is that a bigger task? > > I don't know the answer, but perhaps the proper fix is to correctly implement these for PPC? > I found this article: https://lwn.net/Articles/698014/#PowerPC%20Implementation Apparently, this issue was discussed at a conference in 2015. Based on how I read this article, writel() and writel_relaxed() behave exactly the same on PowerPC because the barrier is enforced by the time code is leaving a critical section not during MMIO execution. I also see that __raw_writel() is being heavily used in drivers directory. https://elixir.bootlin.com/linux/latest/ident/__raw_writel I'll change writel_relaxed() with __raw_writel() in the series like you suggested and also look at your other comments. This seems to be the current norm. > >> >> >From API perspective both __raw_writeX() and writeX_relaxed() are >> correct. >> It is just PowerPC doesn't seem the follow the definition yet. > > > > -- 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-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
