On 3/22/2018 4:45 PM, Casey Leedom wrote: > Yes, but ... > > For instance, I see that the x86 writel() has "memory" in its asm(), which > prevents GCC from reordering generated instructions. And it ~looks like~ > arm64 ~sort of~ gets that with the inclusion of __iowmb() (which translates > to wmb() then dsb(st) which finally holds the GCC "memory" barrier). Is > this part of the documented semantic of the writel_relaxed()? The PowerPC > stuff simply defines writel_relaxed() as writel() and I can't find the > bottom of that Rabbit Hole ... > This is changing. See "RFC on writel and writel_relaxed" thread. PowerPC maintainers are looking for a way to implement this. What matters is the description in the barriers document. See also section "MMIO access primitives" here about mmiowb() https://lwn.net/Articles/697539/ > I'm guessing~ that this line in the documentation ~may~ imply the GCC > ordering: > > ... Note that relaxed accesses to > the same peripheral are guaranteed to be ordered with respect to each > other. ... > This can be a compiler barrier for some arches and/or can be architecturally guaranteed as in ARM64's device nGnRE mapping (non-gathering non-reordering with early acknowledgment). Both writel() and writel_relaxed() need to guarantee ordering with respect to what HW observes for writes. They have different guarantees regarding the code surrounding write like you identified. > In any case, we really only have a few places where we (the various Chelsio > drivers) need to worry about this: the "Fast Paths" where we have a lot of > I/O to the device. I think we should leave everything else alone. makes sense > > Casey > -- 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
