[Ksummit-discuss] [TECH TOPIC] Semantics of MMIO mapping attributes accross archs

[Dan Williams]

On Sat, Jul 4, 2015 at 1:17 AM, Benjamin Herrenschmidt
<benh at kernel.crashing.org> wrote:
> Allright, it's that time of year ... So here's my attempt at getting
> myself invited :-)
>
> We've been talking about some of that on-list recently, and it might
> well be that this will trigger a resolution before we even reach KS, but
> I though it might be worthwhile to gather enough people from various
> arch together to hash things out:
>
> We have a pile of mapping attributes (more showing up recently),
> typically used for MMIO mappings (but not necessarily exclusively).
> ioremap_cache and ioremap_nocache are the old/common ones, but we have
> _wc (write combine), _wt (write through) and possibly more around the
> corner.
>
> What are their precise semantics accross all architecture ? This is not
> clear (not documented). For example, we define writel(), readl() and
> friends as being fully ordered vs each other but also vs DMA etc... but
> on what mapping types do they have this property ?
>
> Will _wc() provide the write-combine ability for writel() on all archs ?
> Or does it require writel_relaxed() on some ? Will _wc() bring other
> side effects such as loss of read vs. write ordering ? (on some archs at
> least...). Etc....
>
> There is a growing matrix of MMIO accessors and mapping types whose
> semantics are poorly (if at all) defined. We cannot define them all
> exactly for all architectures as there are too many differences that
> will impact them. But we should be able to guarantee at least *some*,
> ie, whether a given type of ordering is guaranteed or not by a given
> accessor on a given mapping type, whether write combine (if supported at
> all) will happen with a given accessor or not etc...
>
> As for who should participate, well, at least one rep from each major
> arch who is familiar with the intricacies of the architecture memory
> model I would say, possibly others who dabbled in that stuff recently
> such as Luis R. Rodriguez <mcgrof at suse.com> who was proposing patch
> series lately to consolidate the use of _wc.
>

Another side topic that has come up in this space is the desire to
define a "memremap" api to clean up __iomem abuses for cases where
"memory-like" mappings are needed.

https://lkml.org/lkml/2015/6/22/100