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

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Sun, 2015-07-05 at 11:55 -0700, Andy Lutomirski wrote:
> 
> At some point, it would also be nice if the various macros has
> well-defined semantics.  For example, x86 has:
> 
> #define pgprot_noncached(prot)                                          \
>         ((boot_cpu_data.x86 > 3)                                        \
>          ? (__pgprot(pgprot_val(prot) |                                 \
>                      cachemode2protval(_PAGE_CACHE_MODE_UC_MINUS)))     \
>          : (prot))
> 
> Putting aside the pointless boot_cpu_data check (surely the recent PAT
> rework completely obsoletes it), what is
> pgprot_noncached(pgprot_writecombine(x)) supposed to do?  Currently it
> results in garbage.  Should it have well-defined behavior instead?

Can it ? On powerpc it will just mean pgprot_noncached for example,
those macros manipulate the same bits and it's not a bitmask, it's
either unached or uncached with write combining.

> I suspect the other arches all have their own unique glitches here.

Correct. I'm still trying to get feedback on ARM for example.

I don't think we can (or should try) to have completely identical semantics
for everything, but we should try to find the common set that are guaranteed
and, possibly, do a best effort for archs to individually document the
remaining.

Ben.