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

[Andy Lutomirski <luto@amacapital.net>]

On Sun, Jul 5, 2015 at 12:56 PM, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
> 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 think it should mean pgprot_noncached on all arches.  On x86, if it
does, it's purely by luck, since it only sets bits and doesn't clear
them.

>
>> 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.

Agreed.

--Andy