Re: [RFC PATCH v6] Documentation/arch: Add Documentation/arch-features.txt

Ingo Molnar
2015-05-14T10:02:40Z

* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 13 May 2015 09:27:57 -0700 Josh Triplett <josh@joshtriplett.org> wrote:
>
> > If we can't generate this, then the ASCII-art style and right-aligned
> > feature names seems *really* likely to produce spurious conflicts,
> > especially when adding a feature to the list. Even though it would
> > produce a much longer file, would you consider dropping the tables and
> > just having a section per feature?
>
> me2. The patch conflicts are going to be pretty bad.
>
> I'd also prefer a format which allows us to add useful notes - it's
> a bit hostile to say "thou shalt implement X" without providing any
> info about how to do so. Where do we tell maintainers that there's
> a handy test app in tools/testing/selftests which they should use?
>
> This way, I can bug patch submitters with "hey, you forgot to update
> Documentation/arch-features.txt" and they will add useful info while
> it's all still hot in their minds.

Ok, agreed, I've solved these problems by creating a per feature
broken out directory hieararchy, see my next patch submission.

> And there's a ton of stuff which can go in here, much of it not
> immediately apparent.

Yes.

> Just grepping 9 months worth of the stuff I've handled, I'm seeing
> things like
>
> HAVE_ARCH_KASAN

Ok, added.

> __HAVE_ARCH_PMDP_SPLITTING_FLUSH

Ok, added.

> __HAVE_ARCH_PTE_SPECIAL

Ok, added.

> __HAVE_ARCH_GATE_AREA

So this does not appear to be a feature per se: architectures that
don't define __HAVE_ARCH_GATE_AREA fall back to the generic one. Or is
it expected for every architecture to provide its own?

> ARCH_HAVE_ELF_ASLR

Does not seem to be upstream.

> ARCH_HAS_GCOV_PROFILE_ALL

Yeah, that's already included in v6.

> CONFIG_ARCH_USE_BUILTIN_BSWAP

So AFAICS this feature is an arch opt-in, on the basis of whether GCC
does the right thing or not.

We'd need a separate config switch: ARCH_DONT_USE_BUILTIN_BSWAP to
make a distinction between architectures that have made an informed
decision to not support it, versus architectures that have not
bothered so far.

> HAVE_ARCH_HUGE_VMAP

Ok, added.

> ARCH_HAS_SG_CHAIN

Ok, added.

> __HAVE_ARCH_STRNCASECMP

So, no architecture supports this yet- but added.

> ARCH_HAS_ELF_RANDOMIZE

Agreed and v6 already includes this.

> CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID

So this isn't really a feature, but a facility that an architecture
might have to provide if it has a quirk. Only ia64 has that at the
moment.

> ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT

Not upstream yet it appears.

> CONFIG_ARCH_USES_PG_UNCACHED

Ok, added.

> CONFIG_ARCH_HAS_WALK_MEMORY

So this too is a quirk, for PowerPC, which does not maintain the
memory layout in the resource tree.

> and things which don't contain ARCH
>
> HAVE_GENERIC_RCU_GUP

So this is a generic, RCU based fast-GUP facility - but architectures
may implement their own get_user_pages_fast() facilites, such as x86
which does it lock-less.

So I'm not sure what to flag here: perhaps architectures that don't
offer get_user_pages_fast() at all?

> HAVE_RCU_TABLE_FREE

So this is related to HAVE_GENERIC_RCU_GUP: architectures that do RCU
based GUP will want to use HAVE_RCU_TABLE_FREE.

> HAVE_GENERIC_RCU_GUP

double ;-)

> CONFIG_HAVE_CLK

So I think the generic clock framework first needs to be integrated
with core timekeeping before we start requiring it from architectures.

> CONFIG_HAVE_IOREMAP_PROT

Agreed - already in -v6.

> CONFIG_HAVE_MEMBLOCK_NODE_MAP

Ok, added.

> And then there's the increasingly common
>
> arch/include/asm/foo.h:
>
> static inline void wibble(...)
> {
> ...
> }
> #define wibble wibble
>
> include/linux/foo.h:
>
> #ifndef wibble
> static inline void wibble(...)
> {
> ...
> }
> #define wibble
> #endif
>
> which is going to be hard to grep for....

Hm, yes. If you give me a rough list then I can try to map them out as
well.

Once we have the initial feature collection done it will be a lot
easier going forward: anything missing or inaccurate can be added to
or fixed in its own file.

> ugh, this thing's going to be enormous. People will go insane
> reading it, so each section should have a sentence describing what
> the feature does so maintainers can make quick decisions about
> whether they should bother.

I hope you'll like the structure of -v7 better :-)

Thanks,

Ingo