Hi Will,
On Mon, Sep 25, 2017 at 5:21 PM, Will Deacon <[email protected]> wrote:
> On Mon, Sep 25, 2017 at 09:16:22AM +0200, Geert Uytterhoeven wrote:
>> On Wed, Jul 12, 2017 at 7:16 PM, Linux Kernel Mailing List
>> <[email protected]> wrote:
>> > Web:
>> > https://git.kernel.org/torvalds/c/c1004803b40596c1aabbbc78a6b1b33e4dfd96c6
>> > Commit: c1004803b40596c1aabbbc78a6b1b33e4dfd96c6
>> > Parent: 58188afeb727e0f73706f1460707bd3ba6ccc221
>> > Refname: refs/heads/master
>> > Author: Will Deacon <[email protected]>
>> > AuthorDate: Fri Jun 23 11:45:57 2017 +0100
>> > Committer: Will Deacon <[email protected]>
>> > CommitDate: Fri Jun 23 17:58:02 2017 +0100
>> >
>> > iommu/io-pgtable: depend on !GENERIC_ATOMIC64 when using COMPILE_TEST
>> > with LPAE
>> >
>> > The LPAE/ARMv8 page table format relies on the ability to read and
>> > write
>> > 64-bit page table entries in an atomic fashion. With the move to a
>> > lockless
>> > implementation, we also need support for cmpxchg64 to resolve races
>> > when
>> > installing table entries concurrently.
>> >
>> > Unfortunately, not all architectures support cmpxchg64, so the code can
>> > fail to compiler when building for these architectures using
>> > COMPILE_TEST.
>> > Rather than disable COMPILE_TEST altogether, instead check that
>> > GENERIC_ATOMIC64 is not selected, which is a reasonable indication that
>> > the architecture has support for 64-bit cmpxchg.
>> >
>> > Reported-by: kbuild test robot <[email protected]>
>> > Signed-off-by: Will Deacon <[email protected]>
>> > ---
>> > drivers/iommu/Kconfig | 2 +-
>> > 1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>> > index 6ee3a25ae731..c88cfa7522b2 100644
>> > --- a/drivers/iommu/Kconfig
>> > +++ b/drivers/iommu/Kconfig
>> > @@ -23,7 +23,7 @@ config IOMMU_IO_PGTABLE
>> > config IOMMU_IO_PGTABLE_LPAE
>> > bool "ARMv7/v8 Long Descriptor Format"
>> > select IOMMU_IO_PGTABLE
>> > - depends on HAS_DMA && (ARM || ARM64 || COMPILE_TEST)
>> > + depends on HAS_DMA && (ARM || ARM64 || (COMPILE_TEST &&
>> > !GENERIC_ATOMIC64))
>> > help
>> > Enable support for the ARM long descriptor pagetable format.
>> > This allocator supports 4K/2M/1G, 16K/32M and 64K/512M page
>>
>> I can't find where this patch was submitted and discussed, so I'm replying
>> to this email. On which architectures did it fail to compile?
>
> It was in response to a report from the kbuild test robot on m32r, but
> looking back I now see that it didn't go to the lists for some reason. I've
> included the report at the end of this email so you can have a look.
Thanks!
>> cmpxchg64() is defined by include/asm-generic/cmpxchg.h, so I fail to
>> see what's the relation with GENERIC_ATOMIC64, which is related to
>> lib/atomic64.c instead.
>
> Yeah, it's a bit of a hack, but we're basically relying on architectures
> that don't select GENERIC_ATOMIC64 providing their own cmpxchg64
> implementation, which seems to be the case.
>
>> E.g. on m68k, which uses GENERIC_ATOMIC64, it compile-tested fine before.
>
> FWIW, the lock-based atomics wouldn't be sufficient at runtime, but I
> appreciate that we're only talking about COMPILE_TEST here.
Correct.
>> Perhaps there's another (SMP vs UP?) dependency, as
>> include/asm-generic/cmpxchg.h cannot be used on SMP?
>> Should it be COMPILE_TEST && (!GENERIC_ATOMIC64 || !SMP)?
>
> I don't see how that helps. Are you seeing build failures on a non-SMP
> arch?
I'm not seeing build failures.
I see reduced build coverage on m68k due to this change.
So m32r fails to provide a definition for cmpxchg64(), which has nothing to
do with GENERIC_ATOMIC64.
Probably m32r just needs the same treatment as in commit feb20ec2bb859d1d
("m68k: Add missing cmpxchg64() if CONFIG_RMW_INSNS=y")
Worse, people are now adding more depends on !GENERIC_ATOMIC64 for
various IOMMU drivers, to fix up "selects IOMMU_IO_PGTABLE_LPAE which
has unmet direct dependencies" warnings :-(
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
