Re: [PATCH] MIPS: cavium_octeon: Fix syncw generation.

[Mark Tomlinson]

Hi Phil,

On Mon, 2020-02-17 at 01:22 +0100, Philippe Mathieu-Daudà wrote:
> Hi Mark,
> 
> On Tue, Feb 11, 2020 at 10:42 PM Mark Tomlinson
> <mark.tomlinson@xxxxxxxxxxxxxxxxxxx> wrote:
> > 
> > The Cavium Octeon CPU uses a special sync instruction for implementing
> > wmb, and due to a CPU bug, the instruction must appear twice. A macro
> > had been defined to hide this:
> > 
> >  #define __SYNC_rpt(type)     (1 + (type == __SYNC_wmb))
> > 
> > which was intended to evaluate to 2 for __SYNC_wmb, and 1 for any other
> > type of sync. However, this expression is evaluated by the assembler,
> > and not the compiler, and the result of '==' in the assembler is 0 or
> > -1, not 0 or 1 as it is in C. The net result was wmb() producing no code
> > at all. The simple fix in this patch is to change the '+' to '-'.
> 
> Isn't this particular to the assembler implementation?
> Can you explicit the assembler you are using in the commit description?
> Assuming we have to look at your commit in 3 years from now, we'll
> wonder what assembler you were using.
> 
> Thanks,
> 
> Phil.

Yes, it is tied to the assembler. But the Linux kernel is tied to GCC,
and GCC (I believe) is tied to GNU as. I can't see the specification of
GNU as changing, since that could break anything written for it.


> > Fixes: bf92927251b3 ("MIPS: barrier: Add __SYNC() infrastructure")
> > Signed-off-by: Mark Tomlinson <mark.tomlinson@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  arch/mips/include/asm/sync.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/mips/include/asm/sync.h b/arch/mips/include/asm/sync.h
> > index 7c6a1095f5..aabd097933 100644
> > --- a/arch/mips/include/asm/sync.h
> > +++ b/arch/mips/include/asm/sync.h
> > @@ -155,9 +155,11 @@
> >   * effective barrier as noted by commit 6b07d38aaa52 ("MIPS: Octeon: Use
> >   * optimized memory barrier primitives."). Here we specify that the affected
> >   * sync instructions should be emitted twice.
> > + * Note that this expression is evaluated by the assembler (not the compiler),
> > + * and that the assembler evaluates '==' as 0 or -1, not 0 or 1.
> >   */
> >  #ifdef CONFIG_CPU_CAVIUM_OCTEON
> > -# define __SYNC_rpt(type)      (1 + (type == __SYNC_wmb))
> > +# define __SYNC_rpt(type)      (1 - (type == __SYNC_wmb))
> >  #else
> >  # define __SYNC_rpt(type)      1
> >  #endif
> > --
> > 2.25.0
> >