RE: [PATCH 3/7] microblaze: Define SMP safe bit operations

Stefan Asserhall
2020-02-13T09:01:26Z

> On 12. 02. 20 16:53, Peter Zijlstra wrote:
> > On Wed, Feb 12, 2020 at 04:42:25PM +0100, Michal Simek wrote:
> >> From: Stefan Asserhall <stefan.asserhall@xilinx.com>
> >>
> >> For SMP based system there is a need to have proper bit operations.
> >> Microblaze is using exclusive load and store instructions.
> >>
> >> Signed-off-by: Stefan Asserhall <stefan.asserhall@xilinx.com>
> >> Signed-off-by: Michal Simek <michal.simek@xilinx.com>
> >
> >> +/*
> >> + * clear_bit doesn't imply a memory barrier */
> >> +#define smp_mb__before_clear_bit() smp_mb()
> >> +#define smp_mb__after_clear_bit() smp_mb()
> >
> > These macros no longer exist.
>
> ok. Easy to remove.
>
> >
> > Also, might I draw your attention to:
> >
> > include/asm-generic/bitops/atomic.h
> >
> > This being a ll/sc arch, I'm thinking that if you do your atomic_t
> > implementation right, the generic atomic bitop code should be near
> > optimal.
> >
>
> Based on my look it looks like that I can replace implementations in this file by
> sourcing which will be using atomic operations.
>
> #include <asm-generic/bitops/atomic.h>
> #include <asm-generic/bitops/lock.h>
>
> Correct?
>
> Would be good to run any testsuite to prove that all operations works as
> expected. Is there any testsuite I can use to confirm it?
>
> Thanks,
> Michal

The comment in the generic bitops.h says "You should recode these in the
native assembly language, if at all possible". I don't think using the generic
implementation will be as efficient as the current arch specific one.

My recommendation is to stick with the arch specific implementation.

Thanks,
Stefan