Re: [v3,11/41] mips: reuse asm-generic/barrier.h

[Leonid Yegoshin]

On 01/14/2016 02:55 PM, Paul E. McKenney wrote:
> OK, so it looks like Will was asking not about WRC+addr+addr, but instead
> about WRC+sync+addr.
(He actually asked twice about this and that too but skip this)

> I am guessing that the manual's "Older instructions which must be globally
> performed when the SYNC instruction completes" provides the equivalent
> of ARM/Power A-cumulativity, which can be thought of as transitivity
> backwards in time.  This leads me to believe that your smp_mb() needs
> to use SYNC rather than SYNC_MB, as was the subject of earlier spirited
> discussion in this thread.

Don't be fooled here by words "ordered" and "completed" - it is HW 
design items and actually written poorly.
Just assume that SYNC_MB is absolutely the same as SYNC for any CPU and 
coherent device (besides performance). The difference can be in 
non-coherent devices because SYNC actually tries to make a barrier for 
them too. In some SoCs it is just the same because there is no need to 
barrier a non-coherent device (device register access usually strictly 
ordered... if there is no bridge in between).

> Suppose you have something like this:
> ...
> Does your hardware guarantee that it is not possible for all of r0,
> r1, r2, and r3 to be equal to zero at the end of the test, assuming
> that a, b, c, and d are all initially zero, and the four functions
> above run concurrently?

It is assumed to be so from Arch point of view. HW bugs are possible, of 
course.

> Another (more academic) case is this one, with x and y initially zero:
>
> ...
> Does SYNC_MB() prohibit r1 == 1 && r2 == 0 && r3 == 1 && r4 == 0?

It is assumed to be so from Arch point of view. HW bugs are possible, of 
course.

Note: I am not sure about ANY past MIPS R2 CPU because that stuff is 
implemented some time but nobody made it in Linux kernel (it was used by 
some vendor for non-Linux system). For that reason my patch for 
lightweight SYNCs has an option - implement it or implement a generic 
SYNC. It is possible that some vendor did it in different way but nobody 
knows or test it. But as a minimum - SYNC must be implemented in 
spinlocks/atomics/bitops, in recent P5600 it is proven that read can 
pass write in atomics.

MIPS R6 is a different story, I verified lightweight SYNCs from the 
beginning and it also should use SYNCs.

- Leonid.