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

From: Leonid Yegoshin <[email-masked]>
Date: Thu, 14 Jan 2016 17:07:14 -0800

On 01/14/2016 04:47 PM, Paul E. McKenney wrote:
> On Thu, Jan 14, 2016 at 03:33:40PM -0800, Leonid Yegoshin wrote:
>> 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).
> So smp_mb() can be SYNC_MB.  However, mb() needs to be SYNC for MMIO
> purposes, correct?

Absolutely. For MIPS R2 which is not Octeon.

>> 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.
> So you need to build a different kernel for some types of MIPS systems?
> Or do you do boot-time rewriting, like a number of other arches do?

I don't know. I would like to have responses. Ralf asked Maciej about 
old systems and that came nowhere. Even rewrite - don't know what to do 
with that: no lightweight SYNC or no SYNC at all - yes, it is still 
possible that SYNC on some systems can be too heavy or even harmful, 
nobody tested that.

- Leonid.