Re: [PATCH 1/3] MIPS: R6: Use lightweight SYNC instruction in smp_* memory barriers

From: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
To: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Paul Burton <paul.burton@imgtec.com>
Cc: <linux-mips@linux-mips.org>, <benh@kernel.crashing.org>,
	<will.deacon@arm.com>, <linux-kernel@vger.kernel.org>,
	<ralf@linux-mips.org>, <markos.chandras@imgtec.com>,
	<macro@linux-mips.org>, <Steven.Hill@imgtec.com>,
	<alexander.h.duyck@redhat.com>, <davem@davemloft.net>
Subject: Re: [PATCH 1/3] MIPS: R6: Use lightweight SYNC instruction in smp_* memory barriers
Date: Tue, 2 Jun 2015 11:20:56 -0700	[thread overview]
Message-ID: <556DF408.2040003@imgtec.com> (raw)
In-Reply-To: <20150602121227.GA1474@macpro.local>

On 06/02/2015 05:12 AM, Luc Van Oostenryck wrote:
> On Tue, Jun 02, 2015 at 11:08:35AM +0100, Paul Burton wrote:
>
>> I think this would read better as something like:
>>
>>    If a processor does not implement the lightweight sync operations then
>>    the architecture requires that they interpret the corresponding sync
>>    instructions as the typical heavyweight "sync 0". Therefore this
>>    should be safe to enable on all CPUs implementing release 2 or
>>    later of the MIPS architecture.
>>
> Is it really the case for release 2?
>
> I'm asking because recently I needed to do something similar and I couldn't
> find this garantee in the revision 2.00 of the manual.
Yes. MD00086/MD00084/MD00087 Rev 2.60 are technically MIPS R2. And this 
revision explicitly lists optional codes and it has a clear statement:

> Implementations that do not use any of the non-zero values of stype to 
> define different barriers, such as ordering bar-
> riers, must make those stype values act the same as stype zero.

(don't blame me that Rev 2.60 is 5 years after initial 2.00, it is still 
MIPS R2).

- Leonid.