Re: [PATCH 5/6] MIPS: CONFIG_MIPS_MT_SMP should depend upon CPU_MIPSR2

[Ralf Baechle <ralf@linux-mips.org>]

On Sat, Sep 12, 2015 at 08:11:31AM -0700, Paul Burton wrote:
> Date:   Sat, 12 Sep 2015 08:11:31 -0700
> From: Paul Burton <paul.burton@imgtec.com>
> To: Ralf Baechle <ralf@linux-mips.org>
> CC: linux-mips@linux-mips.org, Markos Chandras
>  <markos.chandras@imgtec.com>, stable@vger.kernel.org,
>  linux-kernel@vger.kernel.org
> Subject: Re: [PATCH 5/6] MIPS: CONFIG_MIPS_MT_SMP should depend upon
>  CPU_MIPSR2
> Content-Type: text/plain; charset="utf-8"
> 
> On Sat, Sep 12, 2015 at 12:16:39PM +0200, Ralf Baechle wrote:
> > >  config MIPS_MT_SMP
> > >  	bool "MIPS MT SMP support (1 TC on each available VPE)"
> > > -	depends on SYS_SUPPORTS_MULTITHREADING
> > > +	depends on SYS_SUPPORTS_MULTITHREADING && CPU_MIPSR2
> > 
> > Right now this line is
> > 
> > depends on SYS_SUPPORTS_MULTITHREADING && !CPU_MIPSR6
> > 
> > which I believe is correct.  The MT SMP support aka VSMP had been
> > carefully crafted to work on older ASEs that is all use of MIPS MT
> > instructions or features was carefully protected by cpu_has_mipsmt
> > or similar.
> 
> I disagree. The "background" section in the introduction to the MT ASE
> spec (MD00376, revision 1.12) reads:
> 
> > Multi-threading, or the concurrent presence of multiple active threads
> > or contexts of execution on the same CPU, is an increasingly
> > widely-used technique for tolerating memory and execution latency and
> > for getting higher utilization out of processor functional units. The
> > MIPS� Multi-threading (MT) Module is an extension to Release 2 (and
> > newer) of the MIPS32� Architecture which provides a framework for
> > multi-threading the MIPS processor architecture.
> 
> MT is quite clearly an extension to r2. The MT bit in Config3 has this
> note in the MIPS32 PRA (MD00088, revision 6.01):
> 
> > For Release 6 and MIPS after, this bit must be 0.
> 
> Thus MT is an option from r2 <= ISA < r6. The current !CPU_MIPSR6
> constraint in Kconfig only enforces half of that. Depending upon
> CPU_MIPSR2 would enforce the whole.

Correct.  Still, VSMP's use of MT instruction is very limited to a few
particular places in the kernel which makes a simple exervise to support
a kernel that is capable of supporting VSMP yet will support MIPS IV,
MIPS 32R1 CPUs or MIPS 32R2+ without MT.

  Ralf