Re: [PATCH V3 1/2] MIPS: Loongson-3: Enable COP2 usage in kernel

[Thomas Bogendoerfer <tsbogend@alpha.franken.de>]

On Fri, Aug 07, 2020 at 09:25:25PM +0800, Jiaxun Yang wrote:
> 
> 
> 在 2020/8/7 21:13, Thomas Bogendoerfer 写道:
> >On Wed, Aug 05, 2020 at 09:51:44PM +0800, Jiaxun Yang wrote:
> >>>yes there is. Since this COP2 is a total black box to me, it would be
> >>>really helpfull to get some docs for it or at least some information what
> >>>it exactly does and how you want to use it in kernel code.
> >>FYI:
> >>Loongson doesn't have any CU2 register. It just reused LWC2 & LDC2 opcode
> >>to define some load & store instructions (e.g. 128bit load to two GPRs).
> >>
> >>I have a collection of these instructions here[1].
> >>
> >> From GS464E (3A2000+), execuating these instruction won't produce COP2
> >>unusable
> >>exception. But older Loongson cores (GS464) will still produce COP2
> >>exception, thus
> >>we should have CU2 enabled in kernel. That would allow us use to these
> >>instructions
> >>to optimize kernel.
> >thank you that makes things a little bit clearer.
> >
> >How will this be used in kernel code ? Special assembler routines or
> >by enabling gcc options ?
> 
> Via special assembly routines, as -msoft-float will disable generation of
> these instructions in GCC.
> 
> I knew Huacai have out-of-tree memcpy optimization and Xuerui have
> RAID5 optimiztion with these instructions.
> 
> >
> >>>And finally what I stil don't like is the splittering of more
> >>>#ifdef LOONGSON into common code. I'd prefer a more generic way
> >>>to enable COPx for in kernel usage. Maybe a more generic config option
> >>>or a dynamic solution like the one for user land.
> >>Agreed. some Kconfig options or cpuinfo_mips.options can be helpful.
> >let's see whether this really is needed.
> >
> >To me it looks like the COP2 exception support for loongson makes
> >thing worse than it helps. How about the patch below ? There is still
> >a gap between starting the kernel and COP2 enabled for which I'm not
> >sure, if we are hitting COP2 instructions.
> 
> Yes, the exception does not really make sense.
> What's your opinion Huacai?
> 
> For in-kernel usage, we still have to enable it in genex.

you shouldn't need any other changes. ST0_CU2 will be set in
configure_status() and not touched after that.

Thomas.

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