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

[Huacai Chen <chenhc@lemote.com>]

Hi, Jiaxun

On Thu, Aug 6, 2020 at 1:39 AM Jiaxun Yang <jiaxun.yang@flygoat.com> wrote:
>
>
>
> 在 2020/8/5 20:10, Thomas Bogendoerfer 写道:
> > On Sat, Aug 01, 2020 at 03:59:41PM +0800, Huacai Chen wrote:
> >> On Wed, May 6, 2020 at 1:30 PM Jiaxun Yang <jiaxun.yang@flygoat.com> wrote:
> >>>
> >>>
> >>> 于 2020年5月2日 GMT+08:00 下午12:55:43, Huacai Chen <chenhc@lemote.com> 写到:
> >>>> Loongson-3's COP2 is Multi-Media coprocessor, it is disabled in kernel
> >>>> mode by default. However, gslq/gssq (16-bytes load/store instructions)
> >>>> overrides the instruction format of lwc2/swc2. If we wan't to use gslq/
> >>>> gssq for optimization in kernel, we should enable COP2 usage in kernel.
> >>>>
> >>>> Please pay attention that in this patch we only enable COP2 in kernel,
> >>>> which means it will lose ST0_CU2 when a process go to user space (try
> >>>> to use COP2 in user space will trigger an exception and then grab COP2,
> >>>> which is similar to FPU). And as a result, we need to modify the context
> >>>> switching code because the new scheduled process doesn't contain ST0_CU2
> >>>> in its THERAD_STATUS probably.
> >>>>
> >>>> Signed-off-by: Huacai Chen <chenhc@lemote.com>
> >>> Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
> >>>
> >> Does this patch have some unresolved problems, or there is something unclear?
> > 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.
>
> >
> > Looking closer at the patch I realized, that there is already support
> > for usage of COP2 in user land, which I thought isn't the case (at least
> > I understood that from one of your mails). So is there enough state
> > saving to support this ?
>
> Actually there is no CU2 state for Loongson to save.
>
> >
> > 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.
>
So make those codes depend on CONFIG_CPU_LOONGSON3_WORKAROUNDS instead
of CONFIG_CPU_LOONGSON64?

Huacai

> Thanks.
>
> - Jiaxun
>
> [1]: https://github.com/FlyGoat/loongson-insn/
> >
> > Thomas.
> >