Re: [PATCH V7 2/3] MIPS: Loongson-3: Enable COP2 usage in kernel

From: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To: Huacai Chen <chenhc@lemote.com>
Cc: linux-mips@vger.kernel.org, Fuxin Zhang <zhangfx@lemote.com>,
	Huacai Chen <chenhuacai@gmail.com>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>
Subject: Re: [PATCH V7 2/3] MIPS: Loongson-3: Enable COP2 usage in kernel
Date: Mon, 21 Sep 2020 14:15:54 +0200	[thread overview]
Message-ID: <20200921121554.GA8642@alpha.franken.de> (raw)
In-Reply-To: <1600679548-29154-2-git-send-email-chenhc@lemote.com>

On Mon, Sep 21, 2020 at 05:12:27PM +0800, Huacai Chen wrote:
> 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.
> 
> For zboot, we disable gslq/gssq be generated by toolchain.
> 
> Signed-off-by: Huacai Chen <chenhc@lemote.com>
> ---
> V3: Stop using ST0_MM and use ST0_CU2 instead (Thank Thomas and Maciej).
> V4: Adopt Thomas's suggestion to improve coding style.
> V5: Use ST0_KERNEL_CUMASK in all possible places to avoid #ifdefs.
> V6: Modify switch_to() and don't touch r4k_switch.S.
> V7: For zboot, disable gslq/gssq be generated by toolchain.
> 
>  arch/mips/boot/compressed/Makefile | 5 +++++
>  arch/mips/include/asm/mipsregs.h   | 7 +++++++
>  arch/mips/include/asm/stackframe.h | 6 +++---
>  arch/mips/kernel/head.S            | 2 +-
>  arch/mips/kernel/process.c         | 4 ++--
>  arch/mips/kernel/traps.c           | 2 +-
>  6 files changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/mips/boot/compressed/Makefile b/arch/mips/boot/compressed/Makefile
> index 9a9ba77..2c491c1 100644
> --- a/arch/mips/boot/compressed/Makefile
> +++ b/arch/mips/boot/compressed/Makefile
> @@ -22,6 +22,11 @@ KBUILD_CFLAGS := $(filter-out -pg, $(KBUILD_CFLAGS))
>  
>  KBUILD_CFLAGS := $(filter-out -fstack-protector, $(KBUILD_CFLAGS))
>  
> +# Disable lq/sq in zboot
> +ifdef CONFIG_CPU_LOONGSON64
> +KBUILD_CFLAGS := $(filter-out -march=loongson3a, $(KBUILD_CFLAGS)) -march=mips64r2
> +endif
> +

thanks for doing this, I'm going to apply this later.

This remind of another question, what about loongson2ef ? I'm getting
kbuild failure because of enabled loongson-mmi mails. Are we are missing
something like cflags-y += $(call cc-option,-mno-loongson-mmi) in Platform
file ? Who is taking care of loongson2ef ?

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]