linux-kernel - Re: [RFC] mips: Add MXU context switching support

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Date:	Tue, 5 Jul 2016 15:37:21 +0530
From:	PrasannaKumar Muralidharan <prasannatsmkumar@...il.com>
To:	"Maciej W. Rozycki" <macro@...tec.com>
Cc:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	John Stultz <john.stultz@...aro.org>, mguzik@...hat.com,
	athorlton@....com, mhocko@...e.com, ebiederm@...ssion.com,
	gorcunov@...nvz.org, luto@...nel.org, cl@...ux.com,
	serge.hallyn@...ntu.com, Kees Cook <keescook@...omium.org>,
	jslaby@...e.cz, Andrew Morton <akpm@...ux-foundation.org>,
	Florian Fainelli <f.fainelli@...il.com>, mingo@...nel.org,
	alex.smith@...tec.com, markos.chandras@...tec.com,
	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	david.daney@...ium.com, zhaoxiu.zeng@...il.com, chenhc@...ote.com,
	Zubair.Kakakhel@...tec.com, James Hogan <james.hogan@...tec.com>,
	Paul Burton <paul.burton@...tec.com>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [RFC] mips: Add MXU context switching support

On 5 July 2016 at 04:00, Maciej W. Rozycki <macro@...tec.com> wrote:
> On Sat, 25 Jun 2016, PrasannaKumar Muralidharan wrote:
>
>> diff --git a/arch/mips/include/asm/mxu.h b/arch/mips/include/asm/mxu.h
>> new file mode 100644
>> index 0000000..cf77cbd
>> --- /dev/null
>> +++ b/arch/mips/include/asm/mxu.h
>> @@ -0,0 +1,157 @@
>> +/*
>> + * Copyright (C) Ingenic Semiconductor
>> + * File taken from Ingenic Semiconductor's linux repo available in github
>> + *
>> + * This program is free software; you can redistribute it and/or modify it
>> + * under the terms of the GNU General Public License as published by the
>> + * Free Software Foundation;  either version 2 of the  License, or (at your
>> + * option) any later version.
>> + */
>> +#ifndef _ASM_MXU_H
>> +#define _ASM_MXU_H
>> +
>> +#include <asm/cpu.h>
>> +#include <asm/cpu-features.h>
>> +#include <asm/hazards.h>
>> +#include <asm/mipsregs.h>
>> +
>> +static inline void __enable_mxu(void)
>> +{
>> +     unsigned int register val asm("t0");
>> +     val = 3;
>> +     asm volatile(".word     0x7008042f\n\t"::"r"(val));
>
>  Can you please document your manually generated machine code, i.e. what
> instruction 0x7008042f actually is?

I have taken this header from vendor kernel. This instruction saves 3
to xr16 register. I will document them in the next revision.

>  Also our convention has been to separate asm operands with spaces, and
> there's no need for a new line or a tab character at the end of an
> inline as GCC will add these automatically as needed, i.e.:
>
>         asm volatile(".word     0x7008042f" : : "r" (val));
>
> Likewise throughout.

Will follow the convention.

>> +static inline void __save_mxu(void *tsk_void)
>> +{
>> +     struct task_struct *tsk = tsk_void;
>> +
>> +     register unsigned int reg_val asm("t0");
>> +
>> +     asm volatile(".word     0x7008042e\n\t");
>> +     tsk->thread.mxu.xr[0] = reg_val;
>> +     asm volatile(".word     0x7008006e\n\t");
>> +     tsk->thread.mxu.xr[1] = reg_val;
>> +     asm volatile(".word     0x700800ae\n\t");
>> +     tsk->thread.mxu.xr[2] = reg_val;
>> +     asm volatile(".word     0x700800ee\n\t");
>> +     tsk->thread.mxu.xr[3] = reg_val;
>> +     asm volatile(".word     0x7008012e\n\t");
>> +     tsk->thread.mxu.xr[4] = reg_val;
>> +     asm volatile(".word     0x7008016e\n\t");
>> +     tsk->thread.mxu.xr[5] = reg_val;
>> +     asm volatile(".word     0x700801ae\n\t");
>> +     tsk->thread.mxu.xr[6] = reg_val;
>> +     asm volatile(".word     0x700801ee\n\t");
>> +     tsk->thread.mxu.xr[7] = reg_val;
>> +     asm volatile(".word     0x7008022e\n\t");
>> +     tsk->thread.mxu.xr[8] = reg_val;
>> +     asm volatile(".word     0x7008026e\n\t");
>> +     tsk->thread.mxu.xr[9] = reg_val;
>> +     asm volatile(".word     0x700802ae\n\t");
>> +     tsk->thread.mxu.xr[10] = reg_val;
>> +     asm volatile(".word     0x700802ee\n\t");
>> +     tsk->thread.mxu.xr[11] = reg_val;
>> +     asm volatile(".word     0x7008032e\n\t");
>> +     tsk->thread.mxu.xr[12] = reg_val;
>> +     asm volatile(".word     0x7008036e\n\t");
>> +     tsk->thread.mxu.xr[13] = reg_val;
>> +     asm volatile(".word     0x700803ae\n\t");
>> +     tsk->thread.mxu.xr[14] = reg_val;
>> +     asm volatile(".word     0x700803ee\n\t");
>> +     tsk->thread.mxu.xr[15] = reg_val;
>> +}
>
>  Not using an output operand with asms here?

I think the instruction saves the xr* register value to reg_val
without need for output operand.

Thanks for your review, appreciate it.