linux-kernel - Re: [PATCH 1/2] MIPS: Add basic support for Loongson1B (UPDATED)

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Date:	Mon, 26 Sep 2011 09:40:48 +0800
From:	wu zhangjin <wuzhangjin@...il.com>
To:	Shane McDonald <mcdonald.shane@...il.com>
Cc:	keguang.zhang@...il.com, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org, ralf@...ux-mips.org,
	r0bertz@...too.org, chenj@...ote.com
Subject: Re: [PATCH 1/2] MIPS: Add basic support for Loongson1B (UPDATED)

On Sun, Sep 25, 2011 at 9:40 PM, Shane McDonald
<mcdonald.shane@...il.com> wrote:
>>> diff --git a/arch/mips/include/asm/mach-loongson1/regs-clk.h b/arch/mips/include/asm/mach-loongson1/regs-clk.h
>>> new file mode 100644
>>> index 0000000..7a09d6a
>>> --- /dev/null
>>> +++ b/arch/mips/include/asm/mach-loongson1/regs-clk.h
>>> @@ -0,0 +1,32 @@
>>> +/*
>>> + * Copyright (c) 2011 Zhang, Keguang <keguang.zhang@...il.com>
>>> + *
>>> + * Loongson1 Clock Register Definitions.
>>> + *
>>> + * 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_MACH_LOONGSON1_REGS_CLK_H
>>> +#define __ASM_MACH_LOONGSON1_REGS_CLK_H
>>> +
>>> +#define LS1_CLK_REG(x)         ((void __iomem *)(LOONGSON1_CLK_BASE + (x)))
>>
>> "volatile" keyword may be required for __iomem access, the same to the
>> following similar usage.
>>
>> Considering a scene is(LS1_XXX_REG(X) doesn't really exist):
>>
>> LS1_XXX_REG(X) = 0;              /* put cpu into idle and wait interrupt */
>> LS1_XXX_REG(X) = 7;              /* recover the cpu frequency to the highest */
>>
>> If no "volatile" keyword indicated, the first line will be
>> intelligently but wrongly removed by compiler.
>
> No -- please see Documentation/volatile-considered-harmful.txt,
> particularly the paragraph starting at line 49.  This macro
> is only being used as an argument to __raw_readl,
> as it should be.

Yeah, __raw_readl/writel() will use volatile to prevent it from
optimization, thanks ;)

"within the kernel, I/O memory  accesses are always done through
accessor functions;
 accessing I/O memory directly through pointers is frowned upon and
does not work on all
 architectures.  Those accessors are written to prevent unwanted
 optimization,
....
  - The above-mentioned accessor functions might use volatile on
     architectures where direct I/O memory access does work.  Essentially,
     each accessor call becomes a little critical section on its own and
     ensures that the access happens as expected by the programmer.
...
Patches to remove volatile variables are generally welcome - as long as
 they come with a justification which shows that the concurrency issues have
 been properly thought through.
"

Thanks & Regards,
Wu Zhangjin

>
> Shane
>
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