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    SubjectRe: [PATCH 1/2] MIPS: Add basic support for Loongson1B (UPDATED)
    From
    On Sun, Sep 25, 2011 at 9:40 PM, Shane McDonald
    <mcdonald.shane@gmail.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@gmail.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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