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Date:   Fri, 15 Oct 2010 15:25:24 +0400
From:   "Gleb O. Raiko" <raiko@niisi.msk.ru>
Organization: NIISI RAN
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To:     David Daney <ddaney@caviumnetworks.com>
CC:     linux-mips@linux-mips.org, ralf@linux-mips.org
Subject: Re: [PATCH] MIPS: Implement __read_mostly
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On 14.10.2010 23:36, David Daney wrote:
> diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
> --- a/arch/mips/kernel/setup.c
> +++ b/arch/mips/kernel/setup.c
> @@ -69,7 +69,7 @@ static char __initdata builtin_cmdline[COMMAND_LINE_SIZE] = CONFIG_CMDLINE;
>    * mips_io_port_base is the begin of the address space to which x86 style
>    * I/O ports are mapped.
>    */
> -const unsigned long mips_io_port_base __read_mostly = -1;
> +const unsigned long mips_io_port_base = -1;
>   EXPORT_SYMBOL(mips_io_port_base);

While we're here could we eliminate mips_io_port_base for boards that 
don't need it.

Now, it almost might be done by defining something like
__swizzle_addr_[bwlq](port)	((port) - mips_io_port_base)

Unfortunately, __swizzle_addr_[bwlq] is also used for memory acceeses 
too (in read/write[bwlq]), so definition above doesn't work.

By providing two variants, e.g. __swizzle_io_addr_[bwlq] and 
__swizzle_mem_addr_[bwlq] we can eliminate unnecessary loads of 
mips_io_port_base.

BTW, in recent kernels the trick with mips_io_port_base doesn't work 
well anyway. The solely purpose of the trick was to prevent loading of 
mips_io_port_base across function calls. Now drivers tend to use 
ioread/iowrite that don't use mips_io_port_base at all or use its own 
wrappers for in/out[bwlq] that _do_ load mips_io_port_base on every call.

Gleb.