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Date:   Mon, 01 Feb 2010 13:42:53 -0800
From:   David Daney <ddaney@caviumnetworks.com>
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To:     Guenter Roeck <guenter.roeck@ericsson.com>
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Subject: Re: [PATCH v2] Virtual memory size detection for 64 bit MIPS CPUs
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Guenter Roeck wrote:
[...]
>  
> +static inline void cpu_set_vmbits(struct cpuinfo_mips *c)
> +{
> +	if (cpu_has_64bits) {
> +		unsigned long zbits;
> +
> +		asm volatile(".set mips64\n"
> +			     "and %0, 0\n"
> +			     "dsubu %0, 1\n"
> +			     "dmtc0 %0, $10, 0\n"
> +			     "dmfc0 %0, $10, 0\n"
> +			     "dsll %0, %0, 2\n"
> +			     "dsra %0, %0, 2\n"
> +			     "dclz %0, %0\n"
> +			     ".set mips0\n"
> +			     : "=r" (zbits));
> +		c->vmbits = 64 - zbits;
> +	} else
> +		c->vmbits = 32;
> +}
> +

It should be possible to express this in 'pure' C using 
read_c0_entryhi()/write_c0_entryhi(), also you need to be sure you are 
not writing 1s to any reserved bits of the register.


David Daney