Re: [PATCH v2] Virtual memory size detection for 64 bit MIPS CPUs

[Guenter Roeck <guenter.roeck@ericsson.com>]

On Mon, 2010-02-01 at 16:42 -0500, David Daney wrote:
> 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 

Sure, no problem.

> not writing 1s to any reserved bits of the register.
> 
That may be tricky, since the upper bits are reserved in some
architectures. For example, the 20Kc core specification says that bits
61:40 are reserved and must be written with 0.

I can write, say, 0x3fffffffffff0000 to avoid writing into lower
reserved bits, but that won't help for any upper reserved bits. Would
that be acceptable / better ?

Guenter