[prev in list] [next in list] [prev in thread] [next in thread] 

List:       linux-mips
Subject:    Re: 74K/1074K support
From:       Leonid Yegoshin <Leonid.Yegoshin () imgtec ! com>
Date:       2013-09-30 21:29:02
Message-ID: 5249ED1E.4050106 () imgtec ! com
[Download RAW message or body]

On 09/24/2013 10:27 PM, Ralf Baechle wrote:
> Commit 006a851b10a395955c153a145ad8241494d43688 adds 74K support in c-r4k.c:
> 
> +static inline void alias_74k_erratum(struct cpuinfo_mips *c)
> +{
> +       /*
> +        * Early versions of the 74K do not update the cache tags on a
> +        * vtag miss/ptag hit which can occur in the case of KSEG0/KUSEG
> +        * aliases. In this case it is better to treat the cache as always
> +        * having aliases.
> +        */
> +       if ((c->processor_id & 0xff) <= PRID_REV_ENCODE_332(2, 4, 0))
> +               c->dcache.flags |= MIPS_CACHE_VTAG;
> +       if ((c->processor_id & 0xff) == PRID_REV_ENCODE_332(2, 4, 0))
> +               write_c0_config6(read_c0_config6() | MIPS_CONF6_SYND);
> +       if (((c->processor_id & 0xff00) == PRID_IMP_1074K) &&
> +           ((c->processor_id & 0xff) <= PRID_REV_ENCODE_332(1, 1, 0))) {
> +               c->dcache.flags |= MIPS_CACHE_VTAG;
> +               write_c0_config6(read_c0_config6() | MIPS_CONF6_SYND);
> +       }
> +}
> 
> But MIPS D-caches are never virtually tagged, so there is nothing in
> the kernel that ever tests the MIPS_CACHE_VTAG flag in a D-cache
> descriptor.
> 
> Cargo cult programming at its finest?  Or was MIPS_CACHE_ALIASES what
> really was meant to be set?
> 
> Ralf

There is a problem on early versions of 74K/1074K which can be effectively cured by \
setting MIPS_CACHE_VTAG. It enforces the needed cache handling.
I hope it will go away as customers replace RTL for newer versions.
  
But I prefer the patch version from Maciej W. Rozycki, it is more clear.

- Leonid.


[prev in list] [next in list] [prev in thread] [next in thread]