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Re: 74K/1074K support

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: 74K/1074K support
From: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Date: Mon, 30 Sep 2013 14:29:02 -0700
Cc: <linux-mips@linux-mips.org>, "Steven J. Hill" <Steven.Hill@imgtec.com>
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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.



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