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Re: PATCH: Fix ll/sc for mips (take 3)
- From: Hartvig Ekner <hartvige at mips dot com>
- To: macro at ds2 dot pg dot gda dot pl (Maciej W. Rozycki)
- Cc: ralf at oss dot sgi dot com (Ralf Baechle), hartvige at mips dot com (Hartvig Ekner), justinca at ri dot cmu dot edu (Justin Carlson), dan at debian dot org (Daniel Jacobowitz), hjl at lucon dot org (H . J . Lu), dom at algor dot co dot uk (Dominic Sweetman), libc-alpha at sources dot redhat dot com (GNU C Library), linux-mips at oss dot sgi dot com
- Date: Tue, 5 Feb 2002 13:38:34 +0100 (MET)
- Subject: Re: PATCH: Fix ll/sc for mips (take 3)
Some of MIPS's cores do externalize the event of a "LL" and make it
visible on the bus interface. Similarly, the SC is externalized and
requires a go/nogo response from the system logic. Think of it as
putting a shared LLAddr & LLBit outside the processor. The SC will
only succeed if the internal LLBit is ok *and* the external logic gives
the go-ahead as well.
The reasoning behind all this is that one can then utilize LL/SC in
multi CPU systems without full coherency support being required.
But then again, this might not be relevant for MIPS/Linux as it will not
run without full HW coherency on multiple CPUs?
Maciej W. Rozycki writes:
> On Tue, 5 Feb 2002, Ralf Baechle wrote:
> > Some of the processor manuals explicitly note that the execution of a
> > ll instruction may not be visible at all externally. That's the case if
> > the address is already in a primary cache line in exclusive (ll) or
> > dirty (sc) state. That makes even if a processor supports full coherency
> > since there is no reason to indicate the update to any other external
> > agent. Or am I missing something?
> By definition, apart from an ordinary load, an "ll" does only the
> following two additional actions:
> 1. Loads the LLAddr register (it's visible in CP0, at least in certain
> implementations) to set up the ll comparator.
> 2. Sets the LLbit flip-flop to set the ll state to valid initially.
> None of the actions should normally involve externally visible activity.
> + Maciej W. Rozycki, Technical University of Gdansk, Poland +
> + e-mail: email@example.com, PGP key available +
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