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Re: a quick question regarding CONFIG_MIPS_UNCACHED..

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: a quick question regarding CONFIG_MIPS_UNCACHED..
From: Carsten Langgaard <carstenl@mips.com>
Date: Fri, 29 Nov 2002 14:49:37 +0100
Cc: "Maciej W. Rozycki" <macro@ds2.pg.gda.pl>, atul srivastava <atulsrivastava9@rediffmail.com>, linux-mips@linux-mips.org
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References: <20021128171519.A18165@linux-mips.org> <Pine.GSO.3.96.1021128172026.8D-100000@delta.ds2.pg.gda.pl> <20021129134230.A11704@linux-mips.org>
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Ralf Baechle wrote:

> On Fri, Nov 29, 2002 at 12:56:10PM +0100, Maciej W. Rozycki wrote:
>
> > > We've talked about this before - the specification of the ll/sc
> > > instructions says they only work ok on cached memory.  In the real world
> > > they seem to work also in uncached memory but I'd not bet the farm on
> > > that, too many implementations out there, too many chances for subtle
> > > bugs.
> >
> >  Indeed -- CONFIG_MIPS_UNCACHED should either be removed or imply
> > CONFIG_CPU_HAS_LLSC=n.  I suppose there is some interest in the option, so
> > the latter is preferable.  That would imply moving the option into the CPU
> > configuration section as now it's set very late, long after
> > CONFIG_CPU_HAS_LLSC is set.  Or it could be set up the other way, i.e. the
> > option would only become available if CONFIG_CPU_HAS_LLSC had been set to
> > n.  There would be no need to move it then.
> >
> >  What do you think?
>
> I had the same thought.  So far there are the following groups of people
> using this option:
>
>   - Hardware people using it so the the entire program is becoming visible
>     externally.
>
>     Becoming obsoleted by more advanced, less intrusive hardware debugging
>     methods.
>
>   - Software people kluding around bugs in their software.
>
>     Don't expect me to support this.  Note with a line of code.
>
>   - For testing if the cache code is actually working.
>
>     Will only be useful to show big fat bugs.  For the more subtle bugs
>     there is no way at all around understanding the entire cache managment
>     thing including all subtilities.  Actually a good reason to not support
>     this option either.
>
> It's been my observation that hardly any user is aware of these consquences
> nor is the Linux code making a good attempt at complying with all the
> additional restrictions of running uncached.  So in my oppinion
> CONFIG_MIPS_UNCACHED should go.  But I don't feel very strong about it so
> I'm going to wait for a few days so others have a chance to raise their
> voice.
>

I have used this option a lot, it has been very useful in hardware (CPU)
debugging.


>
>   Ralf

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