From: Ralf Baechle <ralf@linux-mips.org>
To: "Maciej W. Rozycki" <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Subject: Re: [PATCH] MIPS: Tell R4k SC and MC variations apart
Date: Tue, 24 Sep 2013 10:48:45 +0200 [thread overview]
Message-ID: <20130924084845.GA21257@linux-mips.org> (raw)
In-Reply-To: <alpine.LFD.2.03.1309222307440.16797@linux-mips.org>
On Sun, Sep 22, 2013 at 11:30:59PM +0100, Maciej W. Rozycki wrote:
> There is no reliable way to tell R4000/R4400 SC and MC variations apart,
> however simple heuristic should give good results. Only the MC version
> supports coherent caching so we can rely on such a mode having been set
> for KSEG0 by the power-on firmware to reliably indicate an MC processor.
> SC processors reportedly hang on coherent cached memory accesses and Linux
> is linked to a cached load address so the firmware has to use the correct
> caching mode to download the kernel image in a cached mode successfully.
>
> OTOH if the firmware chooses to use either the non-coherent cached or the
> uncached mode for KSEG0 on an MC processor, then the SC variant will be
> reported, just as we currently do, so no regression here.
>
> Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
> ---
> Ralf,
>
> I believe we discussed this once long ago and you had some concerns about
> such an approach although I don't recall exactly what they were. I
> maintain that this heuristic is reasonable, has no drawbacks and has a
> potential to make some optimisations or errata workarounds easier. Also
> we can collect data about systems affected to see what their firmware does
> -- R4000SC/R4400SC DECstations definitely get CP0.Config.K0 right.
I'm fairly sure it gets k0 right - otherwise it'd likely not work at all!
My reservations may have been about userland reading /proc/cpuinfo and
looking at the CPU type. Some software may know how to handle the
PC/SC variants but not the MC versions. But this seems to be a fairly
weak concern - and I trust you checked gcc's parsing of /proc/cpuinfo.
Ralf
next prev parent reply other threads:[~2013-09-24 8:48 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-22 22:30 [PATCH] MIPS: Tell R4k SC and MC variations apart Maciej W. Rozycki
2013-09-23 11:37 ` Jonas Gorski
2013-09-23 12:35 ` [PATCH v2] " Maciej W. Rozycki
2013-09-23 12:41 ` Jonas Gorski
2013-09-23 13:01 ` [PATCH v3] " Maciej W. Rozycki
2013-09-24 9:11 ` Ralf Baechle
2013-09-24 8:48 ` Ralf Baechle [this message]
2013-09-24 23:37 ` [PATCH] " Maciej W. Rozycki
2013-09-25 11:02 ` Ralf Baechle
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