[U-Boot] [PATCH 3/9] arm: Move CP15 init out of cpu_init_crit()
Albert ARIBAUD
albert.u.boot at aribaud.net
Sat Nov 5 09:09:45 CET 2011
Hi Simon et al.,
Thanks to you and others for their patience. Let me summarize the thread:
- ARM7TDMI definitely lacks a cp15, as AN99 explicitely states;
- ARM provides no other, reliable, way to identify an ARM7TDMI reliably;
- Identification through a manufacturer-specific method just defeats the
goal.
This makes it impossible to support ARM7TDMI (as well as others listed
in AM99) with a start.S code that contains cp15 instructions.
OTOH, I've gone through a number of ARM TRMs for cores that have
caches/MMU and they all state that these are disabled on reset. I will
also assume that when U-Boot starts indirectly through SPL or any other
mechanism, and if this mechanism uses caches/MMU, then it will disable
them properly before passing control to U-Boot, just like U-Boot
disables caches etc before passing control to e.g. Linux.
I understand that this initialization is done so that the the cache/MMU
is in a correct state, and this must still be ensured, but I now think
that ensuring this is the burden of whatever passed control to the start
label (this includes true resets, branching from SPL or any other IPL,
and --ugh-- branching to start from inside U-Boot).
Based on this, I revise my opinion that the caches and MMU cleaning code
in start.S should be kept there, and actually I now consider that they
should be removed entirely.
However, removing cp15 inits would mean a *third* revision of your
patchset, plus it could cause unexpected issues which I don't want to
happen at this point of the release cycle.
Therefore, I'll go back to your patch as posted on oct 13th.
Apologies for the long discussion.
Amicalement,
--
Albert.
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