Subject: G2 and dcbi
To: None <firstname.lastname@example.org>
From: Neil Ludban <email@example.com>
Date: 06/28/2005 11:47:35
According to many footnotes in the G2 PowerPC Core Reference Manual,
"the dcbi instruction should never be used on the G2 core". The only
explanation given is here:
188.8.131.52.1 Supervisor-Level Cache Management Instruction
The supervisor-level cache management instruction in the PowerPC
architecture, dcbi, should not be used on the G2 core. If it is
used it can cause a data storage interrupt. The user-level dcbf
instruction, described in Section 184.108.40.206, "Memory Control
Instructions--VEA" and Section 4.8, "Cache Control Instructions,"
should be used when the program needs to invalidate cache blocks.
This change in sys/arch/powerpc/powerpc/bus_dma.c:_bus_dmamap_sync():
[ dcbf partial leading cache line ]
[ dcbf partial trailing cache line ]
/* FALLTHROUGH */
* The contents will have changed, make sure to remove
* them from the cache. Note: some implementation
* implement dcbi identically to dcbf. Thus if the
* cacheline has data, it will be written to memory.
* If the DMA is updating the same cacheline at the
* time, bad things can happen.
- dcbi(addr, seglen, dcache_line_size);
+ dcbf(addr, seglen, dcache_line_size);
appears to have fixed a whole range of stability issues on an MPC8272
based system including corrupt kernel memory while mounting NFS root,
wireless cards hanging, DIAGNOSTIC assertions failing (usually related
to pmap and pool_cache), core dumps and MCHK exceptions when compiling.
Does this look like a correct fix? I'm still suspicious because dcbi
is apparently working for others (right?), and it's presumably
slower to flush the cache than invalidate, so maybe this is just
avoiding a race condition somewhere else.