Subject: Re: [RFC PATCH 03/19] powerpc: gamecube: bootwrapper bits

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 26 Nov 2009 19:46:04 +1100

On Thu, 2009-11-26 at 09:17 +0100, Gabriel Paubert wrote:
> 
> > They should hopefully... as long as you don't rely on the reservation
> > blowing as a result of a DMA write.
> 
> Hmm, this really depends on whether the DMA transfers generate bus cycles
> that require coherency or not. Not the other way around. M=1 only forces
> bus cycles to be snooped by other processors (asserting the GBL signal
> on 603/604/750 busses).

You are absolutely right. Which makes it even more likely that
lwarx/stwcx. won't care unless the L2 cache plays tricks.

> The host bridge is free to systematically snoop processor accesses (to make 
> sure that data queued in the bridge and not yet written to memory is seen
> in the coherent memory domain even if, for example, interrupts propagate 
> so fast that DMA target addresses are accessed before it is written to RAM).
> 
> On memory coherent systems, the host bridge has to assert the GBL signal,
> to force data to be written to memory (for most DMA accesses), or to  
> invalidate caches (for full line writes from devices). 

Cheers,
Ben.