netdev - Re: [PATCH] ucc_geth: Rework the TX logic.

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Message-ID: <OFEAA9D1E3.4B2F3270-ONC125758A.00307236-C125758A.00322239@transmode.se>
Date:	Tue, 31 Mar 2009 11:07:36 +0200
From:	Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Li Yang <leoli@...escale.com>, linuxppc-dev@...abs.org,
	netdev@...r.kernel.org, pku.leo@...il.com
Subject: Re: [PATCH] ucc_geth: Rework the TX logic.

Scott Wood <scottwood@...escale.com> wrote on 30/03/2009 21:32:23:
> 
> Joakim Tjernlund wrote:
> >> different since descriptors are in MURAM which is ioremap()ed -- 
though 
> >> switching to a cacheable mapping with barriers should be a 
performance 
> >> improvement.
> > 
> > I always thought that MURAM was very fast. The whole reason to have 
BDs in
> > MURAM is that it is faster than normal RAM, at least that is what I 
> > thought.
> 
> Yeah, on second thought it probably wouldn't be worth it.  There's also 
> the question of under what circumstances the QE's MURAM accesses will be 

> cache-coherent.

I am a bit confused, what isn't worth it? Currently MURAM isn't used by 
ucc_geth, but
it is easy to change. Swap MEM_PART_SYSTEM to MEM_PART_MURAM, however, 
just
tried that and the driver stopped working. I known this worked earlier 
because
I tried it and I even think I sent a patch to Leo.

What choices do we have, I see three:

1) MEM_PART_SYSTEM, as today.
2) MEM_PART_MURAM. I guess this should be uncacheable memory?
3) as gianfar, dma_alloc_coherent(). I presume this is uncacheable memory?

My guess would be 2 or 3. Do they have the same synchronization
sematics?

> 
> As for the CPU not reordering guarded+cache inhibited accesses, that 
> initially seemed to be true for the new arch stuff (book3e/book3s, but 
> not really, see below), but the classic arch documentation only 
> guarantees stores to such regions to be in-order (and the 
> explicitly-specified operation of eieio on I+G accesses wouldn't make 
> much sense if they were already guaranteed to be in-order).
> 
> Then I looked at the EREF to see what older book E documents had to say 
> on the issue, and it suggests that when the architecture document says 
> "out of order", it really means "speculative" (and reading the arch 
> doc's definition of "out of order" seems to confirm this -- redefining 
> terms is bad, m'kay?).  So it seems that the simple answer is no, 
> guarded storage is not guaranteed to be in order, unless the only thing 
> that can cause an out-of-order access is speculative execution.

Very informative, thanks.

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