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Date: Tue, 12 Sep 2006 00:30:06 -0400 From: "Albert Cahalan" <acahalan@...il.com> To: benh@...nel.crashing.org, jbarnes@...tuousgeek.org, alan@...rguk.ukuu.org.uk, davem@...emloft.net, jeff@...zik.org, paulus@...ba.org, torvalds@...l.org, linux-kernel@...r.kernel.org, akpm@...l.org, segher@...nel.crashing.org Subject: Re: Opinion on ordering of writel vs. stores to RAM Benjamin Herrenschmidt writes: > On Mon, 2006-09-11 at 11:08 -0700, Jesse Barnes wrote: >> Ok, that's fine, though I think you'd only want the very weak >> semantics (as provided by your __raw* routines) on write >> combined memory typically? > > Well, that and memory with no side effects (like frame buffers) Oh no, it's great for regular device driver work. I used this type of system all the time on a different PowerPC OS. Suppose you need to set up a piece of hardware. Assume that the hardware isn't across some nasty bridge. You do this: hw->x = 42; hw->y = 19; eieio(); hw->p = 11; hw->q = 233; hw->r = 87; eieio() hw->n = 101; hw->m = 5; eieio() In that ficticious example, I get 7 writes to the hardware device with only 3 "eieio" operations. It's not hard at all. Sometimes a "sync" is used instead, also explicitly. To get even more speed, you can mark memory as non-coherent. You can even do this for RAM. There are cache control instructions to take care of any problems; simply ask the CPU to write things out as needed. Linux should probably do this: Plain stuff is like x86. If you want the performance of loose ordering, ask for it when you get the mapping and use read/write functions that have a "_" prefix. If you mix the "_" versions with a plain x86-like mapping or the other way, the behavior you get will be an arch-specific middle ground. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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