Re: [PATCH] ARM MMU: add strongly-ordered memory type

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, 2008-08-08 at 14:19 +0100, Russell King - ARM Linux wrote:
> On Fri, Aug 08, 2008 at 12:44:49PM +0100, Catalin Marinas wrote:
> > I need to check in ARM when people come from holidays but a simple LDR
> > might not be enough to guarantee that a CPSIE etc. happens after it. You
> > may need to add either an LDR + CMP (or some other usage of the loaded
> > register) or LDR + DSB. I agree that DSB alone is not enough.
> 
> Okay, I give up on this issue.  Weak memory ordering seems to be a
> very very big can of worms.  And then there's this:
> 
> 14:07 < rmk> so we're back to making readl() itself do something with the
>              data... which brings us back to that question about why bother
>              with weak ordering
> 14:10 < willy> you can't have weak ordering for device control registers
> 14:11 < rmk> yes you can, provided they're ordered wrt each other.
> 14:11 < willy> weak ordering works great for SMP or for just covering up latency
> 14:12 < willy> no, you can't.  see writel(); readl(); udelay(1); writel();.
>                You didn't wait for 1 microsecond before accessing the device
>                again.
> 
> Or, to put it another way, it seems that on Linux _all_ devices must
> be strongly ordered or be seen to Linux as being strongly ordered
> (iow, readl and writel and friends _must_ have a barrier.)

Why not put a barrier in udelay? The delay functions are mainly used for
device access.

> TBH, this is becoming soo much of a joke, it's untrue.

For performance in a multi-stage pipeline model, you need weaker
ordering. I'm not a hardware engineer to comment more on this.

> Let's go back to having a strongly ordered memory model.  Please.

On pre-ARMv6, updates to CPSR are guaranteed to take place in program
order with a strongly-ordered memory access. This is still true on
ARMv6/v7 for backward compatibility but the feature is deprecated and it
might not be true for future architecture versions. At some point
barriers might be needed.

-- 
Catalin