linux-kernel - Re: [RFC PATCH 1/3] pstore-ram: use write-combine mappings

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Message-ID: <20130416134856.GB30292@arm.com>
Date:	Tue, 16 Apr 2013 14:48:56 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Will Deacon <Will.Deacon@....com>,
	Colin Cross <ccross@...roid.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Anton Vorontsov <cbouatmailru@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Tony Luck <tony.luck@...el.com>
Subject: Re: [RFC PATCH 1/3] pstore-ram: use write-combine mappings

On Tue, Apr 16, 2013 at 01:58:27PM +0100, Rob Herring wrote:
> On 04/16/2013 03:44 AM, Will Deacon wrote:
> > On Tue, Apr 16, 2013 at 01:43:09AM +0100, Colin Cross wrote:
> >> On Mon, Apr 15, 2013 at 4:59 PM, Rob Herring <robherring2@...il.com> wrote:
> >>> Exclusive accesses still have further restrictions. From section 3.4.5:
> >>>
> >>> • It is IMPLEMENTATION DEFINED whether LDREX and STREX operations can be
> >>> performed to a memory region
> >>>    with the Device or Strongly-ordered memory attribute. Unless the
> >>> implementation documentation explicitly
> >>>   states that LDREX and STREX operations to a memory region with the
> >>> Device or Strongly-ordered attribute are
> >>>  permitted, the effect of such operations is UNPREDICTABLE.
> >>>
> >>>
> >>> Given that it is implementation defined, I don't see how Linux can rely
> >>> on that behavior.
> >>
> >> I see, the problem is that while noncached and writecombined appear to
> >> be similar mappings, noncached is mapped in PRRR to strongly-ordered,
> >> while writecombined is mapped to unbufferable normal memory.
> >>
> >> I think adding a wmb() to persistent_ram_write is going to be
> >> expensive on cpus with outer caches like the L2X0, where wmb() will
> >> result in a spinlock.  Is there a real SoC where this doesn't work?
> > 
> > A real SoC where exclusives don't work to memory not mapped as normal? Take
> > your pick...
> 
> This patch doesn't actually fix problems for me. Exclusives to DDR work
> for any memory type for me as the DDR controller has an exclusive
> monitor. It takes write-thru cache mapping to get internal RAM to work.

I can't find any reference in the ARM ARM but I think you would need
cacheable memory for the exclusives to work. A9 for example uses the
cacheline exclusiveness to emulate the global monitor.

-- 
Catalin
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