linux-kernel - Re: [PATCH] ramoops appears geared to not support ARM

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Date:	Sat, 29 Oct 2011 14:42:42 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	linux-arm-kernel@...ts.infradead.org, msb@...omium.org,
	linux-kernel@...r.kernel.org, sergiu@...omium.org,
	Bryan Freed <bfreed@...omium.org>, akpm@...ux-foundation.org,
	seiji.aguchi@....com
Subject: Re: [PATCH] ramoops appears geared to not support ARM

Il 29/10/2011 13:55, Russell King - ARM Linux ha scritto:
> On Sat, Oct 29, 2011 at 01:04:30PM +0200, Marco Stornelli wrote:
>> About the ioremap problem. It seems there is a problem on some ARM arch
>> to use ioremap (ramoops driver) to remap a piece of RAM even if it's not
>> used by kernel (reserved at boot with mem option, Bryan can you
>> confirm?).
>
> It's all very simple.
>
> We have three major 'memory types' - 'normal memory' which must be used
> for things like RAM that we execute code from and use atomic operations
> within.  This can be prefetched and reordered at will.
>
> 'device memory' is for devices, which tighter restrictions on reordering
> and guarantees concerning reads and writes.
>
> 'strongly ordered memory' is much like device memory.
>
> It is absolutely not permitted to map the same physical addresses with
> different types - this is a stronger requirement than getting the cache
> attributes the same.
>
> System memory is mapped using 'normal memory' - obviously because we need
> to be able to execute code and have working atomic operations throughout
> kernel memory.
>
> Now, ioremap creates device memory - because its main function is to
> dynamically map memory regions in devices.
>
> Now think: if we have system memory mapped as 'normal memory', and then
> we try to use ioremap() to remap some of that memory, that will create
> a new 'device memory' mapping with the existing 'normal memory' mapping
> still present.  Now look at the paragraph 'It is absolutely not permitted'
> and realise that the requirements for the architecture are being violated
> if we permitted this to occur.
>
> Also realise that if that condition is violated, 'unpredictable behaviour'
> will occur - not to the extent that the CPU will hang, but it could cause
> data errors which could influence overall system stability.
>
> So, the whole idea of using plain ioremap() with system memory is one
> that is just completely unsupportable on ARM without _first_ removing
> memory from the system mapping, which in turn means updating the page
> tables for every task in the system.
>

Ok, I understand, but other question: isn't there any way to reserve 
normal memory? Or at least, hasn't the mem kernel option any effect from 
that point of view?
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