linux-kernel - Re: [PATCH 1/3] ARM: Premit ioremap() to map reserved pages

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Date:	Wed, 22 Jan 2014 19:55:40 +0800
From:	Wang Nan <wangnan0@...wei.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	<kexec@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
	Geng Hui <hui.geng@...wei.com>, <linux-mm@...ck.org>,
	Eric Biederman <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/3] ARM: Premit ioremap() to map reserved pages

On 2014/1/22 19:42, Russell King - ARM Linux wrote:
> On Wed, Jan 22, 2014 at 07:25:14PM +0800, Wang Nan wrote:
>> This patch relaxes the restriction set by commit 309caa9cc, which
>> prohibit ioremap() on all kernel managed pages.
>>
>> Other architectures, such as x86 and (some specific platforms of) powerpc,
>> allow such mapping.
>>
>> ioremap() pages is an efficient way to avoid arm's mysterious cache control.
>> This feature will be used for arm kexec support to ensure copied data goes into
>> RAM even without cache flushing, because we found that flush_cache_xxx can't
>> reliably flush code to memory.
> 
> Yes, let's bypass the check and allow this in violation of the
> architecture specification by allowing mapping the same memory with
> different types, which leads to unpredictable behaviour.  Yes, that's
> a very good idea, because what we want to do is far more important than
> following the requirements of the architecture.
> 
> So... NAK.
> 
> Yes, flush_cache_xxx() doesn't flush back to physical RAM, that's not
> what it's defined to do - it's defined that it flushes enough of the
> cache to ensure that page table updates are safe (such as when tearing
> down a page mapping.)  So it's hardly surprising that doesn't work.
> 
> If you want to be able to have DMA access to memory, then you need to
> use an API which has been designed for that purpose, and if there isn't
> one, then you need to discuss your requirements, rather than trying to
> hack around the problem.

So what is correct API which is designed for this propose?

> 
> The issue here will be that the APIs we currently have for DMA become
> extremely expensive when you want to deal with (eg) all system RAM.
> Or, there's flush_cache_all() which should flush all levels of cache
> in the system, and thus push all data back to RAM.
> 
> Now, why are you copying your patches to the stable people?  That makes
> no sense - they haven't been reviewed and they haven't been integrated
> into an existing kernel.  So, they don't meet the basic requirements
> for stable tree submission...
> 


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