linux-kernel - Re: [RFC PATCH 1/2] memremap: add arch specific hook for MEMREMAP

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Date:	Tue, 23 Feb 2016 13:03:14 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>, Nicolas Pitre <nico@...aro.org>
Subject: Re: [RFC PATCH 1/2] memremap: add arch specific hook for MEMREMAP_WB mappings

On 23 February 2016 at 12:58, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Mon, Feb 22, 2016 at 09:35:24PM +0100, Ard Biesheuvel wrote:
>> OK, thanks for the historical context.
>>
>> So what is your opinion on this series, i.e., to wire up memremap() to
>> remap arbitrary memory regions into the vmalloc area with MT_MEMORY_RW
>> attributes, and at the same time lift the restriction that the region
>> must be disjoint from memory covered by lowmem or kmap?
>
> The historical context is still present, because pxa2xx-flash has
> been converted to use memremap() from ioremap_cache() - possibly
> inappropriately.
>
> I've already described the semantics of ioremap_cache(), which are
> to always create a cacheable mapping irrespective of the system
> memory mapping type.  However, memremap() says that MEMREMAP_WB
> matches system RAM, which on ARM it doesn't right now.
>

Indeed. Hence this series, to decouple memremap(MEMREMAP_WB) from
ioremap_cache() for ARM

> Changing it to MT_MEMORY_RW would satisfy that comment against
> memremap(), but at the same time changes what happens with
> pxa2xx-flash - the memory region (which is not system RAM) then
> changes with the cache status of system RAM.
>
> So, I'm not that happy about the memremap() stuff right now, and
> I don't like the idea of making memremap() conform to its stated
> requirements without first preventing pxa2xx-flash being affected
> by such a change.
>

Actually, my change fixes this issue, since it will cause memremap()
to always create MT_MEMORY_RW mappings, and not fallback to
ioremap_cache() for ranges that are not covered by lowmem.

> Perhaps we need to reinstate the original ioremap_cached() API for
> pxa2xx-flash, and then switch memremap() to MT_MEMORY_RW - that
> would seem to result in the expected behaviour by all parties.
>

I think we can simply revert the change to pxa2xx-flash if it is
deemed inappropriate.