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

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Message-ID: <CAPcyv4gtzzSBTUdd6_5P0C=bJBgbb1q035bQX9OcxUUT1ifJeA@mail.gmail.com>
Date:	Mon, 22 Feb 2016 11:55:35 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"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 Mon, Feb 22, 2016 at 11:17 AM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> On 22 February 2016 at 20:05, Dan Williams <dan.j.williams@...el.com> wrote:
>> On Mon, Feb 22, 2016 at 6:02 AM, Ard Biesheuvel
>> <ard.biesheuvel@...aro.org> wrote:
>>> Currently, the memremap code serves MEMREMAP_WB mappings directly from
>>> the kernel direct mapping, unless the region is in high memory, in which
>>> case it falls back to using ioremap_cache(). However, the semantics of
>>> ioremap_cache() are not unambiguously defined, and on ARM, it will
>>> actually result in a mapping type that differs from the attributes used
>>> for the linear mapping, and for this reason, the ioremap_cache() call
>>> fails if the region is part of the memory managed by the kernel.
>>>
>>> So instead, implement an optional hook 'arch_memremap_wb' whose default
>>> implementation calls ioremap_cache() as before, but which can be
>>> overridden by the architecture to do what is appropriate for it.
>>>
>>
>> Acked-by: Dan Williams <dan.j.williams@...el.com>
>>
>> I still have patches pending to delete ioremap_cache() from ARM and
>> require memremap() to be used for cacheable mappings.  Do you see any
>> use for ioremap_cache() on ARM after this change?
>
> I am not exactly sure why ioremap_cache() does not use MT_MEMORY_RW
> attributes, but the ARM architecture simply does not allow mismatched
> attributes, so we cannot simply replace each instance of
> ioremap_cache() with memremap()
>
> Perhaps Russell can explain?

>From the x86 perspective mismatched mappings are also disallowed.  The
goal of deprecating ioremap_cache() in favor of memremap() is simply
that the __iomem annotation is misplaced [1], and to clean up the
calling convention to be explicit without silent fallbacks to
different mapping types.

[1]: https://lwn.net/Articles/653585/