linux-kernel - Re: [PATCH v2 08/25] arch: introduce memremap()

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Date:	Mon, 27 Jul 2015 17:32:54 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Christoph Hellwig <hch@....de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 08/25] arch: introduce memremap()

On Mon, Jul 27, 2015 at 4:43 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> On Mon, Jul 27, 2015 at 04:31:09PM -0700, Dan Williams wrote:
>> On Mon, Jul 27, 2015 at 4:17 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
>> > On Fri, Jul 24, 2015 at 10:38:42PM -0400, Dan Williams wrote:
>> >> Existing users of ioremap_cache() are mapping memory that is known in
>> >> advance to not have i/o side effects.  These users are forced to cast
>> >> away the __iomem annotation, or otherwise neglect to fix the sparse
>> >> errors thrown when dereferencing pointers to this memory.  Provide
>> >> memremap() as a non __iomem annotated ioremap_*() in the case when
>> >> ioremap is otherwise a pointer to memory.
>> >
>> > Ok so a special use case.
>> >
>> >> Outside of ioremap() and
>> >> ioremap_nocache(), the expectation is that most calls to
>> >> ioremap_<type>() are seeking memory-like semantics (e.g.  speculative
>> >> reads, and prefetching permitted).  These callsites can be moved to
>> >> memremap() over time.
>> >
>> > Such memory-like smantics are not well defined yet and this has caused
>> > issues over expectations over a slew of APIs. As you note above
>> > your own defined 'semantics' so far for memremap are just that there are
>> > no i/o side effects, when the mapped memory is just a pointer to memory,
>> > as such I do not think its fair to set the excpectations that we'll
>> > switch all other ioremap_*() callers to the same memremap() API. Now,
>> > it may be a good idea to use something similar, ie, to pass flags,
>> > but setting the expectations outright to move to memremap() without having
>> > any agreement having been made over semantics seems uncalled for at this
>> > point in time, specially when you are noting that the expectations for
>> > both sets of calls are different.
>> >
>> > So perhaps:
>> >
>> > "
>> > Outside of ioremap() and ioremap_nocache(), all other ioremap_<type>()
>> > variant calls are seeking memory-like semantics (e.g.  speculative
>> > reads, and prefetching permitted) and all calls expecations currently
>> > differ depending on architecture. Once and if we get agreement on such
>> > semantics we may be able to move such ioremap_*() variant calls to
>> > a similar API where the semantics required are clearly spelled out
>> > and well defined and passed as arguments.
>>
>> I still think ioremap_wc(), and now ioremap_uc(), are special and are
>> not obvious candidates for conversion to memremap().
>
> OK great, then we're in strong agreement, so removing the "Outside of
> ioremap() and... over time" might be best then? Otherwise what I posted
> seems to reflect the state of affairs better?
>

Ah yes, I need to clean that up.  Thanks!
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