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

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Date:	Wed, 29 Jul 2015 15:11:14 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Dan Williams <dan.j.williams@...el.com>,
	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	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>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 Wed, 2015-07-29 at 15:00 -0600, Toshi Kani wrote:
> On Wed, 2015-07-29 at 11:33 -0700, Dan Williams wrote:
> > On Wed, Jul 29, 2015 at 11:27 AM, Luis R. Rodriguez <mcgrof@...e.com> 
> > wrote:
> > > On Wed, Jul 29, 2015 at 08:50:04AM +0200, Christoph Hellwig wrote:
> > > > On Mon, Jul 27, 2015 at 04:26:03PM -0700, Dan Williams wrote:
> > > > > Oh, because all we have at this point is ioremap_cache() which
> > > > > silently falls back.  It's not until the introduction of
> > > > > arch_memremp() where we update the arch code to break that 
> > > > > behavior.
> > > > 
> > > > Ok, makes sense.  Might be worth to document in the changelog.
> > > > 
> > > > > That said, I think it may be beneficial to allow a fallback if 
> > > > > the
> > > > > user cares.  So maybe memremap() can call plain ioremap() if
> > > > > MEMREMAP_STRICT is not set and none of the other mapping types 
> > > > > are
> > > > > satisfied.
> > > > 
> > > > Is there a real use case for it?  Fallback APIs always seem 
> > > > confusing
> > > > and it might make more sense to do this in the caller(s) that 
> > > > actually
> > > > need it.
> > > 
> > > It seems semantics-wise we are trying to separate these two really, 
> > > so 
> > > I agree
> > > with this. Having a fallback would onloy make things more complicated 
> > > 
> > > for any
> > > sanitizer / checker / etc, and I don't think the practical gains of 
> > > having a
> > > fallback outweight the gains of having a clear semantic separation on 
> > > 
> > > intended
> > > memory type and interactions with it.
> > > 
> > 
> > Yup, consider it dropped.  Drivers that want fallback behavior can do
> > it explicitly.
> 
> I agree in general.  However, for the drivers to be able to fall back
> properly, they will need to know the cache type they can fall back with. 
>  ioremap() falls back to the cache type of an existing mapping to avoid
> aliasing.

Never mind...  In this context, we are talking about fallback in case of "
API not implemented" on arch.  I was talking about fallback in case of
aliasing.

Thanks,
-Toshi





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