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

Toshi Kani
2015-07-29T21:12:35Z

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@suse.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