[PATCH 0/3] ARM: mvebu: disable I/O coherency on !SMP

[arnd@arndb.de (Arnd Bergmann)]

On Thursday 17 July 2014 15:27:20 Will Deacon wrote:
> On Thu, Jul 17, 2014 at 02:12:24PM +0100, Russell King - ARM Linux wrote:
> > On Thu, Jul 17, 2014 at 02:39:42PM +0200, Arnd Bergmann wrote:
> > > On Thursday 17 July 2014 09:33:42 Russell King - ARM Linux wrote:
> > > > No - the problem is that we're running from the page table in question
> > > > with global mappings, and we need to switch all these mappings, including
> > > > the ones we're currently using to execute from.
> > > > 
> > > > We can't even create a new page table and switch to it because the
> > > > mappings in question are global mappings.
> > > > 
> > > > The only way to do that safely from an architectural point of view would
> > > > be to turn the MMU off, and drop back to assembly code to change the
> > > > page tables, and re-enable the MMU.  For something as obscure as Marvell's
> > > > coherency stuff, that's not something I want to see in core code.
> > > 
> > > Is this different from what we do in the LPAE version of
> > > early_paging_init()? That code already adjusts all the page
> > > table entries on a per-platform setting and should be very
> > > easy to extend for a modified procinfo->__cpu_mm_mmu_flags,
> > > and possibly able to extend for traditional (non-LPAE)
> > > page tables without a lot of duplication.
> > 
> > I'm going to have to profess no knowledge of how the LPAE stuff works.
> > LPAE is something I can't run, and something that I was not involved
> > in its creation.  Therefore, I know very little about it and zero
> > experience thereof.
> > 
> > From a glance over that function, I think that is unsafe for all the
> > reasons I've stated.  However, I'll pass this over to Will Deacon
> > to comment further on.
> 
> Just an FYI, but I've had to check internally to clarify the behaviour
> around TLB conflict aborts. We're changing live page tables here, but the
> *only* thing to differ is the output address, which I would hope is ok but
> need to check to be sure.

The two cases are slightly different:

- The existing keystone code changes the output address but not the flags
- The hypothetical mvebu code needs to change the flags but not the address.

	Arnd