[RFC v2 4/4] ARM: keystone: dma-coherent with safe fallback

[Arnd Bergmann]

On Monday, June 6, 2016 9:56:27 AM CEST Mark Rutland wrote:
> [adding devicetree]
> 
> On Sun, Jun 05, 2016 at 11:20:29PM -0400, Bill Mills wrote:
> > Keystone2 can do DMA coherency but only if:
> > 1) DDR3A DMA buffers are in high physical addresses (0x8_0000_0000)
> >     (DDR3B does not have this constraint)
> > 2) Memory is marked outer shared
> > 3) DMA Master marks transactions as outer shared
> >     (This is taken care of in bootloader)
> > 
> > Use outer shared instead of inner shared.
> > This choice is done at early init time and uses the attr_mod facility
> > 
> > If the kernel is not configured for LPAE and using high PA, or if the
> > switch to outer shared fails, then we fail to meet this criteria.
> > Under any of these conditions we veto any dma-coherent attributes in
> > the DTB.
> 
> I very much do not like this. As I previously mentioned [1],
> dma-coherent has de-facto semantics today. This series deliberately
> changes that, and inverts the relationship between DT and kernel (as the
> describption in the DT would now depend on the configuration of the
> kernel).
> 
> I would prefer that we have a separate property (e.g.
> "dma-outer-coherent") to describe when a device can be coherent with
> Normal, Outer Shareable, Inner Write-Back, Outer Write-Back memory.
> Then the kernel can figure out whether or not device can be used
> coherently, depending on how it is configured.

I share your concern, but I don't think the dma-outer-coherent attribute
would be a good solution either.

The problem really is that keystone is a platform that is sometimes
coherent, depending purely on what kernel we run, and not at all on
anything we can describe in devicetree, and I don't see any good way
to capture the behavior of the hardware in generic DT bindings.

So far, the assumption has been:

- when running a non-LPAE kernel, keystone is not coherent, and we
  must ignore both the dma-coherent properties in devices and the
  dma-ranges properties in bus nodes.
- when running an LPAE kernel, keystone is coherent, and we must
  respect both of those.

My interpretation of Bill's description above is that we now have
an additional requirement that at least I was not aware of before,
regarding the outer-sharable attribute. I don't think there is
much value in making this a boot-time option, since everyone would
want to run this platform in a cache-coherent way if at all possible.

We already have special hacks to detect the case of keystone running
in LPAE mode, in order to do the special rewrite-all-page-tables
hack at boot time for relocating the physical address, and we could
use the same hack to change the page table attributes.

The question is how to communicate the requirement for outer-sharable
for a platform. If we think it's a safe assumption that there will
not be future 32-bit platforms with this requirement (or maybe one
or two more at most), we could leave it in the special keystone hack.
Alternatively, a DT property in an appropriate node could indicate
that a particular platform requires it.

	Arnd