Subject: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency

From: [email-masked] (Will Deacon)
Date: Thu, 30 Apr 2015 12:07:18 +0100

On Thu, Apr 30, 2015 at 11:47:46AM +0100, Arnd Bergmann wrote:
> On Thursday 30 April 2015 11:41:02 Will Deacon wrote:
> >    - 0x0: The device is not coherent. Therefore:
> >      * Cache maintenance is required for memory shared with the
> >        device that is mapped on CPUs as IWB-OWB-ISH.
> 
> This still seems insufficient. I guess this excludes having to
> synchronize external bridges or write buffers, but it does not specify
> what cache maintenance is required. Should there be an "outer-flush"?
> Should the CPU cache be invalidated or flushed (or both), and do
> we need to care about caches inside of the device or just inside of
> the CPU?

See the note below:

> > [1] Note: Caching operations described in this document apply to the CPU
> >     caches and any other caches in the system where device memory accesses
> >     can hit.'

So for the CPU caches we'd do the usual clean to push dirty lines to the device
and (clean+)invalidate before reading data from the device. For the "other
caches in the system" we currently assume (for ARM64) that cache maintenance
will be broadcast and therefore I wouldn't anticipate doing anything extra.

If people want to build system caches that don't respect broadcast cache
maintenance and require explicit management (e.g outer_flush), then I
consider that a broken system and we should try to disable the cache before
entering the kernel. ARMv8 explicitly prohibits this type of cache in the
architecture (type 1 below):

  `Conceptually, three classes of system cache can be envisaged:

   1. System caches which lie before the point of coherency and cannot
      be managed by any cache maintenance instructions. Such systems
      fundamentally undermine the concept of cache maintenance
      instructions operating to the point of coherency, as they imply
      the use of non-architecture mechanisms to manage coherency. The
      use of such systems in the ARM architecture is explicitly
      prohibited.

   2. System caches which lie before the point of coherency and can be
      managed by cache maintenance by address instructions that apply to
      the point of coherency, but cannot be managed by cache maintenance
      by set/way instructions. Where maintenance of the entirety of such
      a cache must be performed, as in the case for power management, it
      must be performed using non-architectural mechanisms.

   3. System caches which lie beyond the point of coherency and so are
      invisible to the software. The management of such caches is
      outside the scope of the architecture.'

(sorry to keep throwing the book at you!)

Will