Re: [RFC PATCH 3/7] arm64: mm: use nGnRnE instead of nGnRE on Apple processors

[Marc Zyngier]

On 2021-01-21 15:12, Mohamed Mediouni wrote:
> Please ignore that patch.
>
> It turns out that the PCIe controller on Apple M1 expects posted
> writes and so the memory range for it ought to be set nGnRE.
> So, we need to use nGnRnE for on-chip MMIO and nGnRE for PCIe BARs.
>
> The MAIR approach isn’t adequate for such a thing, so we’ll have to
> look elsewhere.

Well, there isn't many alternative to having a memory type defined
in MAIR if you want to access your PCIe devices with specific
semantics.

It probably means defining a memory type for PCI only, but:
- we only have a single free MT entry, and I'm not sure we can
  afford to waste this on a specific platform (can we re-purpose
  GRE instead?),
- we'd need to teach the PCI code to use this...

Thanks,

        M.
--
Jazz is not dead. It just smells funny...