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

[Marc Zyngier <maz@kernel.org>]

On 2021-01-21 17:55, Will Deacon wrote:
> On Thu, Jan 21, 2021 at 04:25:54PM +0000, Marc Zyngier wrote:
>> 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 already have an nGnRnE MAIR for config space accesses.

I'm confused. If M1 needs nGnRE for PCI, and overrides nGnRE to nE
for its in-SoC accesses, where does nGnRE goes?

Or do you propose that it is the page tables that get a different
MT index?

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