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Date:   Thu, 21 Jan 2021 18:15:06 +0000
From:   Marc Zyngier <maz@kernel.org>
To:     Will Deacon <will@kernel.org>
Cc:     Mohamed Mediouni <mohamed.mediouni@caramail.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Hector Martin <marcan@marcan.st>, linux-kernel@vger.kernel.org,
        Stan Skowronek <stan@corellium.com>
Subject: Re: [RFC PATCH 3/7] arm64: mm: use nGnRnE instead of nGnRE on Apple
 processors
In-Reply-To: <20210121175525.GB22963@willie-the-truck>
References: <20210120132717.395873-1-mohamed.mediouni@caramail.com>
 <20210120132717.395873-4-mohamed.mediouni@caramail.com>
 <20210121112725.GA21750@willie-the-truck>
 <fdc49d125ef9f520254196509f6c0aa2@kernel.org>
 <20210121124742.GA22123@willie-the-truck>
 <39067FDD-F7D3-4F37-8B43-1A9FCF80EEE5@caramail.com>
 <195e2bfe3a5e5503d9988b517159300e@kernel.org>
 <20210121175525.GB22963@willie-the-truck>
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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...