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

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Message-ID: <20210121182236.GC22963@willie-the-truck>
Date:   Thu, 21 Jan 2021 18:22:37 +0000
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Hector Martin <marcan@...can.st>, linux-kernel@...r.kernel.org,
        Stan Skowronek <stan@...ellium.com>
Subject: Re: [RFC PATCH 3/7] arm64: mm: use nGnRnE instead of nGnRE on Apple
 processors

On Thu, Jan 21, 2021 at 06:15:06PM +0000, Marc Zyngier wrote:
> 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?

Right, I'm just saying that we already have an nGnRnE MAIR configuration
so there's no need to worry about running out of entries if we need both
nGnRE and nGnRnE to co-exist. The nasty part is how to plumb this into
the mappings only for on-chip MMIO; I guess either a new API or we get
ioremap() to pick the memory type based on the address :/

Will