linux-kernel - Re: [RFT PATCH v3 27/27] arm64: apple: Add initial Apple Mac mini (M1, 2020) devicetree

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Date:   Sat, 6 Mar 2021 01:50:20 +0900
From:   Hector Martin <marcan@...can.st>
To:     Mark Kettenis <mark.kettenis@...all.nl>
Cc:     krzysztof.kozlowski@...onical.com,
        linux-arm-kernel@...ts.infradead.org, maz@...nel.org,
        robh@...nel.org, arnd@...nel.org, olof@...om.net, tony@...mide.com,
        mohamed.mediouni@...amail.com, stan@...ellium.com, graf@...zon.com,
        will@...nel.org, linus.walleij@...aro.org, mark.rutland@....com,
        andy.shevchenko@...il.com, gregkh@...uxfoundation.org,
        corbet@....net, catalin.marinas@....com, hch@...radead.org,
        davem@...emloft.net, devicetree@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFT PATCH v3 27/27] arm64: apple: Add initial Apple Mac mini
 (M1, 2020) devicetree

On 06/03/2021 00.59, Mark Kettenis wrote:
> It may be better to handle the memory reserved by the firmware using a
> "/reserved-memory" node.  I think the benefit of that could be that it
> communicates the entire range of physical memory to the kernel, which
> means it could use large mappings in the page tables.  Unless the
> "/reserved-memory" node defines a region that has the "no-map"
> property of course.

We actually need no-map, because otherwise the CPU could speculate its 
way into these carveouts (it's not just firmware, there's stuff in here 
the CPU really can't be allowed to touch, e.g. the SEP carveout). It 
also breaks simplefb mapping the framebuffer. I thought of the 
reserved-memory approach, but then figured it wouldn't buy us anything 
for this reason.

-- 
Hector Martin (marcan@...can.st)
Public Key: https://mrcn.st/pub