Re: [PATCH v3 2/2] irqchip/apple-aic: Add support for A7-A11 SoCs

From: Nick Chan <towinchenmi@gmail.com>
To: Sven Peter <sven@svenpeter.dev>,
	Konrad Dybcio <konrad.dybcio@somainline.org>,
	asahi@lists.linux.dev, linux-arm-kernel@lists.infradead.org
Cc: Hector Martin <marcan@marcan.st>,
	Alyssa Rosenzweig <alyssa@rosenzweig.io>,
	Thomas Gleixner <tglx@linutronix.de>,
	Marc Zyngier <maz@kernel.org>, Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	linux-kernel@vger.kernel.org, devicetree@vger.kernel.org
Subject: Re: [PATCH v3 2/2] irqchip/apple-aic: Add support for A7-A11 SoCs
Date: Thu, 6 Oct 2022 00:43:31 +0800	[thread overview]
Message-ID: <be98b367-f29f-25d1-de01-be189ae8057a@gmail.com> (raw)
In-Reply-To: <3398859f-e872-4f1d-8a03-4dcb1e46e010@app.fastmail.com>


On 4/10/2022 23:56, Sven Peter wrote:
> Hi,
> 
> 
> On Tue, Oct 4, 2022, at 13:27, Konrad Dybcio wrote:
>> Add support for A7-A11 SoCs by if-ing out some features only present
>> on:
>>
>> * A11 & newer (implementation-defined IPI & UNCORE registers)
>> * A11[1] & newer (fast IPI support).
>>
>> UNCORE/UNCORE2 and IPI registers conveniently both first appeared on
>> A11, so introduce just one check for that.
>>
>> Knowing whether the SoC supports the latter is necessary, as they are
>> written to, even if fast IPI is disabled.
> 
> AFAIK that's only an artifact in this driver: It was added to prevent an FIQ
> storm in case there were pending fast ipis (i.e. the bootloader was broken ;))
> when this driver didn't support fast ipis yet.
> 
>> This in turn causes a crash
>> on older platforms, as the implemention-defined registers either do
>> something else or are not supposed to be touched - definitely not a
>> NOP though.
>>
>> [1] A11 is supposed to use this feature, but it currently doesn't work
>> for reasons unknown and hence remains disabled. It can easily be enabled
>> on A11 only, as there is a SoC-specific compatible in the DT with a
>> fallback to apple,aic. That said, it is not yet necessary, especially
>> with only one core up, and it has worked a-ok so far.
> 
> Just to make sure I understand this correctly - we have the following three situations:
> 
> - base: no fastipi, no uncore, will work on A11 and M1 though
> - A11: fastipi and uncore but fastipi is broken (possibly due to HW errata or some bug in this driver that only happens on A11)
> - M1 (or maybe even A12 already, doesn't matter though): fastipi and uncore support
> 
> If we figured out _why_ fastipi is broken on A11 we would only need a single
> feature flag to enable both uncore and fastipi but for now we need two to
> disable fastipi for A11

The previous issues with fast IPI does not seem to be reproducible with thisversion of patches anymore, at least when modifying the device tree to useapple,t8103-aic the device still boots. Tested on iPhone X.
> 
> I'm also curious: What are the symptoms when you enable fastipi on A11?
> 
> 
> Sven


Nick Chan

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