Re: [PATCH] arm64: cache: Lower ARCH_DMA_MINALIGN to 64 (L1_CACHE_BYTES)

From: Marc Zyngier <maz@kernel•org>
To: Arnd Bergmann <arnd@arndb•de>
Cc: Will Deacon <will@kernel•org>,
	Robin Murphy <robin.murphy@arm•com>,
	Catalin Marinas <catalin.marinas@arm•com>,
	Yassine Oudjana <y.oudjana@protonmail•com>,
	Ard Biesheuvel <ardb@kernel•org>,
	Android Kernel Team <kernel-team@android•com>,
	Linux ARM <linux-arm-kernel@lists•infradead.org>,
	Mark Rutland <mark.rutland@arm•com>,
	Vincent Whitchurch <vincent.whitchurch@axis•com>,
	linux-arm-msm <linux-arm-msm@vger•kernel.org>
Subject: Re: [PATCH] arm64: cache: Lower ARCH_DMA_MINALIGN to 64 (L1_CACHE_BYTES)
Date: Tue, 06 Jul 2021 15:46:04 +0100	[thread overview]
Message-ID: <87zguz7b6b.wl-maz@kernel.org> (raw)
In-Reply-To: <CAK8P3a2xWTvj6HjsC6gH44Ad13adKjK0wR7UxFFQ1i=XYixvQA@mail.gmail.com>

On Tue, 06 Jul 2021 15:30:34 +0100,
Arnd Bergmann <arnd@arndb•de> wrote:
> 
> On Tue, Jul 6, 2021 at 3:44 PM Marc Zyngier <maz@kernel•org> wrote:
> >
> > On 2021-07-06 14:33, Will Deacon wrote:
> > > On Tue, Jul 06, 2021 at 02:29:07PM +0100, Robin Murphy wrote:
> > >
> > > I can't find much information about the original Kryo core at all...
> >
> > I have similar issues with my QDF2400. The UART, RTC and DMA controllers
> > are all screaming at me. I'm confident that the UART doesn't do any
> > DMA (it is handled by the SBSA driver), but the DMA controllers are
> > probably doing what it says on the tin.
> 
> But that's a server chip, surely the DMA controller is fully cache
> coherent as required by SBSA? (please?)

Remember that there is a SBSA level for each broken implementation
(even my XGene is SBSA compliant!), so that doesn't mean much.

> Maybe just a misannotation on the device node?

Maybe. But given that whoever wrote the ACPI tables made sure that
everything else was annotated as coherent, I doubt the DMA controllers
being advertised as non-coherent is an accident.

> > Do we know whether Falkor and Kryo share any part of their design?
> 
> I'm fairly sure the Snapdragon 821 / msm8996 is not cache coherent.
> 
> I can only speculate on how much got reused between the two, but
> as Falkor was released only after they had already given up on
> the full-custom Kryo core, it's plausible that it incorporates bits from
> that one. In particular the cache controller is probably easy to reuse
> even if the rest of it was a new design.

I guess we'll never find out, and I'm probably one of the few still
having some access to this HW (not even sure for how long anyway).

I won't cry if we decide to pull the plug on it.

	M.

-- 
Without deviation from the norm, progress is not possible.

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