linux-kernel - Re: [PATCH v7 1/9] arm64: dts: exynos: Add dts files for 64-bit Exynos5433 SoC

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Date:	Tue, 7 Apr 2015 11:25:27 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Chanwoo Choi <cw00.choi@...sung.com>
Cc:	"kgene@...nel.org" <kgene@...nel.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"arnd@...db.de" <arnd@...db.de>, "olof@...om.net" <olof@...om.net>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	"inki.dae@...sung.com" <inki.dae@...sung.com>,
	"chanho61.park@...sung.com" <chanho61.park@...sung.com>,
	"sw0312.kim@...sung.com" <sw0312.kim@...sung.com>,
	"jh80.chung@...sung.com" <jh80.chung@...sung.com>,
	"ideal.song@...sung.com" <ideal.song@...sung.com>,
	"a.kesavan@...sung.com" <a.kesavan@...sung.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 1/9] arm64: dts: exynos: Add dts files for 64-bit
 Exynos5433 SoC

> >>> I'm very worried about adding a DT that's known broken, especially when
> >>> we have no idea as to if/when the FW will be fixed judging from prior
> >>> replies.
> >>
> >> As I replied, I can not fix the FW because I don't have any code of FW.
> > 
> > Surely you are able to contact those who do?
> > 
> >> I don't have any solution to fix it on Linux kernel level.
> >>
> >> So, If you agree, I can add the comment of CPU0 hotplug issue on DT file.
> > 
> > I disagree. I do not want to add a DT that is known to be broken;
> > especially when we have no idea how to fix it. It creates long-term
> > maintenance pain for everyone, and marginal gain for few. A comment does
> > nothing to aid the end-user.
> > 
> > So NAK to the PSCI node and PSCI enable method in this dts until we
> > either have a working firmware, or a reasonable mechanism to handle the
> > deficiency.
> 
> There is only CPU0 hotplug issue. Excpet CPU{1-7} are well working.

I understand that, but the issue with CPU0 is still a blocker from my
PoV.

> To fix this issue, we must need the help of firmware developer.
> But, We never get the any help.

Previously you said that you did not have access to the source code
rather than not having help from the relevant firmware engineers. I take
it you have informed them of the issue with CPU0?

> Also, as I mentioned on previous mail, all Exynos SoCs can not turn
> off the CPU0. I've never seen Exynos SoC that CP0 hotplug is possible.

While that may be the case, PSCI is a more generic standard, and it is
used on systems where CPU0 can be hot unplugged. So Exynos-specific
details cannot dictate how the kernel PSCI driver should behave.

Is there a particular reason that CPU0 cannot be hotplugged?

In PSCI 0.2 and later it's possible to determine whether a trusted OS
prohibits a core from being hotplugged, but this mechanism doesn't exist
in earlier versions. I am not averse to adding a property to PSCI 0.1
to mark a CPU as not being hotpluggable if there is a fundamental reason
(i.e. not simply a bug) for this being the case.

Thanks,
Mark.
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