linux-kernel - Re: [PATCH 2/3] arm64: dts: add the Alpine v2 EVP

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Message-ID: <CABM=7k=-Y+eFB2nUMfMjd3saTSNDsEyfHyYNf-ef8qQc5jCsxA@mail.gmail.com>
Date:	Tue, 9 Feb 2016 12:13:04 +0200
From:	Tsahee Zidenberg <tsahee@...apurnalabs.com>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Antoine Tenart <antoine.tenart@...e-electrons.com>,
	catalin.marinas@....com, will.deacon@....com,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Ronen Shitrit <rshitrit@...apurnalabs.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Barak Wasserstrom <barak@...apurnalabs.com>
Subject: Re: [PATCH 2/3] arm64: dts: add the Alpine v2 EVP

On 9 February 2016 at 11:30, Marc Zyngier <marc.zyngier@....com> wrote:
>
> On 09/02/16 09:14, Tsahee Zidenberg wrote:
> >
> >
> > On 9 February 2016 at 11:09, Marc Zyngier <marc.zyngier@....com
> > <mailto:marc.zyngier@....com>> wrote:
> >
> >     On 09/02/16 09:01, Antoine Tenart wrote:
> >     > On Tue, Feb 09, 2016 at 09:56:33AM +0100, Antoine Tenart wrote:
> >     >> Hi Marc,
> >     >>
> >     >> On Mon, Feb 08, 2016 at 03:29:33PM +0000, Marc Zyngier wrote:
> >     >>> On 08/02/16 09:11, Antoine Tenart wrote:
> >     >>>
> >     >>>> +          gic: gic@...00000 {
> >     >>>> +                  compatible = "arm,gic-v3";
> >     >>>> +                  reg = <0x0 0xf0200000 0x0 0x10000>,     /* GIC Dist */
> >     >>>> +                        <0x0 0xf0280000 0x0 0x200000>,    /* GICR */
> >     >>>> +                        <0x0 0xf0100000 0x0 0x2000>;      /* GICC */
> >     >>>> +                  interrupt-controller;
> >     >>>> +                  #interrupt-cells = <3>;
> >     >>>> +          };
> >     >>>
> >     >>> Something is wrong here. Either you are missing GICH and GICV (assuming
> >     >>> you have legacy support), or you have an extra GICC region (which
> >     >>> doesn't make sense on its own).
> >     >>
> >     >> I'll add the missing regions.
> >     >
> >     > Hmm, in fact the GICC region shouldn't be there. I'll make some tests
> >     > and remove it.
> >
> >     If you have a GICv3 with legacy support, you will probably have GICC,
> >     GICH and GICV. Linux itself will only use GICD and GICR, but it needs at
> >     least GICV to be able to virtualize GICv2 guests. And GICV is not
> >     allowed to exist without GICC and GICH, so I really recommend that you
> >     keep GICC around.
> >
> >
> > We use the GIC without legacy support (we disable it in early boot
> > stages), so I think removing the GICC region is the better solution.
>
> Disabling legacy support doesn't mean that:
> - the HW isn't present
> - the associated regions are not useful
>
By "disabling lecgacy support in early boot" I don't just mean that
ARE bit will be set, but it will actually be RAO/WI. There will be no
way for SW to enable it and use these registers (which, sadly, means
that there will be no way to enable gicv2 virtualization). If you
insist - I will dig up the supposed location of GICV and GICH - yet it
will be both untested and entirely unusable.

We will add an entry for the maintenance interrupt, as it really can
be used by future configurations.

> Here, we describe the HW, not the usage you make of it.
>
> And unless you think that having working virtualization is completely
> useless, I strongly suggest that you properly document the HW in the
> device tree.
>
> Thanks,
>
>         M.
> --
> Jazz is not dead. It just smells funny...