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    SubjectRe: [linux-sunxi] [PATCH 2/2] ARM: sun6i: Add SMP support for the Allwinner A31
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    On Sun, 2013-11-10 at 11:03 +0100, Maxime Ripard wrote:
    > Hi Ian,
    >
    > On Fri, Nov 08, 2013 at 10:25:55AM +0000, Ian Campbell wrote:
    > > On Fri, 2013-11-08 at 09:40 +0100, Maxime Ripard wrote:
    > > > > I'm trying to work out if we can make this work with the requirement
    > > > > which both Xen and KVM have to enter the kernel in NS-HYP mode.
    > > > >
    > > > > The way this works on e.g. vexpress is (roughly) that u-boot wakes up
    > > > > the secondary CPUs from the lowlevel firmware and places them into its
    > > > > own holding pen, which has the same wake up protocol as the firmware so
    > > > > the kernel can just use the same code. If u-boot never gets to run on
    > > > > secondary CPUs that isn't going to help much.
    > > > >
    > > > > My concern is that the sequence here appears to involve resetting the
    > > > > secondary CPU, which I figure will probably defeat that strategy by
    > > > > kicking the CPU back into the lowlevel firmware in the reset state,
    > > > > meaning it can't be done by a u-boot only change.
    > > >
    > > > I think this is where we're headed for the A20, Marc was interested in
    > > > doing that,
    > >
    > > Marc Zyngier is that?
    >
    > Ah yes. I forgot to put it in CC...
    >
    > > > since we already have pretty much this in u-boot already,
    > > > however, this is not the case for the A31.
    > >
    > > > As far as I know, the Allwinner's bootloader that we currently use
    > > > isn't bringing up the secondary CPUs, and we don't have any port of
    > > > some sort of u-boot yet that we could work on.
    > >
    > > Ah, OK. I'd assumed that A20 and A31 (indeed, most sunxi platforms) were
    > > mostly equivalent as far as u-boot support went.
    >
    > No. The A31 has no current support at all in u-boot(-sunxi, that is),
    > so the only bootloader we can use is Allwinner's one.
    >
    > It's one my TODO list somewhere, but as usual, time is lacking :)
    >
    > > > So, I guess we don't really have much choice in that case, even though
    > > > eventually I'd like to have this for the A31 too.
    > >
    > > Right, I suppose it makes sense to consider what we want to do on the
    > > A20 now and keep in mind that A31 may want to follow in the future.
    > >
    > > > > Hrm, what to do ... perhaps a DT driven selection between this mechanism
    > > > > and sev to kick a wfe loop reading the private register?
    > > >
    > > > We can discuss this whenever we will actually have that choice to
    > > > make, but maybe a kernel parameter would be better?
    > >
    > > I don't think so -- u-boot would then have to munge the command line to
    > > say that it had/had not brought up secondaries. DTB seems more natural
    > > to me. e.g. on ARMv8 there is already a requirement to provide a per-CPU
    > > property describing the bringup protocol ("PSCI" and "spintable" are the
    > > options there).
    >
    > Then I guess we can assume that we have to do all the CPU bring up
    > work if this property is missing?

    Yes, I think that is a fair enough assumption.

    > > Anyway, once I get to the point of being able to do something I'll
    > > coordinate with Marc etc and figure out what to do. In the meantime I
    > > think having the kernel do the bringup (like this patch does) is
    > > sensible. It's very likely to be what we want to do in the absence of
    > > any instruction to the contrary (DTB or otherwise) in the future anyway.
    >
    > Yep.
    >
    > A part from the discussion on the approach, do you have any comments
    > on the patches themselves?

    I know approximately diddly about how one is supposed to bring up these
    processors, but I did correlate what you were doing as best I could with
    the A20 manual for the registers and it looked sensible to me, modulo
    the fact that I was looking at the manual for a slightly different
    processor ;-)

    Not a terribly strong statement, sorry.

    Ian.



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