Hi Ian, On Mon, Nov 04, 2013 at 04:53:17PM +0000, Ian Campbell wrote: > Not a comment on the patch, or even A31, but a question about how the > SMP boot process works on sunxi generally... > > On Sun, 2013-11-03 at 10:30 +0100, Maxime Ripard wrote: > > The A31 is a quad Cortex-A7. Add the logic to use the IPs used to > > control the CPU configuration and the CPU power so that we can bring up > > secondary CPUs at boot. > > [...] > > + /* Set CPU boot address */ > > + writel(virt_to_phys(sun6i_secondary_startup), > > + cpucfg_membase + CPUCFG_PRIVATE0_REG); > > Does the secondary CPU jump straight here from the lowlevel firmware or > does it go through the bootloader which reads the reg and does the jump? > I can't see any reference to this reg in u-boot so I guess the former?
There's no reference of this either in the A31 datasheet (or at least, none I could find), so I assume the former too. > 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, 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. 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. > 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? Thanks, Maxime -- Maxime Ripard, Free Electrons Embedded Linux, Kernel and Android engineering http://free-electrons.com
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