Re: [PATCH v2 02/31] arm64: Kernel booting and initialisation

[Shilimkar, Santosh]

On Fri, Aug 17, 2012 at 3:35 PM, Catalin Marinas
<catalin.marinas@xxxxxxx> wrote:
>
> On Fri, Aug 17, 2012 at 10:41:10AM +0100, Santosh Shilimkar wrote:
> > On Tuesday 14 August 2012 11:22 PM, Catalin Marinas wrote:
> > > +The boot loader is expected to enter the kernel on each CPU in the
> > > +following manner:
> > > +
> > > +- The primary CPU must jump directly to the first instruction of the
> > > +  kernel image.  The device tree blob passed by this CPU must contain
> > > +  for each CPU node:
> > > +
> > > +    1. An 'enable-method' property. Currently, the only supported
> > > value
> > > +       for this field is the string "spin-table".
> > > +
> > > +    2. A 'cpu-release-addr' property identifying a 64-bit,
> > > +       zero-initialised memory location.
> > > +
> > > +  It is expected that the bootloader will generate these device tree
> > > +  properties and insert them into the blob prior to kernel entry.
> > > +
> > > +- Any secondary CPUs must spin outside of the kernel in a reserved
> > > area
> > > +  of memory (communicated to the kernel by a /memreserve/ region in
> > > the
> > > +  device tree) polling their cpu-release-addr location, which must be
> > > +  contained in the reserved region.  A wfe instruction may be
> > > inserted
> > > +  to reduce the overhead of the busy-loop and a sev will be issued by
> > > +  the primary CPU.  When a read of the location pointed to by the
> > > +  cpu-release-addr returns a non-zero value, the CPU must jump
> > > directly
> > > +  to this value.
> >
> > So you expect all the secondary CPUs to be in wakeup state and probably
> > looping in WFE for a signal from kernel to boot. There is one issue
> > with this requirement though. For large CPU system, you need to reset
> > all the CPUs and hit this waiting loop. This will lead to large inrush
> > current need at bootup which may be not be supported. To avoid this
> > issue, secondary CPUs are kept in OFF state and then they are woken
> > up from kernel one by one whenever they need to be brought into the
> > system. This requirement should be considered.
>
> I agree, this part will be extended. That's one method that we currently
> support and suitable to the model.
>
> The better method is the SMC standardisation that Charles Garcia-Tobin
> has written (to be made available soon) and was presented at the last
> Linaro Connect in HK. Given that the CPU power is usually controlled by
> the secure side, we'll ask for an SMC to be issued for waking up
> secondary CPUs, so it's up to the secure firmware to write the correct
> hardware registers.
>
Thanks for the information. SMC standardization would indeed help
to overcome some of these. Will wait for that information before
next set of questions.

> > > --- /dev/null
> > > +++ b/arch/arm64/kernel/head.S
> > [..]
> > > +   /*
> > > +    * DO NOT MODIFY. Image header expected by Linux boot-loaders.
> > > +    */
> > > +   b       stext                           // branch to kernel start,
> > > magic
> > > +   .long   0                               // reserved
> > > +   .quad   TEXT_OFFSET                     // Image load offset from
> > > start of RAM
> > > +   .quad   0                               // reserved
> > > +   .quad   0                               // reserved
> > > +
> >
> > Minor nit. Avoid C++ commenting style "//"  here and rest of the patch.
>
> That's not C++ comment style, it's the *official* assembly comment style
> for AArch64 ('@' is no longer supported).
>
Ok. Thanks for clarifying.

Regards
Santosh
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