Re: [PATCH v3 02/31] arm64: Kernel booting and initialisation

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Sep 12, 2012 at 01:08:58PM +0100, Jean-Christophe PLAGNIOL-VILLARD wrote:
> On 17:11 Mon 10 Sep     , Catalin Marinas wrote:
> > On Sun, Sep 09, 2012 at 06:20:46PM +0100, Jean-Christophe PLAGNIOL-VILLARD wrote:
> > > On 17:26 Fri 07 Sep     , Catalin Marinas wrote:
> > > > +Before jumping into the kernel, the following conditions must be met:
> > > > +
> > > > +- Quiesce all DMA capable devices so that memory does not get
> > > > +  corrupted by bogus network packets or disk data.  This will save
> > > > +  you many hours of debug.
> > > > +
> > > > +- Primary CPU general-purpose register settings
> > > > +  x0 = physical address of device tree blob (dtb) in system RAM.
> > > > +  x1 = 0 (reserved for future use)
> > > > +  x2 = 0 (reserved for future use)
> > > > +  x3 = 0 (reserved for future use)
> > > > +
> > > > +- CPU mode
> > > > +  All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError,
> > > > +  IRQ and FIQ).
> > > > +  The CPU must be in either EL2 (RECOMMENDED in order to have access to
> > > > +  the virtualisation extensions) or non-secure EL1.
> > > > +
> > > > +- Caches, MMUs
> > > > +  The MMU must be off.
> > > > +  Instruction cache may be on or off.
> > > > +  Data cache must be off and invalidated.
> > > > +  External caches (if present) must be configured and disabled.
> > > > +
> > > > +- Architected timers
> > > > +  CNTFRQ must be programmed with the timer frequency.
> > > > +  If entering the kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0)
> > > > +  set where available.
> > > can you explain why?
> > 
> > Otherwise the kernel cannot access the generic timer registers (it is
> > described in the AArch64 exception model which isn't public yet).
> I do not like the idea to do too much in the boot loader
> 
> can we drop it and do it the head.S or find an other way

No. The kernel expects some sane setup before it can start. That's the
case already on AArch32. With AArch64 we also mandate single Image from
start. You can have a look at an example boot wrapper that creates an
environment for the AArch64 kernel:

git://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/boot-wrapper-aarch64.git

> > > > +- 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.
> > > do you plan AMP boot?
> > 
> > What do you mean by AMP?
> > 
> > If you only want to use 3 CPUs out of 4 for example, you change the FDT
> > information that gets passed to the kernel accordingly. So the kernel
> > wouldn't touch the 4th one.
> I mean boot a  different kernel in each core as example

That's not on my plan. It may be doable but I don't see any need for it.

-- 
Catalin