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

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Message-ID: <20120912120858.GI31430@game.jcrosoft.org>
Date:	Wed, 12 Sep 2012 14:08:58 +0200
From:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 02/31] arm64: Kernel booting and initialisation

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:
> > > +The device tree blob (dtb) must be no bigger than 2 megabytes in size
> > > +and placed at a 2-megabyte boundary within the first 512 megabytes from
> > > +the start of the kernel image. This is to allow the kernel to map the
> > > +blob using a single section mapping in the initial page tables.
> > why do you want to restrict the DT to be less tahn 2MiB?
> 
> That's a restriction due on the initial memory map. At some point we may
> add support in head.S to parse the dtb and extract the size information.
> Not critical at this stage.
> 
> > > +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

> 
> > > +- 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

Best Regards,
J.
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