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

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Message-ID: <20120815170613.GA2892@quad.lixom.net>
Date:	Wed, 15 Aug 2012 10:06:13 -0700
From:	Olof Johansson <olof@...om.net>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v2 02/31] arm64: Kernel booting and initialisation

On Wed, Aug 15, 2012 at 01:20:02PM +0000, Arnd Bergmann wrote:
> On Tuesday 14 August 2012, Catalin Marinas wrote:
> 
> > +The AArch64 exception model is made up of a number of exception levels
> > +(EL0 - EL3), with EL0 and EL1 having a secure and a non-secure
> > +counterpart.  EL2 is the hypervisor level and exists only in non-secure
> > +mode. EL3 is the highest priority level and exists only in secure mode.
> 
> I'm always confused by a description like this. It sounds like you cannot
> have a hypervisor if you have code running in secure mode in EL3. What
> I instead understand is that you enter non-secure mode by going from
> EL3 into EL2.
> 
> > +2. Setup the device tree
> > +-------------------------
> > +
> > +Requirement: MANDATORY
> > +
> > +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.
> 
> I've seen people put firmware for some peripherals into the device tree,
> so that a device driver can grab a blob from there and load it into the
> device, rather than calling request_firmware() which would fail if the
> OS running on the system does not contain the blob. If such firmware is
> too large, you end up violating the 2 MB limit you impose here.
> 
> Should we keep that limit and declare those use cases as invalid, or
> should we try to make the boot protocol more flexible?
> 
> > diff --git a/arch/arm64/include/asm/setup.h b/arch/arm64/include/asm/setup.h
> > new file mode 100644
> > index 0000000..d766493
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/setup.h
> > @@ -0,0 +1,26 @@
> > +#ifndef __ASM_SETUP_H
> > +#define __ASM_SETUP_H
> > +
> > +#include <linux/types.h>
> > +
> > +#define COMMAND_LINE_SIZE 1024
> > +
> > +#endif
> 
> Is this necessary? The asm-generic version of this file allows 512 bytes,
> which seems plenty.

Chrome OS on my system today uses a 553 character cmdline, in particular
because some of the device mapper arguments are in there (since we boot without
ramdisk). It adds up quickly.

I suggest keeping it common with x86 since those limits are what people
will be used to (2048).


-Olof
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