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    SubjectRe: [PATCH v2 02/31] arm64: Kernel booting and initialisation
    Date
    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.

    > +unsigned int processor_id;
    > +EXPORT_SYMBOL(processor_id);
    > +
    > +unsigned int elf_hwcap __read_mostly;
    > +EXPORT_SYMBOL(elf_hwcap);

    EXPORT_SYMBOL_GPL?

    Neither of these looks like they should be used in drivers.

    Arnd


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