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

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?

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