111 lines
5.5 KiB
ReStructuredText
111 lines
5.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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DeviceTree Booting
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------------------
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During the development of the Linux/ppc64 kernel, and more specifically, the
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addition of new platform types outside of the old IBM pSeries/iSeries pair, it
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was decided to enforce some strict rules regarding the kernel entry and
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bootloader <-> kernel interfaces, in order to avoid the degeneration that had
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become the ppc32 kernel entry point and the way a new platform should be added
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to the kernel. The legacy iSeries platform breaks those rules as it predates
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this scheme, but no new board support will be accepted in the main tree that
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doesn't follow them properly. In addition, since the advent of the arch/powerpc
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merged architecture for ppc32 and ppc64, new 32-bit platforms and 32-bit
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platforms which move into arch/powerpc will be required to use these rules as
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well.
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The main requirement that will be defined in more detail below is the presence
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of a device-tree whose format is defined after Open Firmware specification.
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However, in order to make life easier to embedded board vendors, the kernel
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doesn't require the device-tree to represent every device in the system and only
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requires some nodes and properties to be present. For example, the kernel does
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not require you to create a node for every PCI device in the system. It is a
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requirement to have a node for PCI host bridges in order to provide interrupt
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routing information and memory/IO ranges, among others. It is also recommended
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to define nodes for on chip devices and other buses that don't specifically fit
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in an existing OF specification. This creates a great flexibility in the way the
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kernel can then probe those and match drivers to device, without having to hard
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code all sorts of tables. It also makes it more flexible for board vendors to do
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minor hardware upgrades without significantly impacting the kernel code or
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cluttering it with special cases.
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Entry point
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~~~~~~~~~~~
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There is one single entry point to the kernel, at the start
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of the kernel image. That entry point supports two calling
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conventions:
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a) Boot from Open Firmware. If your firmware is compatible
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with Open Firmware (IEEE 1275) or provides an OF compatible
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client interface API (support for "interpret" callback of
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forth words isn't required), you can enter the kernel with:
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r5 : OF callback pointer as defined by IEEE 1275
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bindings to powerpc. Only the 32-bit client interface
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is currently supported
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r3, r4 : address & length of an initrd if any or 0
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The MMU is either on or off; the kernel will run the
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trampoline located in arch/powerpc/kernel/prom_init.c to
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extract the device-tree and other information from open
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firmware and build a flattened device-tree as described
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in b). prom_init() will then re-enter the kernel using
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the second method. This trampoline code runs in the
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context of the firmware, which is supposed to handle all
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exceptions during that time.
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b) Direct entry with a flattened device-tree block. This entry
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point is called by a) after the OF trampoline and can also be
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called directly by a bootloader that does not support the Open
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Firmware client interface. It is also used by "kexec" to
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implement "hot" booting of a new kernel from a previous
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running one. This method is what I will describe in more
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details in this document, as method a) is simply standard Open
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Firmware, and thus should be implemented according to the
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various standard documents defining it and its binding to the
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PowerPC platform. The entry point definition then becomes:
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r3 : physical pointer to the device-tree block
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(defined in chapter II) in RAM
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r4 : physical pointer to the kernel itself. This is
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used by the assembly code to properly disable the MMU
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in case you are entering the kernel with MMU enabled
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and a non-1:1 mapping.
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r5 : NULL (as to differentiate with method a)
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Note about SMP entry: Either your firmware puts your other
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CPUs in some sleep loop or spin loop in ROM where you can get
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them out via a soft reset or some other means, in which case
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you don't need to care, or you'll have to enter the kernel
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with all CPUs. The way to do that with method b) will be
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described in a later revision of this document.
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Board supports (platforms) are not exclusive config options. An
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arbitrary set of board supports can be built in a single kernel
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image. The kernel will "know" what set of functions to use for a
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given platform based on the content of the device-tree. Thus, you
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should:
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a) add your platform support as a _boolean_ option in
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arch/powerpc/Kconfig, following the example of PPC_PSERIES,
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PPC_PMAC and PPC_MAPLE. The latter is probably a good
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example of a board support to start from.
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b) create your main platform file as
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"arch/powerpc/platforms/myplatform/myboard_setup.c" and add it
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to the Makefile under the condition of your ``CONFIG_``
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option. This file will define a structure of type "ppc_md"
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containing the various callbacks that the generic code will
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use to get to your platform specific code
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A kernel image may support multiple platforms, but only if the
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platforms feature the same core architecture. A single kernel build
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cannot support both configurations with Book E and configurations
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with classic Powerpc architectures.
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