1.. SPDX-License-Identifier: GPL-2.0
   3DeviceTree Booting
   6During the development of the Linux/ppc64 kernel, and more specifically, the
   7addition of new platform types outside of the old IBM pSeries/iSeries pair, it
   8was decided to enforce some strict rules regarding the kernel entry and
   9bootloader <-> kernel interfaces, in order to avoid the degeneration that had
  10become the ppc32 kernel entry point and the way a new platform should be added
  11to the kernel. The legacy iSeries platform breaks those rules as it predates
  12this scheme, but no new board support will be accepted in the main tree that
  13doesn't follow them properly.  In addition, since the advent of the arch/powerpc
  14merged architecture for ppc32 and ppc64, new 32-bit platforms and 32-bit
  15platforms which move into arch/powerpc will be required to use these rules as
  18The main requirement that will be defined in more detail below is the presence
  19of a device-tree whose format is defined after Open Firmware specification.
  20However, in order to make life easier to embedded board vendors, the kernel
  21doesn't require the device-tree to represent every device in the system and only
  22requires some nodes and properties to be present. For example, the kernel does
  23not require you to create a node for every PCI device in the system. It is a
  24requirement to have a node for PCI host bridges in order to provide interrupt
  25routing information and memory/IO ranges, among others. It is also recommended
  26to define nodes for on chip devices and other buses that don't specifically fit
  27in an existing OF specification. This creates a great flexibility in the way the
  28kernel can then probe those and match drivers to device, without having to hard
  29code all sorts of tables. It also makes it more flexible for board vendors to do
  30minor hardware upgrades without significantly impacting the kernel code or
  31cluttering it with special cases.
  34Entry point
  37There is one single entry point to the kernel, at the start
  38of the kernel image. That entry point supports two calling
  41        a) Boot from Open Firmware. If your firmware is compatible
  42        with Open Firmware (IEEE 1275) or provides an OF compatible
  43        client interface API (support for "interpret" callback of
  44        forth words isn't required), you can enter the kernel with:
  46              r5 : OF callback pointer as defined by IEEE 1275
  47              bindings to powerpc. Only the 32-bit client interface
  48              is currently supported
  50              r3, r4 : address & length of an initrd if any or 0
  52              The MMU is either on or off; the kernel will run the
  53              trampoline located in arch/powerpc/kernel/prom_init.c to
  54              extract the device-tree and other information from open
  55              firmware and build a flattened device-tree as described
  56              in b). prom_init() will then re-enter the kernel using
  57              the second method. This trampoline code runs in the
  58              context of the firmware, which is supposed to handle all
  59              exceptions during that time.
  61        b) Direct entry with a flattened device-tree block. This entry
  62        point is called by a) after the OF trampoline and can also be
  63        called directly by a bootloader that does not support the Open
  64        Firmware client interface. It is also used by "kexec" to
  65        implement "hot" booting of a new kernel from a previous
  66        running one. This method is what I will describe in more
  67        details in this document, as method a) is simply standard Open
  68        Firmware, and thus should be implemented according to the
  69        various standard documents defining it and its binding to the
  70        PowerPC platform. The entry point definition then becomes:
  72                r3 : physical pointer to the device-tree block
  73                (defined in chapter II) in RAM
  75                r4 : physical pointer to the kernel itself. This is
  76                used by the assembly code to properly disable the MMU
  77                in case you are entering the kernel with MMU enabled
  78                and a non-1:1 mapping.
  80                r5 : NULL (as to differentiate with method a)
  82Note about SMP entry: Either your firmware puts your other
  83CPUs in some sleep loop or spin loop in ROM where you can get
  84them out via a soft reset or some other means, in which case
  85you don't need to care, or you'll have to enter the kernel
  86with all CPUs. The way to do that with method b) will be
  87described in a later revision of this document.
  89Board supports (platforms) are not exclusive config options. An
  90arbitrary set of board supports can be built in a single kernel
  91image. The kernel will "know" what set of functions to use for a
  92given platform based on the content of the device-tree. Thus, you
  95        a) add your platform support as a _boolean_ option in
  96        arch/powerpc/Kconfig, following the example of PPC_PSERIES,
  97        PPC_PMAC and PPC_MAPLE. The latter is probably a good
  98        example of a board support to start from.
 100        b) create your main platform file as
 101        "arch/powerpc/platforms/myplatform/myboard_setup.c" and add it
 102        to the Makefile under the condition of your ``CONFIG_``
 103        option. This file will define a structure of type "ppc_md"
 104        containing the various callbacks that the generic code will
 105        use to get to your platform specific code
 107A kernel image may support multiple platforms, but only if the
 108platforms feature the same core architecture.  A single kernel build
 109cannot support both configurations with Book E and configurations
 110with classic Powerpc architectures.