1.. SPDX-License-Identifier: GPL-2.0
2
3DeviceTree Booting
4------------------
5
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
16well.
17
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.
32
33
34Entry point
35~~~~~~~~~~~
36
37There is one single entry point to the kernel, at the start
38of the kernel image. That entry point supports two calling
39conventions:
40
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:
45
46              r5 : OF callback pointer as defined by IEEE 1275
47              bindings to powerpc. Only the 32-bit client interface
48              is currently supported
49
50              r3, r4 : address & length of an initrd if any or 0
51
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.
60
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:
71
72                r3 : physical pointer to the device-tree block
73                (defined in chapter II) in RAM
74
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.
79
80                r5 : NULL (as to differentiate with method a)
81
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.
88
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
93should:
94
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 later is probably a good
98        example of a board support to start from.
99
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
106
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.
111