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 latter 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