123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110 |
- .. SPDX-License-Identifier: GPL-2.0
- DeviceTree Booting
- ------------------
- During the development of the Linux/ppc64 kernel, and more specifically, the
- addition of new platform types outside of the old IBM pSeries/iSeries pair, it
- was decided to enforce some strict rules regarding the kernel entry and
- bootloader <-> kernel interfaces, in order to avoid the degeneration that had
- become the ppc32 kernel entry point and the way a new platform should be added
- to the kernel. The legacy iSeries platform breaks those rules as it predates
- this scheme, but no new board support will be accepted in the main tree that
- doesn't follow them properly. In addition, since the advent of the arch/powerpc
- merged architecture for ppc32 and ppc64, new 32-bit platforms and 32-bit
- platforms which move into arch/powerpc will be required to use these rules as
- well.
- The main requirement that will be defined in more detail below is the presence
- of a device-tree whose format is defined after Open Firmware specification.
- However, in order to make life easier to embedded board vendors, the kernel
- doesn't require the device-tree to represent every device in the system and only
- requires some nodes and properties to be present. For example, the kernel does
- not require you to create a node for every PCI device in the system. It is a
- requirement to have a node for PCI host bridges in order to provide interrupt
- routing information and memory/IO ranges, among others. It is also recommended
- to define nodes for on chip devices and other buses that don't specifically fit
- in an existing OF specification. This creates a great flexibility in the way the
- kernel can then probe those and match drivers to device, without having to hard
- code all sorts of tables. It also makes it more flexible for board vendors to do
- minor hardware upgrades without significantly impacting the kernel code or
- cluttering it with special cases.
- Entry point
- ~~~~~~~~~~~
- There is one single entry point to the kernel, at the start
- of the kernel image. That entry point supports two calling
- conventions:
- a) Boot from Open Firmware. If your firmware is compatible
- with Open Firmware (IEEE 1275) or provides an OF compatible
- client interface API (support for "interpret" callback of
- forth words isn't required), you can enter the kernel with:
- r5 : OF callback pointer as defined by IEEE 1275
- bindings to powerpc. Only the 32-bit client interface
- is currently supported
- r3, r4 : address & length of an initrd if any or 0
- The MMU is either on or off; the kernel will run the
- trampoline located in arch/powerpc/kernel/prom_init.c to
- extract the device-tree and other information from open
- firmware and build a flattened device-tree as described
- in b). prom_init() will then re-enter the kernel using
- the second method. This trampoline code runs in the
- context of the firmware, which is supposed to handle all
- exceptions during that time.
- b) Direct entry with a flattened device-tree block. This entry
- point is called by a) after the OF trampoline and can also be
- called directly by a bootloader that does not support the Open
- Firmware client interface. It is also used by "kexec" to
- implement "hot" booting of a new kernel from a previous
- running one. This method is what I will describe in more
- details in this document, as method a) is simply standard Open
- Firmware, and thus should be implemented according to the
- various standard documents defining it and its binding to the
- PowerPC platform. The entry point definition then becomes:
- r3 : physical pointer to the device-tree block
- (defined in chapter II) in RAM
- r4 : physical pointer to the kernel itself. This is
- used by the assembly code to properly disable the MMU
- in case you are entering the kernel with MMU enabled
- and a non-1:1 mapping.
- r5 : NULL (as to differentiate with method a)
- Note about SMP entry: Either your firmware puts your other
- CPUs in some sleep loop or spin loop in ROM where you can get
- them out via a soft reset or some other means, in which case
- you don't need to care, or you'll have to enter the kernel
- with all CPUs. The way to do that with method b) will be
- described in a later revision of this document.
- Board supports (platforms) are not exclusive config options. An
- arbitrary set of board supports can be built in a single kernel
- image. The kernel will "know" what set of functions to use for a
- given platform based on the content of the device-tree. Thus, you
- should:
- a) add your platform support as a _boolean_ option in
- arch/powerpc/Kconfig, following the example of PPC_PSERIES,
- PPC_PMAC and PPC_MAPLE. The later is probably a good
- example of a board support to start from.
- b) create your main platform file as
- "arch/powerpc/platforms/myplatform/myboard_setup.c" and add it
- to the Makefile under the condition of your ``CONFIG_``
- option. This file will define a structure of type "ppc_md"
- containing the various callbacks that the generic code will
- use to get to your platform specific code
- A kernel image may support multiple platforms, but only if the
- platforms feature the same core architecture. A single kernel build
- cannot support both configurations with Book E and configurations
- with classic Powerpc architectures.
|