diff options
Diffstat (limited to 'Documentation/devicetree/usage-model.txt')
| -rw-r--r-- | Documentation/devicetree/usage-model.txt | 23 |
1 files changed, 13 insertions, 10 deletions
diff --git a/Documentation/devicetree/usage-model.txt b/Documentation/devicetree/usage-model.txt index c5a80099b71..2b6b3d3f038 100644 --- a/Documentation/devicetree/usage-model.txt +++ b/Documentation/devicetree/usage-model.txt @@ -106,17 +106,18 @@ In the majority of cases, the machine identity is irrelevant, and the kernel will instead select setup code based on the machine's core CPU or SoC. On ARM for example, setup_arch() in arch/arm/kernel/setup.c will call setup_machine_fdt() in -arch/arm/kernel/devicetree.c which searches through the machine_desc +arch/arm/kernel/devtree.c which searches through the machine_desc table and selects the machine_desc which best matches the device tree data. It determines the best match by looking at the 'compatible' property in the root device tree node, and comparing it with the -dt_compat list in struct machine_desc. +dt_compat list in struct machine_desc (which is defined in +arch/arm/include/asm/mach/arch.h if you're curious). The 'compatible' property contains a sorted list of strings starting with the exact name of the machine, followed by an optional list of boards it is compatible with sorted from most compatible to least. For example, the root compatible properties for the TI BeagleBoard and its -successor, the BeagleBoard xM board might look like: +successor, the BeagleBoard xM board might look like, respectively: compatible = "ti,omap3-beagleboard", "ti,omap3450", "ti,omap3"; compatible = "ti,omap3-beagleboard-xm", "ti,omap3450", "ti,omap3"; @@ -161,7 +162,7 @@ cases. Instead, the compatible list allows a generic machine_desc to provide support for a wide common set of boards by specifying "less -compatible" value in the dt_compat list. In the example above, +compatible" values in the dt_compat list. In the example above, generic board support can claim compatibility with "ti,omap3" or "ti,omap3450". If a bug was discovered on the original beagleboard that required special workaround code during early boot, then a new @@ -191,9 +192,11 @@ Linux it will look something like this: }; The bootargs property contains the kernel arguments, and the initrd-* -properties define the address and size of an initrd blob. The -chosen node may also optionally contain an arbitrary number of -additional properties for platform-specific configuration data. +properties define the address and size of an initrd blob. Note that +initrd-end is the first address after the initrd image, so this doesn't +match the usual semantic of struct resource. The chosen node may also +optionally contain an arbitrary number of additional properties for +platform-specific configuration data. During early boot, the architecture setup code calls of_scan_flat_dt() several times with different helper callbacks to parse device tree @@ -312,7 +315,7 @@ device tree for the NVIDIA Tegra board. }; }; -At .machine_init() time, Tegra board support code will need to look at +At .init_machine() time, Tegra board support code will need to look at this DT and decide which nodes to create platform_devices for. However, looking at the tree, it is not immediately obvious what kind of device each node represents, or even if a node represents a device @@ -347,7 +350,7 @@ later), which will happily live at the base of the Linux /sys/devices tree. Therefore, if a DT node is at the root of the tree, then it really probably is best registered as a platform_device. -Linux board support code calls of_platform_populate(NULL, NULL, NULL) +Linux board support code calls of_platform_populate(NULL, NULL, NULL, NULL) to kick off discovery of devices at the root of the tree. The parameters are all NULL because when starting from the root of the tree, there is no need to provide a starting node (the first NULL), a @@ -375,7 +378,7 @@ platform_devices as more platform_devices is a common pattern, and the device tree support code reflects that and makes the above example simpler. The second argument to of_platform_populate() is an of_device_id table, and any node that matches an entry in that table -will also get its child nodes registered. In the tegra case, the code +will also get its child nodes registered. In the Tegra case, the code can look something like this: static void __init harmony_init_machine(void) |