So far U-Boot uses a 'flat' device tree, which means that it is decoded on the fly as needed. This uses the libfdt library and avoids needing extra memory for additional tables.
For some time there has been discussion about moving U-Boot to use a 'live' tree, where the device tree is decoded at start-up into a set of hierarchical structures with pointers.
The advantages are:
- It is somewhat faster to access (in particular scanning and looking for a parent) - It permits the device tree to be changed at run-time (this is not recommended at present since devices store the offset of their device tree node and updating the tree may invalidate that). This could be useful for overlays, for example. - It allows nodes to be referenced by a single pointer, instead of the current device tree pointer plus offset
The disadvantages are:
- It requires more memory - It takes (a little) time to build the live tree - It adds more complexity under the hood, including an additional abstraction layer
This series is an attempt to introduce a useful live tree feature into U-Boot. There are many options and trade-offs. This series is the culmination of quite a bit of thought and experimentation.
The approach used in this series is:
- Before relocation the flat tree is used, to avoid extra memory usage and time. In general, there is not much access before relocation since most drivers are not started up. So there is little benefit in having a live tree
- After relocation the live tree is built. At this point the CPU should be running quickly and there is plenty of memory. All available devices will be bound so the overhead of building the live tree may be outweighed by its greater efficiency.
As a simplification, this series supports only one tree or the other. When the live tree is active, the flat tree cannot be used. That makes it easy to know the current state and avoids confusion over mixing offset and node pointers.
Some drivers will need to be used both before and after relocation. This means that they must support both the flat and the live tree. To support this, the concept of a node 'reference' is defined. A reference can hold either a node offset (for the flat tree) or a node pointer (for the live tree). This allows drivers to access values from the tree regardless of which tree is in use.
In addition, since most device tree access happens in the context of a device (struct udevice), a new 'dev_read' layer is provided to read device tree configuration associated with a device. This encapsulates the details of exactly how this information is read.
I have taken the view that code compatibility with Linux is desirable. So the of_access.c file brings in code from Linux with very little modification. As new access methods are needed we should be able to bring in more code and avoid writing it ourselves in U-Boot.
Conversion of drivers and subsystems to support the live tree (as well as flat tree) is fairly easy. A patch is included to add support to the GPIO uclass, and the cros_ec device is converted over to provide a driver example.
Once I have comments on this series I will update it to build for all boards, to pass current tests and to tidy up the code. I will also add tests for the live tree and finish conversion for sandbox. So for now I am just looking for high-level comments, rather than a detailed code review.
Simon Glass (22): dm: core: Set return value first in lists_bind_fdt() dm: core: atmel: Dont export dm_scan_fdt_node() Update WARN_ON() to return a value dm: Add livetree definitions dm: Add livetree access functions dm: Add a function to create a 'live' device tree dm: Build a live tree after relocation dm: Add support for device tree references dm: Add a place to put extra device-tree reading functions dm: core: Add device-based functions to access DT properties dm: core: Allow binding a device from a live tree dm: core: Add a method to find a driver for a livetree node dm: core: Scan the live tree when setting up driver model dm: core: Add a way to find a device by its live-tree node dm: core: Add a way to find a device by node reference dm: gpio: Refactor to prepare for live tree support dm: gpio: Add live tree support dm: gpio: Drop blank line in gpio_xlate_offs_flags() comment dm: gpio: sandbox: Use dm_read...() functions to access DT cros_ec: Fix debug() statement in ec_command_inptr() cros_ec: Convert to support live tree sandbox: Move to use live tree
common/board_r.c | 12 ++ configs/sandbox_defconfig | 1 + drivers/core/Makefile | 1 + drivers/core/device.c | 27 +++- drivers/core/lists.c | 80 +++++++++- drivers/core/of_dev.c | 55 +++++++ drivers/core/of_extra.c | 37 +++++ drivers/core/of_ref.c | 218 +++++++++++++++++++++++++ drivers/core/root.c | 61 ++++++- drivers/core/uclass.c | 78 +++++++++ drivers/gpio/atmel_pio4.c | 3 +- drivers/gpio/gpio-uclass.c | 100 ++++++++---- drivers/gpio/sandbox.c | 10 +- drivers/misc/cros_ec.c | 36 ++--- drivers/misc/cros_ec_sandbox.c | 2 +- dts/Kconfig | 11 ++ include/asm-generic/global_data.h | 3 + include/asm-generic/gpio.h | 7 +- include/cros_ec.h | 8 +- include/dm/device-internal.h | 6 + include/dm/device.h | 19 ++- include/dm/lists.h | 5 + include/dm/of.h | 53 ++++++ include/dm/of_access.h | 164 +++++++++++++++++++ include/dm/of_dev.h | 68 ++++++++ include/dm/of_extra.h | 46 ++++++ include/dm/of_ref.h | 306 +++++++++++++++++++++++++++++++++++ include/dm/root.h | 16 -- include/dm/uclass-internal.h | 22 +++ include/dm/uclass.h | 23 +++ include/fdtdec.h | 34 ---- include/linux/compat.h | 8 +- include/livetree.h | 24 +++ lib/Makefile | 1 + lib/fdtdec.c | 32 ---- lib/livetree.c | 320 +++++++++++++++++++++++++++++++++++++ lib/of_access.c | 328 ++++++++++++++++++++++++++++++++++++++ 37 files changed, 2066 insertions(+), 159 deletions(-) create mode 100644 drivers/core/of_dev.c create mode 100644 drivers/core/of_extra.c create mode 100644 drivers/core/of_ref.c create mode 100644 include/dm/of.h create mode 100644 include/dm/of_access.h create mode 100644 include/dm/of_dev.h create mode 100644 include/dm/of_extra.h create mode 100644 include/dm/of_ref.h create mode 100644 include/livetree.h create mode 100644 lib/livetree.c create mode 100644 lib/of_access.c