diff options
| -rw-r--r-- | common/board_f.c | 10 | ||||
| -rw-r--r-- | doc/driver-model/fdt-fixup.txt | 132 | ||||
| -rw-r--r-- | dts/Kconfig | 10 | ||||
| -rw-r--r-- | include/common.h | 1 |
4 files changed, 153 insertions, 0 deletions
diff --git a/common/board_f.c b/common/board_f.c index 7d1ede0404..bb24a633fb 100644 --- a/common/board_f.c +++ b/common/board_f.c @@ -751,6 +751,13 @@ static int setup_reloc(void) return 0; } +#ifdef CONFIG_OF_BOARD_FIXUP +static int fix_fdt(void) +{ + return board_fix_fdt((void *)gd->fdt_blob); +} +#endif + /* ARM calls relocate_code from its crt0.S */ #if !defined(CONFIG_ARM) && !defined(CONFIG_SANDBOX) && \ !CONFIG_IS_ENABLED(X86_64) @@ -1012,6 +1019,9 @@ static const init_fnc_t init_sequence_f[] = { #ifdef CONFIG_SYS_EXTBDINFO setup_board_extra, #endif +#ifdef CONFIG_OF_BOARD_FIXUP + fix_fdt, +#endif INIT_FUNC_WATCHDOG_RESET reloc_fdt, setup_reloc, diff --git a/doc/driver-model/fdt-fixup.txt b/doc/driver-model/fdt-fixup.txt new file mode 100644 index 0000000000..70344bd2c3 --- /dev/null +++ b/doc/driver-model/fdt-fixup.txt @@ -0,0 +1,132 @@ +Pre-relocation device tree manipulation +======================================= + +Contents: + +1. Purpose +2. Implementation +3. Example +4. Work to be done + +1. Purpose +---------- + +In certain markets, it is beneficial for manufacturers of embedded devices to +offer certain ranges of products, where the functionality of the devices within +one series either don't differ greatly from another, or can be thought of as +"extensions" of each other, where one device only differs from another in the +addition of a small number of features (e.g. an additional output connector). + +To realize this in hardware, one method is to have a motherboard, and several +possible daughter boards that can be attached to this mother board. Different +daughter boards then either offer the slightly different functionality, or the +addition of the daughter board to the device realizes the "extension" of +functionality to the device described previously. + +For the software, we obviously want to reuse components for all these +variations of the device. This means that the software somehow needs to cope +with the situation that certain ICs may or may not be present on any given +system, depending on which daughter boards are connected to the motherboard. + +In the Linux kernel, one possible solution to this problem is to employ the +device tree overlay mechanism: There exists one "base" device tree, which +features only the components guaranteed to exist in all varieties of the +device. At the start of the kernel, the presence and type of the daughter +boards is then detected, and the corresponding device tree overlays are applied +to support the components on the daughter boards. + +Note that the components present on every variety of the board must, of course, +provide a way to find out if and which daughter boards are installed for this +mechanism to work. + +In the U-Boot boot loader, support for device tree overlays has recently been +integrated, and is used on some boards to alter the device tree that is later +passed to Linux. But since U-Boot's driver model, which is device tree-based as +well, is being used in more and more drivers, the same problem of altering the +device tree starts cropping up in U-Boot itself as well. + +An additional problem with the device tree in U-Boot is that it is read-only, +and the current mechanisms don't allow easy manipulation of the device tree +after the driver model has been initialized. While migrating to a live device +tree (at least after the relocation) would greatly simplify the solution of +this problem, it is a non-negligible task to implement it, an a interim +solution is needed to address the problem at least in the medium-term. + +Hence, we propose a solution to this problem by offering a board-specific +call-back function, which is passed a writeable pointer to the device tree. +This function is called before the device tree is relocated, and specifically +before the main U-Boot's driver model is instantiated, hence the main U-Boot +"sees" all modifications to the device tree made in this function. Furthermore, +we have the pre-relocation driver model at our disposal at this stage, which +means that we can query the hardware for the existence and variety of the +components easily. + +2. Implementation +----------------- + +To take advantage of the pre-relocation device tree manipulation mechanism, +boards have to implement the function board_fix_fdt, which has the following +signature: + +int board_fix_fdt (void *rw_fdt_blob) + +The passed-in void pointer is a writeable pointer to the device tree, which can +be used to manipulate the device tree using e.g. functions from +include/fdt_support.h. The return value should either be 0 in case of +successful execution of the device tree manipulation or something else for a +failure. Note that returning a non-null value from the function will +unrecoverably halt the boot process, as with any function from init_sequence_f +(in common/board_f.c). + +Furthermore, the Kconfig option OF_BOARD_FIXUP has to be set for the function +to be called: + +Device Tree Control +-> [*] Board-specific manipulation of Device Tree + ++----------------------------------------------------------+ +| WARNING: The actual manipulation of the device tree has | +| to be the _last_ set of operations in board_fix_fdt! | +| Since the pre-relocation driver model does not adapt to | +| changes made to the device tree either, its references | +| into the device tree will be invalid after manipulating | +| it, and unpredictable behavior might occur when | +| functions that rely on them are executed! | ++----------------------------------------------------------+ + +Hence, the recommended layout of the board_fixup_fdt call-back function is the +following: + +int board_fix_fdt(void *rw_fdt_blob) +{ + /* Collect information about device's hardware and store them in e.g. + local variables */ + + /* Do device tree manipulation using the values previously collected */ + + /* Return 0 on successful manipulation and non-zero otherwise */ +} + +If this convention is kept, both an "additive" approach, meaning that nodes for +detected components are added to the device tree, as well as a "subtractive" +approach, meaning that nodes for absent components are removed from the tree, +as well as a combination of both approaches should work. + +3. Example +---------- + +The controlcenterdc board (board/gdsys/a38x/controlcenterdc.c) features a +board_fix_fdt function, in which six GPIO expanders (which might be present or +not, since they are on daughter boards) on a I2C bus are queried for, and +subsequently deactivated in the device tree if they are not present. + +Note that the dm_i2c_simple_probe function does not use the device tree, hence +it is safe to call it after the tree has already been manipulated. + +4. Work to be done +------------------ + +* The application of device tree overlay should be possible in board_fixup_fdt, + but has not been tested at this stage. + +2017-01-06, Mario Six <mario.six@gdsys.cc> diff --git a/dts/Kconfig b/dts/Kconfig index 4b7d8b15cc..3f64eda619 100644 --- a/dts/Kconfig +++ b/dts/Kconfig @@ -14,6 +14,16 @@ config OF_CONTROL This feature provides for run-time configuration of U-Boot via a flattened device tree. +config OF_BOARD_FIXUP + bool "Board-specific manipulation of Device Tree" + help + In certain circumstances it is necessary to be able to modify + U-Boot's device tree (e.g. to delete device from it). This option + make the Device Tree writeable and provides a board-specific + "board_fix_fdt" callback (called during pre-relocation time), which + enables the board initialization to modifiy the Device Tree. The + modified copy is subsequently used by U-Boot after relocation. + config SPL_OF_CONTROL bool "Enable run-time configuration via Device Tree in SPL" depends on SPL && OF_CONTROL diff --git a/include/common.h b/include/common.h index fbbc2cbc52..2cbbd5a60c 100644 --- a/include/common.h +++ b/include/common.h @@ -497,6 +497,7 @@ extern ssize_t spi_write (uchar *, int, uchar *, int); /* $(BOARD)/$(BOARD).c */ int board_early_init_f (void); +int board_fix_fdt (void *rw_fdt_blob); /* manipulate the U-Boot fdt before its relocation */ int board_late_init (void); int board_postclk_init (void); /* after clocks/timebase, before env/serial */ int board_early_init_r (void); |