diff options
Diffstat (limited to 'doc/driver-model/of-plat.rst')
| -rw-r--r-- | doc/driver-model/of-plat.rst | 359 |
1 files changed, 0 insertions, 359 deletions
diff --git a/doc/driver-model/of-plat.rst b/doc/driver-model/of-plat.rst deleted file mode 100644 index 4ef2fe6..0000000 --- a/doc/driver-model/of-plat.rst +++ /dev/null @@ -1,359 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0+ - -Compiled-in Device Tree / Platform Data -======================================= - - -Introduction ------------- - -Device tree is the standard configuration method in U-Boot. It is used to -define what devices are in the system and provide configuration information -to these devices. - -The overhead of adding device tree access to U-Boot is fairly modest, -approximately 3KB on Thumb 2 (plus the size of the DT itself). This means -that in most cases it is best to use device tree for configuration. - -However there are some very constrained environments where U-Boot needs to -work. These include SPL with severe memory limitations. For example, some -SoCs require a 16KB SPL image which must include a full MMC stack. In this -case the overhead of device tree access may be too great. - -It is possible to create platform data manually by defining C structures -for it, and reference that data in a U_BOOT_DRVINFO() declaration. This -bypasses the use of device tree completely, effectively creating a parallel -configuration mechanism. But it is an available option for SPL. - -As an alternative, a new 'of-platdata' feature is provided. This converts the -device tree contents into C code which can be compiled into the SPL binary. -This saves the 3KB of code overhead and perhaps a few hundred more bytes due -to more efficient storage of the data. - -Note: Quite a bit of thought has gone into the design of this feature. -However it still has many rough edges and comments and suggestions are -strongly encouraged! Quite possibly there is a much better approach. - - -Caveats -------- - -There are many problems with this features. It should only be used when -strictly necessary. Notable problems include: - - - Device tree does not describe data types. But the C code must define a - type for each property. These are guessed using heuristics which - are wrong in several fairly common cases. For example an 8-byte value - is considered to be a 2-item integer array, and is byte-swapped. A - boolean value that is not present means 'false', but cannot be - included in the structures since there is generally no mention of it - in the device tree file. - - - Naming of nodes and properties is automatic. This means that they follow - the naming in the device tree, which may result in C identifiers that - look a bit strange. - - - It is not possible to find a value given a property name. Code must use - the associated C member variable directly in the code. This makes - the code less robust in the face of device-tree changes. It also - makes it very unlikely that your driver code will be useful for more - than one SoC. Even if the code is common, each SoC will end up with - a different C struct name, and a likely a different format for the - platform data. - - - The platform data is provided to drivers as a C structure. The driver - must use the same structure to access the data. Since a driver - normally also supports device tree it must use #ifdef to separate - out this code, since the structures are only available in SPL. - - -How it works ------------- - -The feature is enabled by CONFIG OF_PLATDATA. This is only available in -SPL/TPL and should be tested with: - -.. code-block:: c - - #if CONFIG_IS_ENABLED(OF_PLATDATA) - -A new tool called 'dtoc' converts a device tree file either into a set of -struct declarations, one for each compatible node, and a set of -U_BOOT_DRVINFO() declarations along with the actual platform data for each -device. As an example, consider this MMC node: - -.. code-block:: none - - sdmmc: dwmmc@ff0c0000 { - compatible = "rockchip,rk3288-dw-mshc"; - clock-freq-min-max = <400000 150000000>; - clocks = <&cru HCLK_SDMMC>, <&cru SCLK_SDMMC>, - <&cru SCLK_SDMMC_DRV>, <&cru SCLK_SDMMC_SAMPLE>; - clock-names = "biu", "ciu", "ciu_drv", "ciu_sample"; - fifo-depth = <0x100>; - interrupts = <GIC_SPI 32 IRQ_TYPE_LEVEL_HIGH>; - reg = <0xff0c0000 0x4000>; - bus-width = <4>; - cap-mmc-highspeed; - cap-sd-highspeed; - card-detect-delay = <200>; - disable-wp; - num-slots = <1>; - pinctrl-names = "default"; - pinctrl-0 = <&sdmmc_clk>, <&sdmmc_cmd>, <&sdmmc_cd>, <&sdmmc_bus4>; - vmmc-supply = <&vcc_sd>; - status = "okay"; - u-boot,dm-pre-reloc; - }; - - -Some of these properties are dropped by U-Boot under control of the -CONFIG_OF_SPL_REMOVE_PROPS option. The rest are processed. This will produce -the following C struct declaration: - -.. code-block:: c - - struct dtd_rockchip_rk3288_dw_mshc { - fdt32_t bus_width; - bool cap_mmc_highspeed; - bool cap_sd_highspeed; - fdt32_t card_detect_delay; - fdt32_t clock_freq_min_max[2]; - struct phandle_1_arg clocks[4]; - bool disable_wp; - fdt32_t fifo_depth; - fdt32_t interrupts[3]; - fdt32_t num_slots; - fdt32_t reg[2]; - fdt32_t vmmc_supply; - }; - -and the following device declarations: - -.. code-block:: c - - /* Node /clock-controller@ff760000 index 0 */ - ... - - /* Node /dwmmc@ff0c0000 index 2 */ - static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = { - .fifo_depth = 0x100, - .cap_sd_highspeed = true, - .interrupts = {0x0, 0x20, 0x4}, - .clock_freq_min_max = {0x61a80, 0x8f0d180}, - .vmmc_supply = 0xb, - .num_slots = 0x1, - .clocks = {{0, 456}, - {0, 68}, - {0, 114}, - {0, 118}}, - .cap_mmc_highspeed = true, - .disable_wp = true, - .bus_width = 0x4, - .u_boot_dm_pre_reloc = true, - .reg = {0xff0c0000, 0x4000}, - .card_detect_delay = 0xc8, - }; - - U_BOOT_DRVINFO(dwmmc_at_ff0c0000) = { - .name = "rockchip_rk3288_dw_mshc", - .plat = &dtv_dwmmc_at_ff0c0000, - .plat_size = sizeof(dtv_dwmmc_at_ff0c0000), - .parent_idx = -1, - }; - -The device is then instantiated at run-time and the platform data can be -accessed using: - -.. code-block:: c - - struct udevice *dev; - struct dtd_rockchip_rk3288_dw_mshc *plat = dev_get_plat(dev); - -This avoids the code overhead of converting the device tree data to -platform data in the driver. The of_to_plat() method should -therefore do nothing in such a driver. - -Note that for the platform data to be matched with a driver, the 'name' -property of the U_BOOT_DRVINFO() declaration has to match a driver declared -via U_BOOT_DRIVER(). This effectively means that a U_BOOT_DRIVER() with a -'name' corresponding to the devicetree 'compatible' string (after converting -it to a valid name for C) is needed, so a dedicated driver is required for -each 'compatible' string. - -In order to make this a bit more flexible DM_DRIVER_ALIAS macro can be -used to declare an alias for a driver name, typically a 'compatible' string. -This macro produces no code, but it is by dtoc tool. - -The parent_idx is the index of the parent driver_info structure within its -linker list (instantiated by the U_BOOT_DRVINFO() macro). This is used to support -dev_get_parent(). - -During the build process dtoc parses both U_BOOT_DRIVER and DM_DRIVER_ALIAS -to build a list of valid driver names and driver aliases. If the 'compatible' -string used for a device does not not match a valid driver name, it will be -checked against the list of driver aliases in order to get the right driver -name to use. If in this step there is no match found a warning is issued to -avoid run-time failures. - -Where a node has multiple compatible strings, a #define is used to make them -equivalent, e.g.: - -.. code-block:: c - - #define dtd_rockchip_rk3299_dw_mshc dtd_rockchip_rk3288_dw_mshc - - -Converting of-platdata to a useful form ---------------------------------------- - -Of course it would be possible to use the of-platdata directly in your driver -whenever configuration information is required. However this means that the -driver will not be able to support device tree, since the of-platdata -structure is not available when device tree is used. It would make no sense -to use this structure if device tree were available, since the structure has -all the limitations metioned in caveats above. - -Therefore it is recommended that the of-platdata structure should be used -only in the probe() method of your driver. It cannot be used in the -of_to_plat() method since this is not called when platform data is -already present. - - -How to structure your driver ----------------------------- - -Drivers should always support device tree as an option. The of-platdata -feature is intended as a add-on to existing drivers. - -Your driver should convert the plat struct in its probe() method. The -existing device tree decoding logic should be kept in the -of_to_plat() method and wrapped with #if. - -For example: - -.. code-block:: c - - #include <dt-structs.h> - - struct mmc_plat { - #if CONFIG_IS_ENABLED(OF_PLATDATA) - /* Put this first since driver model will copy the data here */ - struct dtd_mmc dtplat; - #endif - /* - * Other fields can go here, to be filled in by decoding from - * the device tree (or the C structures when of-platdata is used). - */ - int fifo_depth; - }; - - static int mmc_of_to_plat(struct udevice *dev) - { - #if !CONFIG_IS_ENABLED(OF_PLATDATA) - /* Decode the device tree data */ - struct mmc_plat *plat = dev_get_plat(dev); - const void *blob = gd->fdt_blob; - int node = dev_of_offset(dev); - - plat->fifo_depth = fdtdec_get_int(blob, node, "fifo-depth", 0); - #endif - - return 0; - } - - static int mmc_probe(struct udevice *dev) - { - struct mmc_plat *plat = dev_get_plat(dev); - - #if CONFIG_IS_ENABLED(OF_PLATDATA) - /* Decode the of-platdata from the C structures */ - struct dtd_mmc *dtplat = &plat->dtplat; - - plat->fifo_depth = dtplat->fifo_depth; - #endif - /* Set up the device from the plat data */ - writel(plat->fifo_depth, ...) - } - - static const struct udevice_id mmc_ids[] = { - { .compatible = "vendor,mmc" }, - { } - }; - - U_BOOT_DRIVER(mmc_drv) = { - .name = "mmc_drv", - .id = UCLASS_MMC, - .of_match = mmc_ids, - .of_to_plat = mmc_of_to_plat, - .probe = mmc_probe, - .priv_auto = sizeof(struct mmc_priv), - .plat_auto = sizeof(struct mmc_plat), - }; - - DM_DRIVER_ALIAS(mmc_drv, vendor_mmc) /* matches compatible string */ - -Note that struct mmc_plat is defined in the C file, not in a header. This -is to avoid needing to include dt-structs.h in a header file. The idea is to -keep the use of each of-platdata struct to the smallest possible code area. -There is just one driver C file for each struct, that can convert from the -of-platdata struct to the standard one used by the driver. - -In the case where SPL_OF_PLATDATA is enabled, plat_auto is -still used to allocate space for the platform data. This is different from -the normal behaviour and is triggered by the use of of-platdata (strictly -speaking it is a non-zero plat_size which triggers this). - -The of-platdata struct contents is copied from the C structure data to the -start of the newly allocated area. In the case where device tree is used, -the platform data is allocated, and starts zeroed. In this case the -of_to_plat() method should still set up the platform data (and the -of-platdata struct will not be present). - -SPL must use either of-platdata or device tree. Drivers cannot use both at -the same time, but they must support device tree. Supporting of-platdata is -optional. - -The device tree becomes in accessible when CONFIG_SPL_OF_PLATDATA is enabled, -since the device-tree access code is not compiled in. A corollary is that -a board can only move to using of-platdata if all the drivers it uses support -it. There would be little point in having some drivers require the device -tree data, since then libfdt would still be needed for those drivers and -there would be no code-size benefit. - -Internals ---------- - -The dt-structs.h file includes the generated file -(include/generated//dt-structs.h) if CONFIG_SPL_OF_PLATDATA is enabled. -Otherwise (such as in U-Boot proper) these structs are not available. This -prevents them being used inadvertently. All usage must be bracketed with -#if CONFIG_IS_ENABLED(OF_PLATDATA). - -The dt-plat.c file contains the device declarations and is is built in -spl/dt-plat.c. - -The dm_populate_phandle_data() function that was previous needed has now been -removed, since dtoc can address the drivers directly from dt-plat.c and does -not need to fix up things at runtime. - -The pylibfdt Python module is used to access the devicetree. - - -Credits -------- - -This is an implementation of an idea by Tom Rini <trini@konsulko.com>. - - -Future work ------------ -- Consider programmatically reading binding files instead of device tree - contents - - -.. Simon Glass <sjg@chromium.org> -.. Google, Inc -.. 6/6/16 -.. Updated Independence Day 2016 -.. Updated 1st October 2020 |