From 39782afb1ae86c15e59b1118278513a1a545652c Mon Sep 17 00:00:00 2001 From: Simon Glass Date: Mon, 4 Jul 2016 11:58:07 -0600 Subject: dm: Add a README for of-platdata Add documentation on how this works, including the benefits and drawbacks. Signed-off-by: Simon Glass --- doc/driver-model/of-plat.txt | 268 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 268 insertions(+) create mode 100644 doc/driver-model/of-plat.txt (limited to 'doc') diff --git a/doc/driver-model/of-plat.txt b/doc/driver-model/of-plat.txt new file mode 100644 index 0000000000..96b9b4642c --- /dev/null +++ b/doc/driver-model/of-plat.txt @@ -0,0 +1,268 @@ +Driver Model 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 referencing that data in a U_BOOT_DEVICE() declaration. This +bypasses the use of device tree completely, but is an available option for +SPL. + +As an alternative, a new 'of-platdata' feature is provided. This converts +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. + + +Caveats +------- + +There are many problems with this features. It should only be used when +stricly necessary. Notable problems include: + + - Device tree does not describe data types but the C code must define a + type for each property. Thesee 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 and 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 SPL_OF_PLATDATA. This is only available +in SPL and should be tested with: + + #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA) + +A new tool called 'dtoc' converts a device tree file either into a set of +struct declarations, one for each compatible node, or a set of +U_BOOT_DEVICE() declarations along with the actual platform data for each +device. As an example, consider this MMC node: + + 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 = ; + 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: + +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_2_cell clocks[4]; + bool disable_wp; + fdt32_t fifo_depth; + fdt32_t interrupts[3]; + fdt32_t num_slots; + fdt32_t reg[2]; + bool u_boot_dm_pre_reloc; + fdt32_t vmmc_supply; +}; + +and the following device declaration: + +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 = {{&dtv_clock_controller_at_ff760000, 456}, {&dtv_clock_controller_at_ff760000, 68}, {&dtv_clock_controller_at_ff760000, 114}, {&dtv_clock_controller_at_ff760000, 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_DEVICE(dwmmc_at_ff0c0000) = { + .name = "rockchip_rk3288_dw_mshc", + .platdata = &dtv_dwmmc_at_ff0c0000, +}; + +The device is then instantiated at run-time and the platform data can be +accessed using: + + struct udevice *dev; + struct dtd_rockchip_rk3288_dw_mshc *plat = dev_get_platdata(dev); + +This avoids the code overhead of converting the device tree data to +platform data in the driver. The ofdata_to_platdata() method should +therefore do nothing in such a driver. + + +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 directly access the platdata struct in its probe() +method. The existing device tree decoding logic should be kept in the +ofdata_to_platdata() and wrapped with #ifdef. + +For example: + + #include + + struct mmc_platdata { + #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA) + /* Put this first */ + struct dtd_mmc dtplat; + #endif + /* + * Other fields can go here, to be filled in by decoding from + * the device tree. They will point to random memory in the + * of-plat case. + */ + int fifo_depth; + }; + + static int mmc_ofdata_to_platdata(struct udevice *dev) + { + #if !CONFIG_IS_ENABLED(SPL_OF_PLATDATA) + struct mmc_platdata *plat = dev_get_platdata(dev); + const void *blob = gd->fdt_blob; + int node = dev->of_offset; + + plat->fifo_depth = fdtdec_get_int(blob, node, "fifo-depth", 0); + #endif + + return 0; + } + + static int mmc_probe(struct udevice *dev) + { + struct mmc_platdata *plat = dev_get_platdata(dev); + #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA) + struct dtd_mmc *dtplat = &plat->dtplat; + + /* Set up the device from the dtplat data */ + writel(dtplat->fifo_depth, ...) + #else + /* Set up the device from the plat data */ + writel(plat->fifo_depth, ...) + #endif + } + + static const struct udevice_id mmc_ids[] = { + { .compatible = "vendor,mmc" }, + { } + }; + + U_BOOT_DRIVER(mmc_drv) = { + .name = "mmc", + .id = UCLASS_MMC, + .of_match = mmc_ids, + .ofdata_to_platdata = mmc_ofdata_to_platdata, + .probe = mmc_probe, + .priv_auto_alloc_size = sizeof(struct mmc_priv), + .platdata_auto_alloc_size = sizeof(struct mmc_platdata), + }; + + +In the case where SPL_OF_PLATDATA is enabled, platdata_auto_alloc_size is +ignored, and the platform data points to the C structure data. In the case +where device tree is used, the platform data is allocated, and starts +zeroed. In this case the ofdata_to_platdata() method should set up the +platform data. + +SPL must use either of-platdata or device tree. Drivers cannot use both. +The device tree becomes in accessible when CONFIG_SPL_OF_PLATDATA is enabled, +since the device-tree access code is not compiled in. + + +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. + +The dt-platdata.c file contains the device declarations and is is built in +spl/dt-platdata.c. + +Some phandles (thsoe that are recognised as such) are converted into +points to platform data. This pointer can potentially be used to access the +referenced device (by searching for the pointer value). This feature is not +yet implemented, however. + +The beginnings of a libfdt Python module are provided. So far this only +implements a subset of the features. + +The 'swig' tool is needed to build the libfdt Python module. + + +Future work +----------- +- Add unit tests +- Add a sandbox_spl functional test +- Consider programmatically reading binding files instead of device tree + contents +- Drop the device tree data from the SPL image +- Complete the phandle feature +- Get this running on a Rockchip board +- Move to using a full Python libfdt module + +-- +Simon Glass +6/6/16 -- cgit v1.2.1 From 1269625177f120d659f66b18de4b532b16c44561 Mon Sep 17 00:00:00 2001 From: Simon Glass Date: Mon, 4 Jul 2016 11:58:42 -0600 Subject: dm: Update the of-platdata README for the new features Revise the content based on the v2 additions. This is kept as a separate patch to avoid confusing those who have already reviewed the v1 series. Signed-off-by: Simon Glass Suggested-by: Tom Rini --- doc/driver-model/of-plat.txt | 110 ++++++++++++++++++++++++++++++------------- 1 file changed, 76 insertions(+), 34 deletions(-) (limited to 'doc') diff --git a/doc/driver-model/of-plat.txt b/doc/driver-model/of-plat.txt index 96b9b4642c..86e5e25300 100644 --- a/doc/driver-model/of-plat.txt +++ b/doc/driver-model/of-plat.txt @@ -19,24 +19,28 @@ 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 referencing that data in a U_BOOT_DEVICE() declaration. This -bypasses the use of device tree completely, but is an available option for -SPL. +for it, and reference that data in a U_BOOT_DEVICE() 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 +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 -stricly necessary. Notable problems include: +strictly necessary. Notable problems include: - - Device tree does not describe data types but the C code must define a - type for each property. Thesee are guessed using heuristics which + - 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 @@ -45,14 +49,15 @@ stricly necessary. Notable problems include: - 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 + 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 and format for the platform data. + 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 @@ -112,7 +117,6 @@ struct dtd_rockchip_rk3288_dw_mshc { fdt32_t interrupts[3]; fdt32_t num_slots; fdt32_t reg[2]; - bool u_boot_dm_pre_reloc; fdt32_t vmmc_supply; }; @@ -125,7 +129,10 @@ static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = { .clock_freq_min_max = {0x61a80, 0x8f0d180}, .vmmc_supply = 0xb, .num_slots = 0x1, - .clocks = {{&dtv_clock_controller_at_ff760000, 456}, {&dtv_clock_controller_at_ff760000, 68}, {&dtv_clock_controller_at_ff760000, 114}, {&dtv_clock_controller_at_ff760000, 118}}, + .clocks = {{&dtv_clock_controller_at_ff760000, 456}, + {&dtv_clock_controller_at_ff760000, 68}, + {&dtv_clock_controller_at_ff760000, 114}, + {&dtv_clock_controller_at_ff760000, 118}}, .cap_mmc_highspeed = true, .disable_wp = true, .bus_width = 0x4, @@ -136,6 +143,7 @@ static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = { U_BOOT_DEVICE(dwmmc_at_ff0c0000) = { .name = "rockchip_rk3288_dw_mshc", .platdata = &dtv_dwmmc_at_ff0c0000, + .platdata_size = sizeof(dtv_dwmmc_at_ff0c0000), }; The device is then instantiated at run-time and the platform data can be @@ -149,15 +157,31 @@ platform data in the driver. The ofdata_to_platdata() method should therefore do nothing in such a driver. +Converting of-platdata to a useful form +--------------------------------------- + +Of course it would be possible use the of-platdata directly in your driver +whenever configuration information is required. However this meands 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 +ofdata_to_platdata() 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 directly access the platdata struct in its probe() -method. The existing device tree decoding logic should be kept in the -ofdata_to_platdata() and wrapped with #ifdef. +Your driver should convert the platdata struct in its probe() method. The +existing device tree decoding logic should be kept in the +ofdata_to_platdata() method and wrapped with #if. For example: @@ -165,13 +189,12 @@ For example: struct mmc_platdata { #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA) - /* Put this first */ + /* 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. They will point to random memory in the - * of-plat case. + * the device tree (or the C structures when of-platdata is used). */ int fifo_depth; }; @@ -179,6 +202,7 @@ For example: static int mmc_ofdata_to_platdata(struct udevice *dev) { #if !CONFIG_IS_ENABLED(SPL_OF_PLATDATA) + /* Decode the device tree data */ struct mmc_platdata *plat = dev_get_platdata(dev); const void *blob = gd->fdt_blob; int node = dev->of_offset; @@ -192,15 +216,15 @@ For example: static int mmc_probe(struct udevice *dev) { struct mmc_platdata *plat = dev_get_platdata(dev); + #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA) + /* Decode the of-platdata from the C structures */ struct dtd_mmc *dtplat = &plat->dtplat; - /* Set up the device from the dtplat data */ - writel(dtplat->fifo_depth, ...) - #else + plat->fifo_depth = dtplat->fifo_depth; + #endif /* Set up the device from the plat data */ writel(plat->fifo_depth, ...) - #endif } static const struct udevice_id mmc_ids[] = { @@ -220,15 +244,26 @@ For example: In the case where SPL_OF_PLATDATA is enabled, platdata_auto_alloc_size is -ignored, and the platform data points to the C structure data. In the case -where device tree is used, the platform data is allocated, and starts -zeroed. In this case the ofdata_to_platdata() method should set up the -platform data. +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 platdata_size which triggers this). -SPL must use either of-platdata or device tree. Drivers cannot use both. -The device tree becomes in accessible when CONFIG_SPL_OF_PLATDATA is enabled, -since the device-tree access code is not compiled in. +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 +ofdata_to_platdata() 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 --------- @@ -236,7 +271,8 @@ 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. +prevents them being used inadvertently. All usage must be bracketed with +#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA). The dt-platdata.c file contains the device declarations and is is built in spl/dt-platdata.c. @@ -249,20 +285,26 @@ yet implemented, however. The beginnings of a libfdt Python module are provided. So far this only implements a subset of the features. -The 'swig' tool is needed to build the libfdt Python module. +The 'swig' tool is needed to build the libfdt Python module. If this is not +found then the Python model is not used and a fallback is used instead, which +makes use of fdtget. + + +Credits +------- + +This is an implementation of an idea by Tom Rini . Future work ----------- -- Add unit tests -- Add a sandbox_spl functional test - Consider programmatically reading binding files instead of device tree contents -- Drop the device tree data from the SPL image - Complete the phandle feature -- Get this running on a Rockchip board - Move to using a full Python libfdt module -- Simon Glass +Google, Inc 6/6/16 +Updated Independence Day 2016 -- cgit v1.2.1