1Driver Model Compiled-in Device Tree / Platform Data 2==================================================== 3 4 5Introduction 6------------ 7 8Device tree is the standard configuration method in U-Boot. It is used to 9define what devices are in the system and provide configuration information 10to these devices. 11 12The overhead of adding device tree access to U-Boot is fairly modest, 13approximately 3KB on Thumb 2 (plus the size of the DT itself). This means 14that in most cases it is best to use device tree for configuration. 15 16However there are some very constrained environments where U-Boot needs to 17work. These include SPL with severe memory limitations. For example, some 18SoCs require a 16KB SPL image which must include a full MMC stack. In this 19case the overhead of device tree access may be too great. 20 21It is possible to create platform data manually by defining C structures 22for it, and reference that data in a U_BOOT_DEVICE() declaration. This 23bypasses the use of device tree completely, effectively creating a parallel 24configuration mechanism. But it is an available option for SPL. 25 26As an alternative, a new 'of-platdata' feature is provided. This converts the 27device tree contents into C code which can be compiled into the SPL binary. 28This saves the 3KB of code overhead and perhaps a few hundred more bytes due 29to more efficient storage of the data. 30 31Note: Quite a bit of thought has gone into the design of this feature. 32However it still has many rough edges and comments and suggestions are 33strongly encouraged! Quite possibly there is a much better approach. 34 35 36Caveats 37------- 38 39There are many problems with this features. It should only be used when 40strictly necessary. Notable problems include: 41 42 - Device tree does not describe data types. But the C code must define a 43 type for each property. These are guessed using heuristics which 44 are wrong in several fairly common cases. For example an 8-byte value 45 is considered to be a 2-item integer array, and is byte-swapped. A 46 boolean value that is not present means 'false', but cannot be 47 included in the structures since there is generally no mention of it 48 in the device tree file. 49 50 - Naming of nodes and properties is automatic. This means that they follow 51 the naming in the device tree, which may result in C identifiers that 52 look a bit strange. 53 54 - It is not possible to find a value given a property name. Code must use 55 the associated C member variable directly in the code. This makes 56 the code less robust in the face of device-tree changes. It also 57 makes it very unlikely that your driver code will be useful for more 58 than one SoC. Even if the code is common, each SoC will end up with 59 a different C struct name, and a likely a different format for the 60 platform data. 61 62 - The platform data is provided to drivers as a C structure. The driver 63 must use the same structure to access the data. Since a driver 64 normally also supports device tree it must use #ifdef to separate 65 out this code, since the structures are only available in SPL. 66 67 68How it works 69------------ 70 71The feature is enabled by CONFIG SPL_OF_PLATDATA. This is only available 72in SPL and should be tested with: 73 74 #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA) 75 76A new tool called 'dtoc' converts a device tree file either into a set of 77struct declarations, one for each compatible node, or a set of 78U_BOOT_DEVICE() declarations along with the actual platform data for each 79device. As an example, consider this MMC node: 80 81 sdmmc: dwmmc@ff0c0000 { 82 compatible = "rockchip,rk3288-dw-mshc"; 83 clock-freq-min-max = <400000 150000000>; 84 clocks = <&cru HCLK_SDMMC>, <&cru SCLK_SDMMC>, 85 <&cru SCLK_SDMMC_DRV>, <&cru SCLK_SDMMC_SAMPLE>; 86 clock-names = "biu", "ciu", "ciu_drv", "ciu_sample"; 87 fifo-depth = <0x100>; 88 interrupts = <GIC_SPI 32 IRQ_TYPE_LEVEL_HIGH>; 89 reg = <0xff0c0000 0x4000>; 90 bus-width = <4>; 91 cap-mmc-highspeed; 92 cap-sd-highspeed; 93 card-detect-delay = <200>; 94 disable-wp; 95 num-slots = <1>; 96 pinctrl-names = "default"; 97 pinctrl-0 = <&sdmmc_clk>, <&sdmmc_cmd>, <&sdmmc_cd>, <&sdmmc_bus4>; 98 vmmc-supply = <&vcc_sd>; 99 status = "okay"; 100 u-boot,dm-pre-reloc; 101 }; 102 103 104Some of these properties are dropped by U-Boot under control of the 105CONFIG_OF_SPL_REMOVE_PROPS option. The rest are processed. This will produce 106the following C struct declaration: 107 108struct dtd_rockchip_rk3288_dw_mshc { 109 fdt32_t bus_width; 110 bool cap_mmc_highspeed; 111 bool cap_sd_highspeed; 112 fdt32_t card_detect_delay; 113 fdt32_t clock_freq_min_max[2]; 114 struct phandle_2_cell clocks[4]; 115 bool disable_wp; 116 fdt32_t fifo_depth; 117 fdt32_t interrupts[3]; 118 fdt32_t num_slots; 119 fdt32_t reg[2]; 120 fdt32_t vmmc_supply; 121}; 122 123and the following device declaration: 124 125static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = { 126 .fifo_depth = 0x100, 127 .cap_sd_highspeed = true, 128 .interrupts = {0x0, 0x20, 0x4}, 129 .clock_freq_min_max = {0x61a80, 0x8f0d180}, 130 .vmmc_supply = 0xb, 131 .num_slots = 0x1, 132 .clocks = {{&dtv_clock_controller_at_ff760000, 456}, 133 {&dtv_clock_controller_at_ff760000, 68}, 134 {&dtv_clock_controller_at_ff760000, 114}, 135 {&dtv_clock_controller_at_ff760000, 118}}, 136 .cap_mmc_highspeed = true, 137 .disable_wp = true, 138 .bus_width = 0x4, 139 .u_boot_dm_pre_reloc = true, 140 .reg = {0xff0c0000, 0x4000}, 141 .card_detect_delay = 0xc8, 142}; 143U_BOOT_DEVICE(dwmmc_at_ff0c0000) = { 144 .name = "rockchip_rk3288_dw_mshc", 145 .platdata = &dtv_dwmmc_at_ff0c0000, 146 .platdata_size = sizeof(dtv_dwmmc_at_ff0c0000), 147}; 148 149The device is then instantiated at run-time and the platform data can be 150accessed using: 151 152 struct udevice *dev; 153 struct dtd_rockchip_rk3288_dw_mshc *plat = dev_get_platdata(dev); 154 155This avoids the code overhead of converting the device tree data to 156platform data in the driver. The ofdata_to_platdata() method should 157therefore do nothing in such a driver. 158 159 160Converting of-platdata to a useful form 161--------------------------------------- 162 163Of course it would be possible use the of-platdata directly in your driver 164whenever configuration information is required. However this meands that the 165driver will not be able to support device tree, since the of-platdata 166structure is not available when device tree is used. It would make no sense 167to use this structure if device tree were available, since the structure has 168all the limitations metioned in caveats above. 169 170Therefore it is recommended that the of-platdata structure should be used 171only in the probe() method of your driver. It cannot be used in the 172ofdata_to_platdata() method since this is not called when platform data is 173already present. 174 175 176How to structure your driver 177---------------------------- 178 179Drivers should always support device tree as an option. The of-platdata 180feature is intended as a add-on to existing drivers. 181 182Your driver should convert the platdata struct in its probe() method. The 183existing device tree decoding logic should be kept in the 184ofdata_to_platdata() method and wrapped with #if. 185 186For example: 187 188 #include <dt-structs.h> 189 190 struct mmc_platdata { 191 #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA) 192 /* Put this first since driver model will copy the data here */ 193 struct dtd_mmc dtplat; 194 #endif 195 /* 196 * Other fields can go here, to be filled in by decoding from 197 * the device tree (or the C structures when of-platdata is used). 198 */ 199 int fifo_depth; 200 }; 201 202 static int mmc_ofdata_to_platdata(struct udevice *dev) 203 { 204 #if !CONFIG_IS_ENABLED(SPL_OF_PLATDATA) 205 /* Decode the device tree data */ 206 struct mmc_platdata *plat = dev_get_platdata(dev); 207 const void *blob = gd->fdt_blob; 208 int node = dev_of_offset(dev); 209 210 plat->fifo_depth = fdtdec_get_int(blob, node, "fifo-depth", 0); 211 #endif 212 213 return 0; 214 } 215 216 static int mmc_probe(struct udevice *dev) 217 { 218 struct mmc_platdata *plat = dev_get_platdata(dev); 219 220 #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA) 221 /* Decode the of-platdata from the C structures */ 222 struct dtd_mmc *dtplat = &plat->dtplat; 223 224 plat->fifo_depth = dtplat->fifo_depth; 225 #endif 226 /* Set up the device from the plat data */ 227 writel(plat->fifo_depth, ...) 228 } 229 230 static const struct udevice_id mmc_ids[] = { 231 { .compatible = "vendor,mmc" }, 232 { } 233 }; 234 235 U_BOOT_DRIVER(mmc_drv) = { 236 .name = "mmc", 237 .id = UCLASS_MMC, 238 .of_match = mmc_ids, 239 .ofdata_to_platdata = mmc_ofdata_to_platdata, 240 .probe = mmc_probe, 241 .priv_auto_alloc_size = sizeof(struct mmc_priv), 242 .platdata_auto_alloc_size = sizeof(struct mmc_platdata), 243 }; 244 245 246In the case where SPL_OF_PLATDATA is enabled, platdata_auto_alloc_size is 247still used to allocate space for the platform data. This is different from 248the normal behaviour and is triggered by the use of of-platdata (strictly 249speaking it is a non-zero platdata_size which triggers this). 250 251The of-platdata struct contents is copied from the C structure data to the 252start of the newly allocated area. In the case where device tree is used, 253the platform data is allocated, and starts zeroed. In this case the 254ofdata_to_platdata() method should still set up the platform data (and the 255of-platdata struct will not be present). 256 257SPL must use either of-platdata or device tree. Drivers cannot use both at 258the same time, but they must support device tree. Supporting of-platdata is 259optional. 260 261The device tree becomes in accessible when CONFIG_SPL_OF_PLATDATA is enabled, 262since the device-tree access code is not compiled in. A corollary is that 263a board can only move to using of-platdata if all the drivers it uses support 264it. There would be little point in having some drivers require the device 265tree data, since then libfdt would still be needed for those drivers and 266there would be no code-size benefit. 267 268Internals 269--------- 270 271The dt-structs.h file includes the generated file 272(include/generated//dt-structs.h) if CONFIG_SPL_OF_PLATDATA is enabled. 273Otherwise (such as in U-Boot proper) these structs are not available. This 274prevents them being used inadvertently. All usage must be bracketed with 275#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA). 276 277The dt-platdata.c file contains the device declarations and is is built in 278spl/dt-platdata.c. 279 280Some phandles (thsoe that are recognised as such) are converted into 281points to platform data. This pointer can potentially be used to access the 282referenced device (by searching for the pointer value). This feature is not 283yet implemented, however. 284 285The beginnings of a libfdt Python module are provided. So far this only 286implements a subset of the features. 287 288The 'swig' tool is needed to build the libfdt Python module. If this is not 289found then the Python model is not used and a fallback is used instead, which 290makes use of fdtget. 291 292 293Credits 294------- 295 296This is an implementation of an idea by Tom Rini <trini@konsulko.com>. 297 298 299Future work 300----------- 301- Consider programmatically reading binding files instead of device tree 302 contents 303- Complete the phandle feature 304- Move to using a full Python libfdt module 305 306-- 307Simon Glass <sjg@chromium.org> 308Google, Inc 3096/6/16 310Updated Independence Day 2016 311