1 Driver Model Compiled-in Device Tree / Platform Data
2 ====================================================
8 Device tree is the standard configuration method in U-Boot. It is used to
9 define what devices are in the system and provide configuration information
12 The overhead of adding device tree access to U-Boot is fairly modest,
13 approximately 3KB on Thumb 2 (plus the size of the DT itself). This means
14 that in most cases it is best to use device tree for configuration.
16 However there are some very constrained environments where U-Boot needs to
17 work. These include SPL with severe memory limitations. For example, some
18 SoCs require a 16KB SPL image which must include a full MMC stack. In this
19 case the overhead of device tree access may be too great.
21 It is possible to create platform data manually by defining C structures
22 for it, and referencing that data in a U_BOOT_DEVICE() declaration. This
23 bypasses the use of device tree completely, but is an available option for
26 As an alternative, a new 'of-platdata' feature is provided. This converts
27 device tree contents into C code which can be compiled into the SPL binary.
28 This saves the 3KB of code overhead and perhaps a few hundred more bytes due
29 to more efficient storage of the data.
35 There are many problems with this features. It should only be used when
36 stricly necessary. Notable problems include:
38 - Device tree does not describe data types but the C code must define a
39 type for each property. Thesee are guessed using heuristics which
40 are wrong in several fairly common cases. For example an 8-byte value
41 is considered to be a 2-item integer array, and is byte-swapped. A
42 boolean value that is not present means 'false', but cannot be
43 included in the structures since there is generally no mention of it
44 in the device tree file.
46 - Naming of nodes and properties is automatic. This means that they follow
47 the naming in the device tree, which may result in C identifiers that
50 - It is not possible to find a value given a property name. Code must use
51 the associated C member variable directly in the code. This makes
52 the code less robust in the face of device-tree changes. It also
53 makes it very unlikely that your driver code will be useful for more
54 than one SoC. Even if the code is common, each SoC will end up with
55 a different C struct and format for the platform data.
57 - The platform data is provided to drivers as a C structure. The driver
58 must use the same structure to access the data. Since a driver
59 normally also supports device tree it must use #ifdef to separate
60 out this code, since the structures are only available in SPL.
66 The feature is enabled by CONFIG SPL_OF_PLATDATA. This is only available
67 in SPL and should be tested with:
69 #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
71 A new tool called 'dtoc' converts a device tree file either into a set of
72 struct declarations, one for each compatible node, or a set of
73 U_BOOT_DEVICE() declarations along with the actual platform data for each
74 device. As an example, consider this MMC node:
76 sdmmc: dwmmc@ff0c0000 {
77 compatible = "rockchip,rk3288-dw-mshc";
78 clock-freq-min-max = <400000 150000000>;
79 clocks = <&cru HCLK_SDMMC>, <&cru SCLK_SDMMC>,
80 <&cru SCLK_SDMMC_DRV>, <&cru SCLK_SDMMC_SAMPLE>;
81 clock-names = "biu", "ciu", "ciu_drv", "ciu_sample";
83 interrupts = <GIC_SPI 32 IRQ_TYPE_LEVEL_HIGH>;
84 reg = <0xff0c0000 0x4000>;
88 card-detect-delay = <200>;
91 pinctrl-names = "default";
92 pinctrl-0 = <&sdmmc_clk>, <&sdmmc_cmd>, <&sdmmc_cd>, <&sdmmc_bus4>;
93 vmmc-supply = <&vcc_sd>;
99 Some of these properties are dropped by U-Boot under control of the
100 CONFIG_OF_SPL_REMOVE_PROPS option. The rest are processed. This will produce
101 the following C struct declaration:
103 struct dtd_rockchip_rk3288_dw_mshc {
105 bool cap_mmc_highspeed;
106 bool cap_sd_highspeed;
107 fdt32_t card_detect_delay;
108 fdt32_t clock_freq_min_max[2];
109 struct phandle_2_cell clocks[4];
112 fdt32_t interrupts[3];
115 bool u_boot_dm_pre_reloc;
119 and the following device declaration:
121 static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = {
123 .cap_sd_highspeed = true,
124 .interrupts = {0x0, 0x20, 0x4},
125 .clock_freq_min_max = {0x61a80, 0x8f0d180},
128 .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}},
129 .cap_mmc_highspeed = true,
132 .u_boot_dm_pre_reloc = true,
133 .reg = {0xff0c0000, 0x4000},
134 .card_detect_delay = 0xc8,
136 U_BOOT_DEVICE(dwmmc_at_ff0c0000) = {
137 .name = "rockchip_rk3288_dw_mshc",
138 .platdata = &dtv_dwmmc_at_ff0c0000,
141 The device is then instantiated at run-time and the platform data can be
145 struct dtd_rockchip_rk3288_dw_mshc *plat = dev_get_platdata(dev);
147 This avoids the code overhead of converting the device tree data to
148 platform data in the driver. The ofdata_to_platdata() method should
149 therefore do nothing in such a driver.
152 How to structure your driver
153 ----------------------------
155 Drivers should always support device tree as an option. The of-platdata
156 feature is intended as a add-on to existing drivers.
158 Your driver should directly access the platdata struct in its probe()
159 method. The existing device tree decoding logic should be kept in the
160 ofdata_to_platdata() and wrapped with #ifdef.
164 #include <dt-structs.h>
166 struct mmc_platdata {
167 #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
169 struct dtd_mmc dtplat;
172 * Other fields can go here, to be filled in by decoding from
173 * the device tree. They will point to random memory in the
179 static int mmc_ofdata_to_platdata(struct udevice *dev)
181 #if !CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
182 struct mmc_platdata *plat = dev_get_platdata(dev);
183 const void *blob = gd->fdt_blob;
184 int node = dev->of_offset;
186 plat->fifo_depth = fdtdec_get_int(blob, node, "fifo-depth", 0);
192 static int mmc_probe(struct udevice *dev)
194 struct mmc_platdata *plat = dev_get_platdata(dev);
195 #if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
196 struct dtd_mmc *dtplat = &plat->dtplat;
198 /* Set up the device from the dtplat data */
199 writel(dtplat->fifo_depth, ...)
201 /* Set up the device from the plat data */
202 writel(plat->fifo_depth, ...)
206 static const struct udevice_id mmc_ids[] = {
207 { .compatible = "vendor,mmc" },
211 U_BOOT_DRIVER(mmc_drv) = {
215 .ofdata_to_platdata = mmc_ofdata_to_platdata,
217 .priv_auto_alloc_size = sizeof(struct mmc_priv),
218 .platdata_auto_alloc_size = sizeof(struct mmc_platdata),
222 In the case where SPL_OF_PLATDATA is enabled, platdata_auto_alloc_size is
223 ignored, and the platform data points to the C structure data. In the case
224 where device tree is used, the platform data is allocated, and starts
225 zeroed. In this case the ofdata_to_platdata() method should set up the
228 SPL must use either of-platdata or device tree. Drivers cannot use both.
229 The device tree becomes in accessible when CONFIG_SPL_OF_PLATDATA is enabled,
230 since the device-tree access code is not compiled in.
236 The dt-structs.h file includes the generated file
237 (include/generated//dt-structs.h) if CONFIG_SPL_OF_PLATDATA is enabled.
238 Otherwise (such as in U-Boot proper) these structs are not available. This
239 prevents them being used inadvertently.
241 The dt-platdata.c file contains the device declarations and is is built in
244 Some phandles (thsoe that are recognised as such) are converted into
245 points to platform data. This pointer can potentially be used to access the
246 referenced device (by searching for the pointer value). This feature is not
247 yet implemented, however.
249 The beginnings of a libfdt Python module are provided. So far this only
250 implements a subset of the features.
252 The 'swig' tool is needed to build the libfdt Python module.
258 - Add a sandbox_spl functional test
259 - Consider programmatically reading binding files instead of device tree
261 - Drop the device tree data from the SPL image
262 - Complete the phandle feature
263 - Get this running on a Rockchip board
264 - Move to using a full Python libfdt module
267 Simon Glass <sjg@chromium.org>