[U-Boot] [PATCH 27/31] dm: Add a README for of-platdata
Simon Glass
sjg at chromium.org
Wed Jun 8 04:48:05 CEST 2016
Add documentation on how this works, including the benefits and drawbacks.
Signed-off-by: Simon Glass <sjg at chromium.org>
---
doc/driver-model/of-plat.txt | 266 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 266 insertions(+)
create mode 100644 doc/driver-model/of-plat.txt
diff --git a/doc/driver-model/of-plat.txt b/doc/driver-model/of-plat.txt
new file mode 100644
index 0000000..a644e9a
--- /dev/null
+++ b/doc/driver-model/of-plat.txt
@@ -0,0 +1,266 @@
+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 at 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:
+
+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 <dt-structs.h>
+
+ 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.
+
+
+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 <sjg at chromium.org>
+6/6/16
--
2.8.0.rc3.226.g39d4020
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