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@@ -19,24 +19,28 @@ SoCs require a 16KB SPL image which must include a full MMC stack. In this
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case the overhead of device tree access may be too great.
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It is possible to create platform data manually by defining C structures
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-for it, and referencing that data in a U_BOOT_DEVICE() declaration. This
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-bypasses the use of device tree completely, but is an available option for
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-SPL.
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+for it, and reference that data in a U_BOOT_DEVICE() declaration. This
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+bypasses the use of device tree completely, effectively creating a parallel
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+configuration mechanism. But it is an available option for SPL.
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-As an alternative, a new 'of-platdata' feature is provided. This converts
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+As an alternative, a new 'of-platdata' feature is provided. This converts the
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device tree contents into C code which can be compiled into the SPL binary.
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This saves the 3KB of code overhead and perhaps a few hundred more bytes due
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to more efficient storage of the data.
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+Note: Quite a bit of thought has gone into the design of this feature.
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+However it still has many rough edges and comments and suggestions are
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+strongly encouraged! Quite possibly there is a much better approach.
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+
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Caveats
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-------
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There are many problems with this features. It should only be used when
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-stricly necessary. Notable problems include:
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+strictly necessary. Notable problems include:
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- - Device tree does not describe data types but the C code must define a
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- type for each property. Thesee are guessed using heuristics which
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+ - Device tree does not describe data types. But the C code must define a
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+ type for each property. These are guessed using heuristics which
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are wrong in several fairly common cases. For example an 8-byte value
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is considered to be a 2-item integer array, and is byte-swapped. A
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boolean value that is not present means 'false', but cannot be
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@@ -45,14 +49,15 @@ stricly necessary. Notable problems include:
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- Naming of nodes and properties is automatic. This means that they follow
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the naming in the device tree, which may result in C identifiers that
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- look a bit strange
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+ look a bit strange.
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- It is not possible to find a value given a property name. Code must use
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the associated C member variable directly in the code. This makes
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the code less robust in the face of device-tree changes. It also
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makes it very unlikely that your driver code will be useful for more
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than one SoC. Even if the code is common, each SoC will end up with
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- a different C struct and format for the platform data.
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+ a different C struct name, and a likely a different format for the
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+ platform data.
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- The platform data is provided to drivers as a C structure. The driver
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must use the same structure to access the data. Since a driver
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@@ -112,7 +117,6 @@ struct dtd_rockchip_rk3288_dw_mshc {
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fdt32_t interrupts[3];
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fdt32_t num_slots;
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fdt32_t reg[2];
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- bool u_boot_dm_pre_reloc;
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fdt32_t vmmc_supply;
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};
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@@ -125,7 +129,10 @@ static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = {
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.clock_freq_min_max = {0x61a80, 0x8f0d180},
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.vmmc_supply = 0xb,
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.num_slots = 0x1,
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- .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}},
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+ .clocks = {{&dtv_clock_controller_at_ff760000, 456},
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+ {&dtv_clock_controller_at_ff760000, 68},
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+ {&dtv_clock_controller_at_ff760000, 114},
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+ {&dtv_clock_controller_at_ff760000, 118}},
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.cap_mmc_highspeed = true,
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.disable_wp = true,
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.bus_width = 0x4,
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@@ -136,6 +143,7 @@ static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = {
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U_BOOT_DEVICE(dwmmc_at_ff0c0000) = {
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.name = "rockchip_rk3288_dw_mshc",
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.platdata = &dtv_dwmmc_at_ff0c0000,
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+ .platdata_size = sizeof(dtv_dwmmc_at_ff0c0000),
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};
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The device is then instantiated at run-time and the platform data can be
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@@ -149,15 +157,31 @@ platform data in the driver. The ofdata_to_platdata() method should
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therefore do nothing in such a driver.
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+Converting of-platdata to a useful form
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+---------------------------------------
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+
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+Of course it would be possible use the of-platdata directly in your driver
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+whenever configuration information is required. However this meands that the
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+driver will not be able to support device tree, since the of-platdata
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+structure is not available when device tree is used. It would make no sense
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+to use this structure if device tree were available, since the structure has
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+all the limitations metioned in caveats above.
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+
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+Therefore it is recommended that the of-platdata structure should be used
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+only in the probe() method of your driver. It cannot be used in the
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+ofdata_to_platdata() method since this is not called when platform data is
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+already present.
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+
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+
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How to structure your driver
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----------------------------
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Drivers should always support device tree as an option. The of-platdata
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feature is intended as a add-on to existing drivers.
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-Your driver should directly access the platdata struct in its probe()
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-method. The existing device tree decoding logic should be kept in the
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-ofdata_to_platdata() and wrapped with #ifdef.
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+Your driver should convert the platdata struct in its probe() method. The
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+existing device tree decoding logic should be kept in the
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+ofdata_to_platdata() method and wrapped with #if.
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For example:
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@@ -165,13 +189,12 @@ For example:
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struct mmc_platdata {
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#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
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- /* Put this first */
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+ /* Put this first since driver model will copy the data here */
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struct dtd_mmc dtplat;
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#endif
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/*
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* Other fields can go here, to be filled in by decoding from
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- * the device tree. They will point to random memory in the
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- * of-plat case.
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+ * the device tree (or the C structures when of-platdata is used).
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*/
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int fifo_depth;
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};
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@@ -179,6 +202,7 @@ For example:
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static int mmc_ofdata_to_platdata(struct udevice *dev)
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{
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#if !CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
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+ /* Decode the device tree data */
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struct mmc_platdata *plat = dev_get_platdata(dev);
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const void *blob = gd->fdt_blob;
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int node = dev->of_offset;
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@@ -192,15 +216,15 @@ For example:
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static int mmc_probe(struct udevice *dev)
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{
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struct mmc_platdata *plat = dev_get_platdata(dev);
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+
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#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
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+ /* Decode the of-platdata from the C structures */
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struct dtd_mmc *dtplat = &plat->dtplat;
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- /* Set up the device from the dtplat data */
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- writel(dtplat->fifo_depth, ...)
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- #else
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+ plat->fifo_depth = dtplat->fifo_depth;
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+ #endif
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/* Set up the device from the plat data */
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writel(plat->fifo_depth, ...)
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- #endif
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}
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static const struct udevice_id mmc_ids[] = {
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@@ -220,15 +244,26 @@ For example:
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In the case where SPL_OF_PLATDATA is enabled, platdata_auto_alloc_size is
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-ignored, and the platform data points to the C structure data. In the case
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-where device tree is used, the platform data is allocated, and starts
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-zeroed. In this case the ofdata_to_platdata() method should set up the
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-platform data.
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+still used to allocate space for the platform data. This is different from
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+the normal behaviour and is triggered by the use of of-platdata (strictly
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+speaking it is a non-zero platdata_size which triggers this).
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-SPL must use either of-platdata or device tree. Drivers cannot use both.
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-The device tree becomes in accessible when CONFIG_SPL_OF_PLATDATA is enabled,
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-since the device-tree access code is not compiled in.
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+The of-platdata struct contents is copied from the C structure data to the
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+start of the newly allocated area. In the case where device tree is used,
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+the platform data is allocated, and starts zeroed. In this case the
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+ofdata_to_platdata() method should still set up the platform data (and the
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+of-platdata struct will not be present).
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+
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+SPL must use either of-platdata or device tree. Drivers cannot use both at
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+the same time, but they must support device tree. Supporting of-platdata is
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+optional.
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+The device tree becomes in accessible when CONFIG_SPL_OF_PLATDATA is enabled,
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+since the device-tree access code is not compiled in. A corollary is that
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+a board can only move to using of-platdata if all the drivers it uses support
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+it. There would be little point in having some drivers require the device
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+tree data, since then libfdt would still be needed for those drivers and
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+there would be no code-size benefit.
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Internals
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---------
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@@ -236,7 +271,8 @@ Internals
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The dt-structs.h file includes the generated file
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(include/generated//dt-structs.h) if CONFIG_SPL_OF_PLATDATA is enabled.
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Otherwise (such as in U-Boot proper) these structs are not available. This
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-prevents them being used inadvertently.
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+prevents them being used inadvertently. All usage must be bracketed with
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+#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA).
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The dt-platdata.c file contains the device declarations and is is built in
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spl/dt-platdata.c.
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@@ -249,20 +285,26 @@ yet implemented, however.
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The beginnings of a libfdt Python module are provided. So far this only
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implements a subset of the features.
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-The 'swig' tool is needed to build the libfdt Python module.
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+The 'swig' tool is needed to build the libfdt Python module. If this is not
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+found then the Python model is not used and a fallback is used instead, which
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+makes use of fdtget.
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+
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+
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+Credits
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+-------
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+
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+This is an implementation of an idea by Tom Rini <trini@konsulko.com>.
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Future work
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-----------
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-- Add unit tests
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-- Add a sandbox_spl functional test
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- Consider programmatically reading binding files instead of device tree
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contents
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-- Drop the device tree data from the SPL image
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- Complete the phandle feature
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-- Get this running on a Rockchip board
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- Move to using a full Python libfdt module
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--
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Simon Glass <sjg@chromium.org>
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+Google, Inc
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6/6/16
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+Updated Independence Day 2016
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