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dm: Add a README for of-platdata

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Add documentation on how this works, including the benefits and drawbacks.

Signed-off-by: Simon Glass <sjg@chromium.org>
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Simon Glass 8 years ago
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      doc/driver-model/of-plat.txt

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doc/driver-model/of-plat.txt
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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 = <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.
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 <sjg@chromium.org>
6/6/16
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