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dm: Update the of-platdata README for the new features

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Revise the content based on the v2 additions. This is kept as a separate
patch to avoid confusing those who have already reviewed the v1 series.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>
This commit is contained in:
Simon Glass 2016-07-04 11:58:42 -06:00
parent b979d3d4c5
commit 1269625177
1 changed files with 76 additions and 34 deletions
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110
doc/driver-model/of-plat.txt
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@ -19,24 +19,28 @@ 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.
for it, and reference that data in a U_BOOT_DEVICE() declaration. This
bypasses the use of device tree completely, effectively creating a parallel
configuration mechanism. But it is an available option for SPL.
As an alternative, a new 'of-platdata' feature is provided. This converts
As an alternative, a new 'of-platdata' feature is provided. This converts the
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.
Note: Quite a bit of thought has gone into the design of this feature.
However it still has many rough edges and comments and suggestions are
strongly encouraged! Quite possibly there is a much better approach.
Caveats
-------
There are many problems with this features. It should only be used when
stricly necessary. Notable problems include:
strictly 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
- Device tree does not describe data types. But the C code must define a
type for each property. These 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
@ -45,14 +49,15 @@ stricly necessary. Notable problems include:
- 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
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.
a different C struct name, and a likely a different 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
@ -112,7 +117,6 @@ struct dtd_rockchip_rk3288_dw_mshc {
fdt32_t interrupts[3];
fdt32_t num_slots;
fdt32_t reg[2];
bool u_boot_dm_pre_reloc;
fdt32_t vmmc_supply;
};
@ -125,7 +129,10 @@ static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = {
.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}},
.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,
@ -136,6 +143,7 @@ static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = {
U_BOOT_DEVICE(dwmmc_at_ff0c0000) = {
.name = "rockchip_rk3288_dw_mshc",
.platdata = &dtv_dwmmc_at_ff0c0000,
.platdata_size = sizeof(dtv_dwmmc_at_ff0c0000),
};
The device is then instantiated at run-time and the platform data can be
@ -149,15 +157,31 @@ platform data in the driver. The ofdata_to_platdata() method should
therefore do nothing in such a driver.
Converting of-platdata to a useful form
---------------------------------------
Of course it would be possible use the of-platdata directly in your driver
whenever configuration information is required. However this meands that the
driver will not be able to support device tree, since the of-platdata
structure is not available when device tree is used. It would make no sense
to use this structure if device tree were available, since the structure has
all the limitations metioned in caveats above.
Therefore it is recommended that the of-platdata structure should be used
only in the probe() method of your driver. It cannot be used in the
ofdata_to_platdata() method since this is not called when platform data is
already present.
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.
Your driver should convert the platdata struct in its probe() method. The
existing device tree decoding logic should be kept in the
ofdata_to_platdata() method and wrapped with #if.
For example:
@ -165,13 +189,12 @@ For example:
struct mmc_platdata {
#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
/* Put this first */
/* Put this first since driver model will copy the data here */
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.
* the device tree (or the C structures when of-platdata is used).
*/
int fifo_depth;
};
@ -179,6 +202,7 @@ For example:
static int mmc_ofdata_to_platdata(struct udevice *dev)
{
#if !CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
/* Decode the device tree data */
struct mmc_platdata *plat = dev_get_platdata(dev);
const void *blob = gd->fdt_blob;
int node = dev->of_offset;
@ -192,15 +216,15 @@ For example:
static int mmc_probe(struct udevice *dev)
{
struct mmc_platdata *plat = dev_get_platdata(dev);
#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
/* Decode the of-platdata from the C structures */
struct dtd_mmc *dtplat = &plat->dtplat;
/* Set up the device from the dtplat data */
writel(dtplat->fifo_depth, ...)
#else
plat->fifo_depth = dtplat->fifo_depth;
#endif
/* Set up the device from the plat data */
writel(plat->fifo_depth, ...)
#endif
}
static const struct udevice_id mmc_ids[] = {
@ -220,15 +244,26 @@ For example:
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.
still used to allocate space for the platform data. This is different from
the normal behaviour and is triggered by the use of of-platdata (strictly
speaking it is a non-zero platdata_size which triggers this).
The of-platdata struct contents is copied from the C structure data to the
start of the newly allocated area. 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 still set up the platform data (and the
of-platdata struct will not be present).
SPL must use either of-platdata or device tree. Drivers cannot use both at
the same time, but they must support device tree. Supporting of-platdata is
optional.
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.
since the device-tree access code is not compiled in. A corollary is that
a board can only move to using of-platdata if all the drivers it uses support
it. There would be little point in having some drivers require the device
tree data, since then libfdt would still be needed for those drivers and
there would be no code-size benefit.
Internals
---------
@ -236,7 +271,8 @@ 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.
prevents them being used inadvertently. All usage must be bracketed with
#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA).
The dt-platdata.c file contains the device declarations and is is built in
spl/dt-platdata.c.
@ -249,20 +285,26 @@ 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.
The 'swig' tool is needed to build the libfdt Python module. If this is not
found then the Python model is not used and a fallback is used instead, which
makes use of fdtget.
Credits
-------
This is an implementation of an idea by Tom Rini <trini@konsulko.com>.
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>
Google, Inc
6/6/16
Updated Independence Day 2016