[U-Boot] [RFC PATCH 17/22] dm: Expand and improve the device lifecycle docs

Simon Glass
2014-05-24T21:21:16Z

The lifecycle of a device is an important part of driver model. Add to the
existing documentation and clarify it.

Thanks for Jon Loeliger <jdl@jdl.com> for helping with the text and
suggesting improvements.

(Jon please comment/adjust to help clarify things further)

Reported-by: Jon Loeliger <jdl@jdl.com>

Signed-off-by: Simon Glass <sjg@chromium.org>
---

doc/driver-model/README.txt | 197 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 191 insertions(+), 6 deletions(-)

diff --git a/doc/driver-model/README.txt b/doc/driver-model/README.txt
index deacfe9..90e0516 100644
--- a/doc/driver-model/README.txt
+++ b/doc/driver-model/README.txt
@@ -95,11 +95,12 @@ are provided in test/dm. To run them, try:
You should see something like this:

<...U-Boot banner...>
- Running 12 driver model tests
+ Running 15 driver model tests
Test: dm_test_autobind
Test: dm_test_autoprobe
Test: dm_test_children
Test: dm_test_fdt
+ Test: dm_test_fdt_pre_reloc
Test: dm_test_gpio
sandbox_gpio: sb_gpio_get_value: error: offset 4 not reserved
Test: dm_test_leak
@@ -109,6 +110,8 @@ You should see something like this:
Test: dm_test_operations
Test: dm_test_ordering
Test: dm_test_platdata
+ Test: dm_test_pre_reloc
+ Test: dm_test_prefer
Test: dm_test_remove
Test: dm_test_uclass
Failures: 0
@@ -222,6 +225,40 @@ device tree) and probe.
Platform Data
-------------

+Platform data is like Linux platform data, if you are familiar with that.
+It provides the board-specific information to start up a device.
+
+Why is this information not just stored in the device driver itself? The
+idea is that the device driver is generic, and can in principle operate on
+any board that has that type of device. For example, with modern
+highly-complex SoCs it is common for the IP to come from an IP vendor, and
+therefore (for example) the MMC controller may be the same on chips from
+different vendors. It makes no sense to write independent drivers for the
+MMC controller on each vendor's SoC, when they are all almost the same.
+Similarly, we may have 6 UARTs in an SoC, all of which are mostly the same,
+but lie at different addresses in the address space.
+
+Using the UART example, we have a single driver and it is instantiated 6
+times by supplying 6 lots of platform data. Each lot of platform data
+gives the driver name and a pointer to a structure containing information
+about this instance - e.g. the address of the register space. It may be that
+one of the UARTS supports RS-485 operation - this can be added as a flag in
+the platform data, which is set for this one port and clear for the rest.
+
+Think of your driver as a generic piece of code which knows how to talk to
+a device, but needs to know where it is, any variant/option information and
+so on. Platform data provides this link between the generic piece of code
+and the specific way it is bound on a particular board.
+
+Examples of platform data include:
+
+ - The base address of the IP block's register space
+ - Configuration options, like:
+ - the SPI polarity and maximum speed for a SPI controller
+ - the I2C speed to use for an I2C device
+ - the number of GPIOs available in a GPIO device
+ - Note this can be parsed from the Device Tree (see below)
+
Where does the platform data come from? See demo-pdata.c which
sets up a table of driver names and their associated platform data.
The data can be interpreted by the drivers however they like - it is
@@ -259,21 +296,30 @@ following device tree fragment:
sides = <4>;
};

+This means that instead of having lots of U_BOOT_DEVICE() declarations in
+the board file, we put these in the device tree. The allows a lot more
+generality, since the same board file can support many types of boards (e,g.
+with the same SoC) just by using different device trees. An added benefit
+is that the Linux device tree can be used, thus further simplifying the
+task of board-bring up either for U-Boot or Linux devs (whoever gets to the
+baord first!).

The easiest way to make this work it to add a few members to the driver:

.platdata_auto_alloc_size = sizeof(struct dm_test_pdata),
.ofdata_to_platdata = testfdt_ofdata_to_platdata,
- .probe = testfdt_drv_probe,

The 'auto_alloc' feature allowed space for the platdata to be allocated
and zeroed before the driver's ofdata_to_platdata method is called. This
-method reads the information out of the device tree and puts it in
-dev->platdata. Then the probe method is called to set up the device.
+method (which the driver writer supplies) should read the information out
+of the device tree and puts it in dev->platdata. Thus when the probe method
+is called later (to set up the device ready for use) the platform data will
+be present.

Note that both methods are optional. If you provide an ofdata_to_platdata
-method then it wlil be called first (after bind). If you provide a probe
-method it will be called next.
+method then it wlil be called first (during activation). If you provide a
+probe method it will be called next. See Driver Lifecycle below for more
+details.

If you don't want to have the platdata automatically allocated then you
can leave out platdata_auto_alloc_size. In this case you can use malloc
@@ -295,6 +341,145 @@ numbering comes from include/dm/uclass.h. To add a new uclass, add to the
end of the enum there, then declare your uclass as above.

+Driver Lifecycle
+----------------
+
+Here are the stages that a device goes through in driver model. Note that all
+methods mentioned here are optional - e.g. if there is no probe() method for
+a device then it will not be called. A simple device may have very few
+methods actually defined.
+
+1. U-Boot scans the U_BOOT_DEVICE() declarations. It looks up the name
+specified by each, to find the appropriate driver. It then calls
+device_bind() to create a new device and bind' it to its driver. This will
+call the device's bind() method.
+
+2. U-Boot scans through top-level nodes in the the device tree. It looks
+at the compatible string in each node and uses the of_match part of the
+U_BOOT_DRIVER() structure to find the right driver for each node. It then
+calls device_bind() to bind the newly-created device to its driver (thereby
+creating a device structure). This will also call the device's bind()
+method.
+
+3. At this point all the devices are known, and bound to their drivers.
+There is a 'struct device' allocated for all devices. However, nothing
+has been activated (except for the root device). Each bound device that
+was created from a U_BOOT_DEVICE() declaration will hold the platdata
+pointer specified in that declaration. For a bound device created from
+the device tree, platdata will be NULL, but of_offset will be the offset
+of the device tree node that caused the device to be created. The uclass
+is set, and the DM_FLAG_PREFER flag is set if the device node has the
+'dm,prefer' property.
+
+Note: The device's bind() method is permitted to perform simple actions,
+but should not scan the device tree node, not initialise hardware, nor set
+up structures or allocate memory. All of these tasks should be left for the
+probe() method. Note that compared to Linux, U-Boot's driver model has a
+separate step of probe/remove which is independent of bind/unbind. This is
+partly because in U-Boot it may be expensive to prove devices and we don't
+want to do it until they are needed, or perhaps until after relocation.
+
+4. When a device needs to be used, U-Boot activates it, by following these
+steps (see device_probe()):
+
+ a. If priv_auto_alloc_size is non-zero, then the device-private space
+ is allocated for the device and zeroed. It will be accessible as
+ dev->priv. The driver can put anything it likes in there, but should use
+ it for run-time information, not platform data (which should be static
+ and known before the device is probed).
+
+ b. If platdata_auto_alloc_size is non-zero, then the platform data space
+ is allocated. This is only useful for device tree operation, since
+ otherwise you would have to specific the platform data in the
+ U_BOOT_DEVICE() declaration. The space is allocated for the device and
+ zeroed. It will be accessible as dev->platdata.
+
+ c. If the device's uclass specifies a non-zero per_device_auto_alloc_size,
+ then this space is allocated and zeroed also. It is allocated for and
+ stored in the device, but it is uclass data. owned by the uclass driver.
+ It is possible for the device to access it.
+
+ d. All parent devices are probed. It is not possible to activate a device
+ unless its parents (all the way up to the root device) are activated.
+ This means (for example) that an I2C driver will require that its bus
+ be activated.
+
+ e. If the driver provides a ofdata_to_platdata() method, then this is
+ called to convert the device tree data into platform data. This should
+ do various calls like fdtdec_get_int(gd->fdt_blob, dev->of_offset, ...)
+ to access the node and store the resulting information into dev->platdata.
+ After this point, the device works the same way whether it was bound
+ using a device tree node or U_BOOT_DEVICE() structure. In either case,
+ the platform data is now stored in the platdata structure. Typically you
+ will use the platdata_auto_alloc_size feature to specify the size of the
+ platform data structure, and U-Boot will automatically allocate and zero
+ it for you before entry to ofdata_to_platdata(). But if not, you can
+ allocate it yourself in ofdata_to_platdata(). Note that it is preferable
+ to do all the device tree decoding in ofdata_to_platdata() rather than
+ in probe(). (Apart from the ugliness of mixing configuration and run-time
+ data, one day it is possible that U-Boot will cache platformat data for
+ devices which are regularly de/activated).
+
+ f. The device's probe() method is called. This should do anything that
+ is required by the device to get it going. This could include checking
+ that the hardware is actually present, setting up clocks for the
+ hardware and setting up hardware registers to initial values. The code
+ in probe() can access:
+
+ - platform data in dev->platdata (for configuration)
+ - private data in dev->priv (for run-time state)
+ - uclass data in dev->uclass_priv (for things the uclass stores
+ about this device)
+
+ Note: If you don't use priv_auto_alloc_size then you will need to
+ allocate the priv space here yourself. The same applies also to
+ platdata_auto_alloc_size. Remember to free them in the remove() method.
+
+ g. The device is marked 'activated'
+
+ h. The uclass's post_probe() method is called, if one exists. This may
+ cause the uclass to do some housekeeping to record the device as
+ activated and 'known' by the uclass.
+
+5. The device is now activated and can be used. From now until it is removed
+all of the above structures are accessible. The device appears in the
+uclass's list of devices (so if the device is in UCLASS_GPIO it will appear
+as a device in the GPIO uclass). This is the 'running' state of the device.
+
+6. When the device is no-longer required, you can call device_remove() to
+remove it. This performs the probe steps in reverse:
+
+ a. The uclass's pre_remove() method is called, if one exists. This may
+ cause the uclass to do some housekeeping to record the device as
+ deactivated and no-longer 'known' by the uclass.
+
+ b. All the device's children are removed. It is not permitted to have
+ an active child device with a non-active parent.
+
+ c. The device's remove() method is called. At this stage nothing has been
+ deallocated so platform data, private data and the uclass data will all
+ still be present. This is where the hardware can be shut down. It is
+ intended that the device be completely inactive at this point, For U-Boot
+ to be sure that no hardware is running, it should be enough to remove
+ all devices.
+
+ d. The device memory is freed (platform data, private data, uclass data).
+
+ Note: for a U_BOOT_DEVICE() declaration, the platform data is supplied as
+ a static pointer and is not allocated. For device tree, the platform
+ data is allocated during activation and freed during dectivation,
+ typically automatically using platdata_auto_alloc_size. But if that value
+ is 0 then U-Boot will not do the allocation/freeing and you will need to
+ do this yourself in your ofdata_to_platdata() and remove() methods. This
+ difference is tracked by the device's DM_FLAG_ALLOC_PDATA flag.
+
+ e. The device is marked inactive. Note that it is still bound, so the
+ device structure itself is not freed at this point. Should the device be
+ activated again, then the cycle starts again at step 4 above.
+
+7. The device is unbound. This is the step that actually destroys the
+
+
Data Structures
---------------

--
1.9.1.423.g4596e3a