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

Jon Loeliger
2014-05-27T17:37:31Z

> +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

Why is the scan only on the "top level"? My GPIO nodes are, for example,
under an SOC node.

Thanks,
jdl

On Sat, May 24, 2014 at 4:21 PM, Simon Glass <sjg@chromium.org> wrote:
> 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
>
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