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| -rw-r--r-- | doc/driver-model/README.txt | 220 |
1 files changed, 213 insertions, 7 deletions
diff --git a/doc/driver-model/README.txt b/doc/driver-model/README.txt index 0b295ac..22c3fcb 100644 --- a/doc/driver-model/README.txt +++ b/doc/driver-model/README.txt @@ -222,7 +222,44 @@ device tree) and probe. Platform Data ------------- -Where does the platform data come from? See demo-pdata.c which +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 + +Where does the platform data come from? It is either held in a structure +which is compiled into U-Boot, or it can be parsed from the Device Tree +(see 'Device Tree' below). + +For an example of how it can be compiled in, 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 basically a communication scheme between the board-specific code and @@ -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. This approach 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 board 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. +and zeroed before the driver's ofdata_to_platdata() method is called. The +ofdata_to_platdata() method, which the driver write supplies, should parse +the device tree node for this device and place 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 will be called first (after bind). If you provide a probe -method it will be called next. +method then it will 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 +341,166 @@ 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. Bind stage + +A device and its driver are bound using one of these two methods: + + - Scan the U_BOOT_DEVICE() definitions. U-Boot 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. + + - Scan through the device tree definitions. U-Boot looks at 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. + +At this point all the devices are known, and bound to their drivers. There +is a 'struct udevice' 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 correctly for +the device. + +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 probe devices and we don't want to do it until +they are needed, or perhaps until after relocation. + +2. Activation/probe + +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 predecessors (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 an 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. + +3. Running stage + +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. + +4. Removal stage + +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. This means that + device_remove() is called for all the children recursively at this point. + + 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: Because the platform data for a U_BOOT_DEVICE() is defined with a + static pointer, it is not de-allocated during the remove() method. For + a device instantiated using the device tree data, the platform data will + be dynamically allocated, and thus needs to be deallocated during the + remove() method, either: + + 1. if the platdata_auto_alloc_size is non-zero, the deallocation + happens automatically within the driver model core; or + + 2. when platdata_auto_alloc_size is 0, both the allocation (in probe() + or preferably ofdata_to_platdata()) and the deallocation in remove() + are the responsibility of the driver author. + + 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 2 above. + +5. Unbind stage + +The device is unbound. This is the step that actually destroys the device. +If a parent has children these will be destroyed first. After this point +the device does not exist and its memory has be deallocated. + + Data Structures --------------- |