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126 lines
5.5 KiB
Text
126 lines
5.5 KiB
Text
The U-Boot Driver Model Project
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===============================
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Driver cores API document
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=========================
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Pavel Herrmann <morpheus.ibis@gmail.com>
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1) Overview
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-----------
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Driver cores will be used as a wrapper for devices of the same type, and as
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an abstraction for device driver APIs. For each driver API (which roughly
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correspond to device types), there will be one driver core. Each driver core
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will implement three APIs - a driver API (which will be the same as API of
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drivers the core wraps around), a core API (which will be implemented by all
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cores) and a command API (core-specific API which will be exposed to
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commands).
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A) Command API
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The command API will provide access to shared functionality for a specific
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device, which is currently located mostly in commands. Commands will be
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rewritten to be more lightweight by using this API. As this API will be
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different for each core, it is out of scope of this document.
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B) Driver API
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The driver API will act as a wrapper around actual device drivers,
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providing a single entrypoint for device access. All functions in this API
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have an instance* argument (probably called "this" or "i"), which will be
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examined by the core, and a correct function for the specified driver will
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get called.
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If the core gets called with a group instance pointer (as discussed in
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design), it will automatically select the instance that is associated
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with this core, and use it as target of the call. if the group contains
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multiple instances of a single type, the caller must explicitly use an
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accessor to select the correct instance.
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This accessor will look like:
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struct instance *get_instance_from_group(struct instance *group, int i)
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When called with a non-group instance, it will simply return the instance.
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C) Core API
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The core API will be implemented by all cores, and will provide
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functionality for getting driver instances from non-driver code. This API
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will consist of following functions:
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int get_count(struct instance *core);
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struct instance* get_instance(struct instance *core, int index);
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int init(struct instance *core);
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int bind(struct instance *core, struct instance *dev, void *ops,
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void *hint);
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int unbind(struct instance *core, instance *dev);
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int replace(struct instance *core, struct_instance *new_dev,
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struct instance *old_dev);
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int destroy(struct instance *core);
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int reloc(struct instance *new_core, struct instance *old_core);
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The 'hint' parameter of bind() serves for additional data a driver can
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pass to the core, to help it create the correct internal state for this
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instance. the replace() function will get called during instance
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relocation, and will replace the old instance with the new one, keeping
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the internal state untouched.
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2) Lifetime of a driver core
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----------------------------
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Driver cores will be initialized at runtime, to limit memory footprint in
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early-init stage, when we have to fit into ~1KB of memory. All active cores
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will be stored in a tree structure (referenced as "Core tree") in global data,
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which provides good tradeoff between size and access time.
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Every core will have a number constant associated with it, which will be used
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to find the instance in Core tree, and to refer to the core in all calls
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working with the Core tree.
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The Core Tree should be implemented using B-tree (or a similar structure)
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to guarantee acceptable time overhead in all cases.
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Code for working with the core (i2c in this example) follows:
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core_init(CORE_I2C);
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This will check whether we already have a i2c core, and if not it creates
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a new instance and adds it into the Core tree. This will not be exported,
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all code should depend on get_core_instance to init the core when
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necessary.
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get_core_instance(CORE_I2C);
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This is an accessor into the Core tree, which will return the instance
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of i2c core, creating it if necessary
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core_bind(CORE_I2C, instance, driver_ops);
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This will get called in bind() function of a driver, and will add the
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instance into cores internal list of devices. If the core is not found, it
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will get created.
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driver_activate(instance *inst);
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This call will recursively activate all devices necessary for using the
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specified device. the code could be simplified as:
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{
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if (is_activated(inst))
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return;
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driver_activate(inst->bus);
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get_driver(inst)->probe(inst);
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}
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The case with multiple parents will need to be handled here as well.
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get_driver is an accessor to available drivers, which will get struct
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driver based on a name in the instance.
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i2c_write(instance *inst, ...);
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An actual call to some method of the driver. This code will look like:
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{
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driver_activate(inst);
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struct instance *core = get_core_instance(CORE_I2C);
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device_ops = get_ops(inst);
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device_ops->write(...);
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}
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get_ops will not be an exported function, it will be internal and specific
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to the core, as it needs to know how are the ops stored, and what type
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they are.
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Please note that above examples represent the algorithm, not the actual code,
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as they are missing checks for validity of return values.
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core_init() function will get called the first time the core is requested,
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either by core_link() or core_get_instance(). This way, the cores will get
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created only when they are necessary, which will reduce our memory footprint.
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