| 1 | =============================
|
| 2 | Device Driver Design Patterns
|
| 3 | =============================
|
| 4 |
|
| 5 | This document describes a few common design patterns found in device drivers.
|
| 6 | It is likely that subsystem maintainers will ask driver developers to
|
| 7 | conform to these design patterns.
|
| 8 |
|
| 9 | 1. State Container
|
| 10 | 2. container_of()
|
| 11 |
|
| 12 |
|
| 13 | 1. State Container
|
| 14 | ~~~~~~~~~~~~~~~~~~
|
| 15 |
|
| 16 | While the kernel contains a few device drivers that assume that they will
|
| 17 | only be probed() once on a certain system (singletons), it is custom to assume
|
| 18 | that the device the driver binds to will appear in several instances. This
|
| 19 | means that the probe() function and all callbacks need to be reentrant.
|
| 20 |
|
| 21 | The most common way to achieve this is to use the state container design
|
| 22 | pattern. It usually has this form::
|
| 23 |
|
| 24 | struct foo {
|
| 25 | spinlock_t lock; /* Example member */
|
| 26 | (...)
|
| 27 | };
|
| 28 |
|
| 29 | static int foo_probe(...)
|
| 30 | {
|
| 31 | struct foo *foo;
|
| 32 |
|
| 33 | foo = devm_kzalloc(dev, sizeof(*foo), GFP_KERNEL);
|
| 34 | if (!foo)
|
| 35 | return -ENOMEM;
|
| 36 | spin_lock_init(&foo->lock);
|
| 37 | (...)
|
| 38 | }
|
| 39 |
|
| 40 | This will create an instance of struct foo in memory every time probe() is
|
| 41 | called. This is our state container for this instance of the device driver.
|
| 42 | Of course it is then necessary to always pass this instance of the
|
| 43 | state around to all functions that need access to the state and its members.
|
| 44 |
|
| 45 | For example, if the driver is registering an interrupt handler, you would
|
| 46 | pass around a pointer to struct foo like this::
|
| 47 |
|
| 48 | static irqreturn_t foo_handler(int irq, void *arg)
|
| 49 | {
|
| 50 | struct foo *foo = arg;
|
| 51 | (...)
|
| 52 | }
|
| 53 |
|
| 54 | static int foo_probe(...)
|
| 55 | {
|
| 56 | struct foo *foo;
|
| 57 |
|
| 58 | (...)
|
| 59 | ret = request_irq(irq, foo_handler, 0, "foo", foo);
|
| 60 | }
|
| 61 |
|
| 62 | This way you always get a pointer back to the correct instance of foo in
|
| 63 | your interrupt handler.
|
| 64 |
|
| 65 |
|
| 66 | 2. container_of()
|
| 67 | ~~~~~~~~~~~~~~~~~
|
| 68 |
|
| 69 | Continuing on the above example we add an offloaded work::
|
| 70 |
|
| 71 | struct foo {
|
| 72 | spinlock_t lock;
|
| 73 | struct workqueue_struct *wq;
|
| 74 | struct work_struct offload;
|
| 75 | (...)
|
| 76 | };
|
| 77 |
|
| 78 | static void foo_work(struct work_struct *work)
|
| 79 | {
|
| 80 | struct foo *foo = container_of(work, struct foo, offload);
|
| 81 |
|
| 82 | (...)
|
| 83 | }
|
| 84 |
|
| 85 | static irqreturn_t foo_handler(int irq, void *arg)
|
| 86 | {
|
| 87 | struct foo *foo = arg;
|
| 88 |
|
| 89 | queue_work(foo->wq, &foo->offload);
|
| 90 | (...)
|
| 91 | }
|
| 92 |
|
| 93 | static int foo_probe(...)
|
| 94 | {
|
| 95 | struct foo *foo;
|
| 96 |
|
| 97 | foo->wq = create_singlethread_workqueue("foo-wq");
|
| 98 | INIT_WORK(&foo->offload, foo_work);
|
| 99 | (...)
|
| 100 | }
|
| 101 |
|
| 102 | The design pattern is the same for an hrtimer or something similar that will
|
| 103 | return a single argument which is a pointer to a struct member in the
|
| 104 | callback.
|
| 105 |
|
| 106 | container_of() is a macro defined in <linux/container_of.h>
|
| 107 |
|
| 108 | What container_of() does is to obtain a pointer to the containing struct from
|
| 109 | a pointer to a member by a simple subtraction using the offsetof() macro from
|
| 110 | standard C, which allows something similar to object oriented behaviours.
|
| 111 | Notice that the contained member must not be a pointer, but an actual member
|
| 112 | for this to work.
|
| 113 |
|
| 114 | We can see here that we avoid having global pointers to our struct foo *
|
| 115 | instance this way, while still keeping the number of parameters passed to the
|
| 116 | work function to a single pointer.
|