1 =============================
2 Device Driver Design Patterns
3 =============================
4
5 This document describes a few common design pa
6 It is likely that subsystem maintainers will a
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
17 only be probed() once on a certain system (sin
18 that the device the driver binds to will appea
19 means that the probe() function and all callba
20
21 The most common way to achieve this is to use
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), GF
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
41 called. This is our state container for this i
42 Of course it is then necessary to always pass
43 state around to all functions that need access
44
45 For example, if the driver is registering an i
46 pass around a pointer to struct foo like this:
47
48 static irqreturn_t foo_handler(int irq, void
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, "
60 }
61
62 This way you always get a pointer back to the
63 your interrupt handler.
64
65
66 2. container_of()
67 ~~~~~~~~~~~~~~~~~
68
69 Continuing on the above example we add an offl
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 *wor
79 {
80 struct foo *foo = container_of(work, str
81
82 (...)
83 }
84
85 static irqreturn_t foo_handler(int irq, void
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(
98 INIT_WORK(&foo->offload, foo_work);
99 (...)
100 }
101
102 The design pattern is the same for an hrtimer
103 return a single argument which is a pointer to
104 callback.
105
106 container_of() is a macro defined in <linux/ke
107
108 What container_of() does is to obtain a pointe
109 a pointer to a member by a simple subtraction
110 standard C, which allows something similar to
111 Notice that the contained member must not be a
112 for this to work.
113
114 We can see here that we avoid having global po
115 instance this way, while still keeping the num
116 work function to a single pointer.
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