117 lines
3.1 KiB
ReStructuredText
117 lines
3.1 KiB
ReStructuredText
=============================
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Device Driver Design Patterns
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=============================
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This document describes a few common design patterns found in device drivers.
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It is likely that subsystem maintainers will ask driver developers to
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conform to these design patterns.
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1. State Container
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2. container_of()
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1. State Container
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~~~~~~~~~~~~~~~~~~
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While the kernel contains a few device drivers that assume that they will
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only be probed() once on a certain system (singletons), it is custom to assume
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that the device the driver binds to will appear in several instances. This
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means that the probe() function and all callbacks need to be reentrant.
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The most common way to achieve this is to use the state container design
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pattern. It usually has this form::
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struct foo {
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spinlock_t lock; /* Example member */
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(...)
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};
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static int foo_probe(...)
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{
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struct foo *foo;
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foo = devm_kzalloc(dev, sizeof(*foo), GFP_KERNEL);
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if (!foo)
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return -ENOMEM;
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spin_lock_init(&foo->lock);
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(...)
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}
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This will create an instance of struct foo in memory every time probe() is
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called. This is our state container for this instance of the device driver.
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Of course it is then necessary to always pass this instance of the
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state around to all functions that need access to the state and its members.
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For example, if the driver is registering an interrupt handler, you would
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pass around a pointer to struct foo like this::
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static irqreturn_t foo_handler(int irq, void *arg)
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{
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struct foo *foo = arg;
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(...)
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}
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static int foo_probe(...)
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{
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struct foo *foo;
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(...)
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ret = request_irq(irq, foo_handler, 0, "foo", foo);
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}
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This way you always get a pointer back to the correct instance of foo in
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your interrupt handler.
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2. container_of()
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~~~~~~~~~~~~~~~~~
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Continuing on the above example we add an offloaded work::
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struct foo {
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spinlock_t lock;
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struct workqueue_struct *wq;
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struct work_struct offload;
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(...)
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};
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static void foo_work(struct work_struct *work)
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{
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struct foo *foo = container_of(work, struct foo, offload);
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(...)
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}
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static irqreturn_t foo_handler(int irq, void *arg)
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{
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struct foo *foo = arg;
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queue_work(foo->wq, &foo->offload);
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(...)
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}
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static int foo_probe(...)
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{
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struct foo *foo;
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foo->wq = create_singlethread_workqueue("foo-wq");
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INIT_WORK(&foo->offload, foo_work);
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(...)
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}
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The design pattern is the same for an hrtimer or something similar that will
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return a single argument which is a pointer to a struct member in the
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callback.
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container_of() is a macro defined in <linux/kernel.h>
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What container_of() does is to obtain a pointer to the containing struct from
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a pointer to a member by a simple subtraction using the offsetof() macro from
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standard C, which allows something similar to object oriented behaviours.
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Notice that the contained member must not be a pointer, but an actual member
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for this to work.
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We can see here that we avoid having global pointers to our struct foo *
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instance this way, while still keeping the number of parameters passed to the
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work function to a single pointer.
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