Date:	Sat, 07 Dec 2013 19:23:12 -0800
From:	Randy Dunlap <rdunlap@...radead.org>
To:	Linus Walleij <linus.walleij@...aro.org>,
	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	linux-doc@...r.kernel.org, Rob Landley <rob@...dley.net>,
	Mark Brown <broonie@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Grant Likely <grant.likely@...aro.org>
Subject: Re: [PATCH] Documentation: start documenting driver design patterns

On 12/04/13 04:22, Linus Walleij wrote:
> After realizing that we tend to tell developers the same thing over
> and over, let's attempt to document some commin design patterns
> used in the device drivers. The idea is that this can be extended
> so I just start out with two well-known design patterns.
> 
> Cc: Rob Landley <rob@...dley.net>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Mark Brown <broonie@...nel.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Grant Likely <grant.likely@...aro.org>
> Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
> ---
> I guess this goes to Greg for merging through the device core
> tree if it is liked, let's see.
> ---
>  Documentation/driver-model/design-patterns.txt | 116 +++++++++++++++++++++++++
>  1 file changed, 116 insertions(+)
>  create mode 100644 Documentation/driver-model/design-patterns.txt
> 
> diff --git a/Documentation/driver-model/design-patterns.txt b/Documentation/driver-model/design-patterns.txt
> new file mode 100644
> index 000000000000..9ef8c1684558
> --- /dev/null
> +++ b/Documentation/driver-model/design-patterns.txt
> @@ -0,0 +1,116 @@
> +
> +Device Driver Design Patterns
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +This document describes a few common design patterns found in device drivers.
> +It is likely that subsystem maintainers will ask driver developers to
> +conform to these design patterns.
> +
> +1. State Container
> +2. container_of()
> +
> +
> +1. State Container
> +~~~~~~~~~~~~~~~~~~
> +
> +While the kernel contains a few device drivers that assume that they will
> +only be probed() once on a certain system (singletons), it is custom to assume
> +that the device the driver binds to will appear in several instances. This
> +means that the probe() function and all callbacks need to be reentrant.
> +
> +The most common way to achieve this is to use the state container design
> +pattern. It usually has this form:
> +
> +struct foo {
> +    spinlock_t lock; /* Example member */
> +    (...)
> +};
> +
> +static int foo_probe(...)
> +{
> +    struct foo *foo;
> +
> +    foo = devm_kzalloc(dev, sizeof(*foo), GFP_KERNEL);
> +    if (!foo)
> +        return -ENOMEM;
> +    spin_lock_init(&foo->lock);
> +    (...)
> +}
> +
> +This will create an instance of struct foo in memory every time probe() is
> +called. This is our state container for this instance of the device driver.
> +Of course it is then necessary to always pass this instance of the
> +state around to all functions that need access to the state and its members.
> +
> +For example, if the driver is registering an interrupt handler, you would
> +pass around a pointer to struct foo like this:
> +
> +static irqreturn_t foo_handler(int irq, void *arg)
> +{
> +    struct foo *foo = arg;
> +    (...)
> +}
> +
> +static int foo_probe(...)
> +{
> +    struct foo *foo;
> +
> +    (...)
> +    ret = request_irq(irq, foo_handler, 0, "foo", foo);
> +}
> +
> +This way you always get a pointer back to the correct instance of foo in
> +your interrupt handler.
> +
> +
> +2. container_of()
> +~~~~~~~~~~~~~~~~~
> +
> +Continuing on the above example we add a offloaded work:

                                          an

> +
> +struct foo {
> +    spinlock_t lock;
> +    struct workqueue_struct *wq;
> +    struct work_struct offload;
> +    (...)
> +};
> +
> +static void foo_work(struct work_struct *work)
> +{
> +    struct foo *foo = container_of(work, struct foo, offload);
> +
> +    (...)
> +}
> +
> +static irqreturn_t foo_handler(int irq, void *arg)
> +{
> +    struct foo *foo = arg;
> +
> +    queue_work(foo->wq, &foo->offload);
> +    (...)
> +}
> +
> +static int foo_probe(...)
> +{
> +    struct foo *foo;
> +
> +    foo->wq = create_singlethread_workqueue("foo-wq");
> +    INIT_WORK(&foo->offload, foo_work);
> +    (...)
> +}
> +
> +The design pattern is the same for a a hrtimer or something similar that will

                                  for an hrtimer

> +return a single argument which is a pointer to a struct member in the
> +callback.
> +
> +container_of() is a macro defined in <linux/kernel.h>
> +
> +What container_of() does is to obtain a pointer to the containing struct from
> +a pointer to a member by a simple subtraction using the offsetof() macro from
> +standard C, which allows something similar to object oriented behaviours.
> +Notice that the contained member must not be a pointer, but an actual member
> +for this to work.
> +
> +We can see here that we avoid having global pointers to our struct foo *
> +instance this way, while still keeping the number of parameters passed to the
> +work function to a single pointer.
> 


-- 
~Randy
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