[PATCH 1/2] Add a common struct clk

Hi Jeremy,

On Wed, Jan 05, 2011 at 11:51:02AM +0800, Jeremy Kerr wrote:

We currently have ~21 definitions of struct clk in the ARM architecture,
each defined on a per-platform basis. This makes it difficult to define
platform- (or architecture-) independent clock sources without making
assumptions about struct clk, and impossible to compile two
platforms with different struct clks into a single image.

This change is an effort to unify struct clk where possible, by defining
a common struct clk, containing a set of clock operations. Different
clock implementations can set their own operations, and have a standard
interface for generic code. The callback interface is exposed to the
kernel proper, while the clock implementations only need to be seen by
the platform internals.

This allows us to share clock code among platforms, and makes it
possible to dynamically create clock devices in platform-independent
code.

Platforms can enable the generic struct clock through
CONFIG_USE_COMMON_STRUCT_CLK. In this case, the clock infrastructure
consists of a common struct clk:

struct clk {
	const struct clk_ops	*ops;
	unsigned int		enable_count;
	int			flags;
	union {
		struct mutex	mutex;
		spinlock_t	spinlock;
	} lock;
};

I'm currently thinking about how to get the locking right with this
approach. In the current i.MX implementation we have a global lock which
protects the clock enable counter and also the register accesses in the
clock code. With the common struct clock we have a lock per clock which
only protects the enable counter, so we have to introduce a second lock
to protect the register accesses.
The problem comes with nested calls to for example clk_enable which
happens when the parent clock gets enabled. currently we do this with
clk->enable(clk->parent) which results in an unlocked clk_enable of the
parent. With common struct clk we would have to call
clk_enable(clk_get_parent(clk) which results in taking the lock a second
time.
Any ideas how to solve this?

Sascha

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