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    SubjectRe: [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
    Any ideas how to solve this?


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