Decoding Samsung’s Kernel Clock Structure: A Deep Dive into /arch/arm/plat‑samsung/clock.h

Clock management in the Linux kernel is tightly coupled with hardware devices, and the Samsung platform provides a concrete example in /arch/arm/plat-samsung/clock.h. The article walks through the relevant structures to show how clocks are registered, queried, and controlled.

Structure of struct clk

The main clock descriptor contains several members that enable unified handling of all clocks through a global linked list. struct list_head node – links the clock into the global clock list. struct module *owner – module that owns the clock. struct clk *parent – pointer to the parent clock, establishing a hierarchy. const char *name – human‑readable clock name. const char *dev_name – name of the device the clock belongs to. int id – identifier used by the device. unsigned int refcnt – reference count for usage tracking. unsigned long rate – current clock frequency. unsigned long flags – flags describing the clock type (e.g., I2C, NAND). struct clk_ops *ops – pointer to a set of operation callbacks. struct clk_lookup *lookup – lookup table entry for name‑based searches.

Clock operation callbacks – struct clk_ops

The clk_ops structure defines the functions that manipulate a clock. The most common callbacks are: int (*set_rate)(struct clk *clk, unsigned long rate) – set the clock frequency. unsigned long (*get_rate)(struct clk *clk) – retrieve the current frequency. int (*round_rate)(struct clk *clk, unsigned long rate) – calculate the nearest supported frequency. int (*set_parent)(struct clk *clk, struct clk *parent) – change the parent clock.

Clock lookup table – struct clk_lookup

To find a clock by name or device ID, the kernel uses a lookup table defined by struct clk_lookup: struct list_head node – links the entry into the lookup list. int dev_id – identifier of the device owning the clock. int con_id – matching condition identifier used during lookup. struct clk *clk – pointer to the actual clock structure.

By registering each clock in the global list and providing appropriate clk_ops and clk_lookup entries, the Samsung platform integrates its hardware clocks into the generic Linux clock framework, enabling uniform configuration and power management across the system.