1 The Common Clk Framework 2 Mike Turquette <mturquette@ti.com> 3 4This document endeavours to explain the common clk framework details, 5and how to port a platform over to this framework. It is not yet a 6detailed explanation of the clock api in include/linux/clk.h, but 7perhaps someday it will include that information. 8 9 Part 1 - introduction and interface split 10 11The common clk framework is an interface to control the clock nodes 12available on various devices today. This may come in the form of clock 13gating, rate adjustment, muxing or other operations. This framework is 14enabled with the CONFIG_COMMON_CLK option. 15 16The interface itself is divided into two halves, each shielded from the 17details of its counterpart. First is the common definition of struct 18clk which unifies the framework-level accounting and infrastructure that 19has traditionally been duplicated across a variety of platforms. Second 20is a common implementation of the clk.h api, defined in 21drivers/clk/clk.c. Finally there is struct clk_ops, whose operations 22are invoked by the clk api implementation. 23 24The second half of the interface is comprised of the hardware-specific 25callbacks registered with struct clk_ops and the corresponding 26hardware-specific structures needed to model a particular clock. For 27the remainder of this document any reference to a callback in struct 28clk_ops, such as .enable or .set_rate, implies the hardware-specific 29implementation of that code. Likewise, references to struct clk_foo 30serve as a convenient shorthand for the implementation of the 31hardware-specific bits for the hypothetical "foo" hardware. 32 33Tying the two halves of this interface together is struct clk_hw, which 34is defined in struct clk_foo and pointed to within struct clk. This 35allows easy for navigation between the two discrete halves of the common 36clock interface. 37 38 Part 2 - common data structures and api 39 40Below is the common struct clk definition from 41include/linux/clk-private.h, modified for brevity: 42 43 struct clk { 44 const char *name; 45 const struct clk_ops *ops; 46 struct clk_hw *hw; 47 char **parent_names; 48 struct clk **parents; 49 struct clk *parent; 50 struct hlist_head children; 51 struct hlist_node child_node; 52 ... 53 }; 54 55The members above make up the core of the clk tree topology. The clk 56api itself defines several driver-facing functions which operate on 57struct clk. That api is documented in include/linux/clk.h. 58 59Platforms and devices utilizing the common struct clk use the struct 60clk_ops pointer in struct clk to perform the hardware-specific parts of 61the operations defined in clk.h: 62 63 struct clk_ops { 64 int (*prepare)(struct clk_hw *hw); 65 void (*unprepare)(struct clk_hw *hw); 66 int (*enable)(struct clk_hw *hw); 67 void (*disable)(struct clk_hw *hw); 68 int (*is_enabled)(struct clk_hw *hw); 69 unsigned long (*recalc_rate)(struct clk_hw *hw, 70 unsigned long parent_rate); 71 long (*round_rate)(struct clk_hw *hw, unsigned long, 72 unsigned long *); 73 int (*set_parent)(struct clk_hw *hw, u8 index); 74 u8 (*get_parent)(struct clk_hw *hw); 75 int (*set_rate)(struct clk_hw *hw, unsigned long); 76 void (*init)(struct clk_hw *hw); 77 }; 78 79 Part 3 - hardware clk implementations 80 81The strength of the common struct clk comes from its .ops and .hw pointers 82which abstract the details of struct clk from the hardware-specific bits, and 83vice versa. To illustrate consider the simple gateable clk implementation in 84drivers/clk/clk-gate.c: 85 86struct clk_gate { 87 struct clk_hw hw; 88 void __iomem *reg; 89 u8 bit_idx; 90 ... 91}; 92 93struct clk_gate contains struct clk_hw hw as well as hardware-specific 94knowledge about which register and bit controls this clk's gating. 95Nothing about clock topology or accounting, such as enable_count or 96notifier_count, is needed here. That is all handled by the common 97framework code and struct clk. 98 99Let's walk through enabling this clk from driver code: 100 101 struct clk *clk; 102 clk = clk_get(NULL, "my_gateable_clk"); 103 104 clk_prepare(clk); 105 clk_enable(clk); 106 107The call graph for clk_enable is very simple: 108 109clk_enable(clk); 110 clk->ops->enable(clk->hw); 111 [resolves to...] 112 clk_gate_enable(hw); 113 [resolves struct clk gate with to_clk_gate(hw)] 114 clk_gate_set_bit(gate); 115 116And the definition of clk_gate_set_bit: 117 118static void clk_gate_set_bit(struct clk_gate *gate) 119{ 120 u32 reg; 121 122 reg = __raw_readl(gate->reg); 123 reg |= BIT(gate->bit_idx); 124 writel(reg, gate->reg); 125} 126 127Note that to_clk_gate is defined as: 128 129#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk) 130 131This pattern of abstraction is used for every clock hardware 132representation. 133 134 Part 4 - supporting your own clk hardware 135 136When implementing support for a new type of clock it only necessary to 137include the following header: 138 139#include <linux/clk-provider.h> 140 141include/linux/clk.h is included within that header and clk-private.h 142must never be included from the code which implements the operations for 143a clock. More on that below in Part 5. 144 145To construct a clk hardware structure for your platform you must define 146the following: 147 148struct clk_foo { 149 struct clk_hw hw; 150 ... hardware specific data goes here ... 151}; 152 153To take advantage of your data you'll need to support valid operations 154for your clk: 155 156struct clk_ops clk_foo_ops { 157 .enable = &clk_foo_enable; 158 .disable = &clk_foo_disable; 159}; 160 161Implement the above functions using container_of: 162 163#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw) 164 165int clk_foo_enable(struct clk_hw *hw) 166{ 167 struct clk_foo *foo; 168 169 foo = to_clk_foo(hw); 170 171 ... perform magic on foo ... 172 173 return 0; 174}; 175 176Below is a matrix detailing which clk_ops are mandatory based upon the 177hardware capabilities of that clock. A cell marked as "y" means 178mandatory, a cell marked as "n" implies that either including that 179callback is invalid or otherwise unnecessary. Empty cells are either 180optional or must be evaluated on a case-by-case basis. 181 182 clock hardware characteristics 183 ----------------------------------------------------------- 184 | gate | change rate | single parent | multiplexer | root | 185 |------|-------------|---------------|-------------|------| 186.prepare | | | | | | 187.unprepare | | | | | | 188 | | | | | | 189.enable | y | | | | | 190.disable | y | | | | | 191.is_enabled | y | | | | | 192 | | | | | | 193.recalc_rate | | y | | | | 194.round_rate | | y | | | | 195.set_rate | | y | | | | 196 | | | | | | 197.set_parent | | | n | y | n | 198.get_parent | | | n | y | n | 199 | | | | | | 200.init | | | | | | 201 ----------------------------------------------------------- 202 203Finally, register your clock at run-time with a hardware-specific 204registration function. This function simply populates struct clk_foo's 205data and then passes the common struct clk parameters to the framework 206with a call to: 207 208clk_register(...) 209 210See the basic clock types in drivers/clk/clk-*.c for examples. 211 212 Part 5 - static initialization of clock data 213 214For platforms with many clocks (often numbering into the hundreds) it 215may be desirable to statically initialize some clock data. This 216presents a problem since the definition of struct clk should be hidden 217from everyone except for the clock core in drivers/clk/clk.c. 218 219To get around this problem struct clk's definition is exposed in 220include/linux/clk-private.h along with some macros for more easily 221initializing instances of the basic clock types. These clocks must 222still be initialized with the common clock framework via a call to 223__clk_init. 224 225clk-private.h must NEVER be included by code which implements struct 226clk_ops callbacks, nor must it be included by any logic which pokes 227around inside of struct clk at run-time. To do so is a layering 228violation. 229 230To better enforce this policy, always follow this simple rule: any 231statically initialized clock data MUST be defined in a separate file 232from the logic that implements its ops. Basically separate the logic 233from the data and all is well. 234 235 Part 6 - Disabling clock gating of unused clocks 236 237Sometimes during development it can be useful to be able to bypass the 238default disabling of unused clocks. For example, if drivers aren't enabling 239clocks properly but rely on them being on from the bootloader, bypassing 240the disabling means that the driver will remain functional while the issues 241are sorted out. 242 243To bypass this disabling, include "clk_ignore_unused" in the bootargs to the 244kernel. 245