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 for easy 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, 72 unsigned long rate, 73 unsigned long *parent_rate); 74 int (*determine_rate)(struct clk_hw *hw, 75 struct clk_rate_request *req); 76 int (*set_parent)(struct clk_hw *hw, u8 index); 77 u8 (*get_parent)(struct clk_hw *hw); 78 int (*set_rate)(struct clk_hw *hw, 79 unsigned long rate, 80 unsigned long parent_rate); 81 int (*set_rate_and_parent)(struct clk_hw *hw, 82 unsigned long rate, 83 unsigned long parent_rate, 84 u8 index); 85 unsigned long (*recalc_accuracy)(struct clk_hw *hw, 86 unsigned long parent_accuracy); 87 void (*init)(struct clk_hw *hw); 88 int (*debug_init)(struct clk_hw *hw, 89 struct dentry *dentry); 90 }; 91 92 Part 3 - hardware clk implementations 93 94The strength of the common struct clk comes from its .ops and .hw pointers 95which abstract the details of struct clk from the hardware-specific bits, and 96vice versa. To illustrate consider the simple gateable clk implementation in 97drivers/clk/clk-gate.c: 98 99struct clk_gate { 100 struct clk_hw hw; 101 void __iomem *reg; 102 u8 bit_idx; 103 ... 104}; 105 106struct clk_gate contains struct clk_hw hw as well as hardware-specific 107knowledge about which register and bit controls this clk's gating. 108Nothing about clock topology or accounting, such as enable_count or 109notifier_count, is needed here. That is all handled by the common 110framework code and struct clk. 111 112Let's walk through enabling this clk from driver code: 113 114 struct clk *clk; 115 clk = clk_get(NULL, "my_gateable_clk"); 116 117 clk_prepare(clk); 118 clk_enable(clk); 119 120The call graph for clk_enable is very simple: 121 122clk_enable(clk); 123 clk->ops->enable(clk->hw); 124 [resolves to...] 125 clk_gate_enable(hw); 126 [resolves struct clk gate with to_clk_gate(hw)] 127 clk_gate_set_bit(gate); 128 129And the definition of clk_gate_set_bit: 130 131static void clk_gate_set_bit(struct clk_gate *gate) 132{ 133 u32 reg; 134 135 reg = __raw_readl(gate->reg); 136 reg |= BIT(gate->bit_idx); 137 writel(reg, gate->reg); 138} 139 140Note that to_clk_gate is defined as: 141 142#define to_clk_gate(_hw) container_of(_hw, struct clk_gate, clk) 143 144This pattern of abstraction is used for every clock hardware 145representation. 146 147 Part 4 - supporting your own clk hardware 148 149When implementing support for a new type of clock it only necessary to 150include the following header: 151 152#include <linux/clk-provider.h> 153 154include/linux/clk.h is included within that header and clk-private.h 155must never be included from the code which implements the operations for 156a clock. More on that below in Part 5. 157 158To construct a clk hardware structure for your platform you must define 159the following: 160 161struct clk_foo { 162 struct clk_hw hw; 163 ... hardware specific data goes here ... 164}; 165 166To take advantage of your data you'll need to support valid operations 167for your clk: 168 169struct clk_ops clk_foo_ops { 170 .enable = &clk_foo_enable; 171 .disable = &clk_foo_disable; 172}; 173 174Implement the above functions using container_of: 175 176#define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw) 177 178int clk_foo_enable(struct clk_hw *hw) 179{ 180 struct clk_foo *foo; 181 182 foo = to_clk_foo(hw); 183 184 ... perform magic on foo ... 185 186 return 0; 187}; 188 189Below is a matrix detailing which clk_ops are mandatory based upon the 190hardware capabilities of that clock. A cell marked as "y" means 191mandatory, a cell marked as "n" implies that either including that 192callback is invalid or otherwise unnecessary. Empty cells are either 193optional or must be evaluated on a case-by-case basis. 194 195 clock hardware characteristics 196 ----------------------------------------------------------- 197 | gate | change rate | single parent | multiplexer | root | 198 |------|-------------|---------------|-------------|------| 199.prepare | | | | | | 200.unprepare | | | | | | 201 | | | | | | 202.enable | y | | | | | 203.disable | y | | | | | 204.is_enabled | y | | | | | 205 | | | | | | 206.recalc_rate | | y | | | | 207.round_rate | | y [1] | | | | 208.determine_rate | | y [1] | | | | 209.set_rate | | y | | | | 210 | | | | | | 211.set_parent | | | n | y | n | 212.get_parent | | | n | y | n | 213 | | | | | | 214.recalc_accuracy| | | | | | 215 | | | | | | 216.init | | | | | | 217 ----------------------------------------------------------- 218[1] either one of round_rate or determine_rate is required. 219 220Finally, register your clock at run-time with a hardware-specific 221registration function. This function simply populates struct clk_foo's 222data and then passes the common struct clk parameters to the framework 223with a call to: 224 225clk_register(...) 226 227See the basic clock types in drivers/clk/clk-*.c for examples. 228 229 Part 5 - Disabling clock gating of unused clocks 230 231Sometimes during development it can be useful to be able to bypass the 232default disabling of unused clocks. For example, if drivers aren't enabling 233clocks properly but rely on them being on from the bootloader, bypassing 234the disabling means that the driver will remain functional while the issues 235are sorted out. 236 237To bypass this disabling, include "clk_ignore_unused" in the bootargs to the 238kernel. 239 240 Part 6 - Locking 241 242The common clock framework uses two global locks, the prepare lock and the 243enable lock. 244 245The enable lock is a spinlock and is held across calls to the .enable, 246.disable and .is_enabled operations. Those operations are thus not allowed to 247sleep, and calls to the clk_enable(), clk_disable() and clk_is_enabled() API 248functions are allowed in atomic context. 249 250The prepare lock is a mutex and is held across calls to all other operations. 251All those operations are allowed to sleep, and calls to the corresponding API 252functions are not allowed in atomic context. 253 254This effectively divides operations in two groups from a locking perspective. 255 256Drivers don't need to manually protect resources shared between the operations 257of one group, regardless of whether those resources are shared by multiple 258clocks or not. However, access to resources that are shared between operations 259of the two groups needs to be protected by the drivers. An example of such a 260resource would be a register that controls both the clock rate and the clock 261enable/disable state. 262 263The clock framework is reentrant, in that a driver is allowed to call clock 264framework functions from within its implementation of clock operations. This 265can for instance cause a .set_rate operation of one clock being called from 266within the .set_rate operation of another clock. This case must be considered 267in the driver implementations, but the code flow is usually controlled by the 268driver in that case. 269 270Note that locking must also be considered when code outside of the common 271clock framework needs to access resources used by the clock operations. This 272is considered out of scope of this document. 273