1======================== 2The Common Clk Framework 3======================== 4 5:Author: Mike Turquette <mturquette@ti.com> 6 7This document endeavours to explain the common clk framework details, 8and how to port a platform over to this framework. It is not yet a 9detailed explanation of the clock api in include/linux/clk.h, but 10perhaps someday it will include that information. 11 12Introduction and interface split 13================================ 14 15The common clk framework is an interface to control the clock nodes 16available on various devices today. This may come in the form of clock 17gating, rate adjustment, muxing or other operations. This framework is 18enabled with the CONFIG_COMMON_CLK option. 19 20The interface itself is divided into two halves, each shielded from the 21details of its counterpart. First is the common definition of struct 22clk which unifies the framework-level accounting and infrastructure that 23has traditionally been duplicated across a variety of platforms. Second 24is a common implementation of the clk.h api, defined in 25drivers/clk/clk.c. Finally there is struct clk_ops, whose operations 26are invoked by the clk api implementation. 27 28The second half of the interface is comprised of the hardware-specific 29callbacks registered with struct clk_ops and the corresponding 30hardware-specific structures needed to model a particular clock. For 31the remainder of this document any reference to a callback in struct 32clk_ops, such as .enable or .set_rate, implies the hardware-specific 33implementation of that code. Likewise, references to struct clk_foo 34serve as a convenient shorthand for the implementation of the 35hardware-specific bits for the hypothetical "foo" hardware. 36 37Tying the two halves of this interface together is struct clk_hw, which 38is defined in struct clk_foo and pointed to within struct clk_core. This 39allows for easy navigation between the two discrete halves of the common 40clock interface. 41 42Common data structures and api 43============================== 44 45Below is the common struct clk_core definition from 46drivers/clk/clk.c, modified for brevity:: 47 48 struct clk_core { 49 const char *name; 50 const struct clk_ops *ops; 51 struct clk_hw *hw; 52 struct module *owner; 53 struct clk_core *parent; 54 const char **parent_names; 55 struct clk_core **parents; 56 u8 num_parents; 57 u8 new_parent_index; 58 ... 59 }; 60 61The members above make up the core of the clk tree topology. The clk 62api itself defines several driver-facing functions which operate on 63struct clk. That api is documented in include/linux/clk.h. 64 65Platforms and devices utilizing the common struct clk_core use the struct 66clk_ops pointer in struct clk_core to perform the hardware-specific parts of 67the operations defined in clk-provider.h:: 68 69 struct clk_ops { 70 int (*prepare)(struct clk_hw *hw); 71 void (*unprepare)(struct clk_hw *hw); 72 int (*is_prepared)(struct clk_hw *hw); 73 void (*unprepare_unused)(struct clk_hw *hw); 74 int (*enable)(struct clk_hw *hw); 75 void (*disable)(struct clk_hw *hw); 76 int (*is_enabled)(struct clk_hw *hw); 77 void (*disable_unused)(struct clk_hw *hw); 78 unsigned long (*recalc_rate)(struct clk_hw *hw, 79 unsigned long parent_rate); 80 long (*round_rate)(struct clk_hw *hw, 81 unsigned long rate, 82 unsigned long *parent_rate); 83 int (*determine_rate)(struct clk_hw *hw, 84 struct clk_rate_request *req); 85 int (*set_parent)(struct clk_hw *hw, u8 index); 86 u8 (*get_parent)(struct clk_hw *hw); 87 int (*set_rate)(struct clk_hw *hw, 88 unsigned long rate, 89 unsigned long parent_rate); 90 int (*set_rate_and_parent)(struct clk_hw *hw, 91 unsigned long rate, 92 unsigned long parent_rate, 93 u8 index); 94 unsigned long (*recalc_accuracy)(struct clk_hw *hw, 95 unsigned long parent_accuracy); 96 int (*get_phase)(struct clk_hw *hw); 97 int (*set_phase)(struct clk_hw *hw, int degrees); 98 void (*init)(struct clk_hw *hw); 99 void (*debug_init)(struct clk_hw *hw, 100 struct dentry *dentry); 101 }; 102 103Hardware clk implementations 104============================ 105 106The strength of the common struct clk_core comes from its .ops and .hw pointers 107which abstract the details of struct clk from the hardware-specific bits, and 108vice versa. To illustrate consider the simple gateable clk implementation in 109drivers/clk/clk-gate.c:: 110 111 struct clk_gate { 112 struct clk_hw hw; 113 void __iomem *reg; 114 u8 bit_idx; 115 ... 116 }; 117 118struct clk_gate contains struct clk_hw hw as well as hardware-specific 119knowledge about which register and bit controls this clk's gating. 120Nothing about clock topology or accounting, such as enable_count or 121notifier_count, is needed here. That is all handled by the common 122framework code and struct clk_core. 123 124Let's walk through enabling this clk from driver code:: 125 126 struct clk *clk; 127 clk = clk_get(NULL, "my_gateable_clk"); 128 129 clk_prepare(clk); 130 clk_enable(clk); 131 132The call graph for clk_enable is very simple:: 133 134 clk_enable(clk); 135 clk->ops->enable(clk->hw); 136 [resolves to...] 137 clk_gate_enable(hw); 138 [resolves struct clk gate with to_clk_gate(hw)] 139 clk_gate_set_bit(gate); 140 141And the definition of clk_gate_set_bit:: 142 143 static void clk_gate_set_bit(struct clk_gate *gate) 144 { 145 u32 reg; 146 147 reg = __raw_readl(gate->reg); 148 reg |= BIT(gate->bit_idx); 149 writel(reg, gate->reg); 150 } 151 152Note that to_clk_gate is defined as:: 153 154 #define to_clk_gate(_hw) container_of(_hw, struct clk_gate, hw) 155 156This pattern of abstraction is used for every clock hardware 157representation. 158 159Supporting your own clk hardware 160================================ 161 162When implementing support for a new type of clock it is only necessary to 163include the following header:: 164 165 #include <linux/clk-provider.h> 166 167To construct a clk hardware structure for your platform you must define 168the following:: 169 170 struct clk_foo { 171 struct clk_hw hw; 172 ... hardware specific data goes here ... 173 }; 174 175To take advantage of your data you'll need to support valid operations 176for your clk:: 177 178 struct clk_ops clk_foo_ops = { 179 .enable = &clk_foo_enable, 180 .disable = &clk_foo_disable, 181 }; 182 183Implement the above functions using container_of:: 184 185 #define to_clk_foo(_hw) container_of(_hw, struct clk_foo, hw) 186 187 int clk_foo_enable(struct clk_hw *hw) 188 { 189 struct clk_foo *foo; 190 191 foo = to_clk_foo(hw); 192 193 ... perform magic on foo ... 194 195 return 0; 196 }; 197 198Below is a matrix detailing which clk_ops are mandatory based upon the 199hardware capabilities of that clock. A cell marked as "y" means 200mandatory, a cell marked as "n" implies that either including that 201callback is invalid or otherwise unnecessary. Empty cells are either 202optional or must be evaluated on a case-by-case basis. 203 204.. table:: clock hardware characteristics 205 206 +----------------+------+-------------+---------------+-------------+------+ 207 | | gate | change rate | single parent | multiplexer | root | 208 +================+======+=============+===============+=============+======+ 209 |.prepare | | | | | | 210 +----------------+------+-------------+---------------+-------------+------+ 211 |.unprepare | | | | | | 212 +----------------+------+-------------+---------------+-------------+------+ 213 +----------------+------+-------------+---------------+-------------+------+ 214 |.enable | y | | | | | 215 +----------------+------+-------------+---------------+-------------+------+ 216 |.disable | y | | | | | 217 +----------------+------+-------------+---------------+-------------+------+ 218 |.is_enabled | y | | | | | 219 +----------------+------+-------------+---------------+-------------+------+ 220 +----------------+------+-------------+---------------+-------------+------+ 221 |.recalc_rate | | y | | | | 222 +----------------+------+-------------+---------------+-------------+------+ 223 |.round_rate | | y [1]_ | | | | 224 +----------------+------+-------------+---------------+-------------+------+ 225 |.determine_rate | | y [1]_ | | | | 226 +----------------+------+-------------+---------------+-------------+------+ 227 |.set_rate | | y | | | | 228 +----------------+------+-------------+---------------+-------------+------+ 229 +----------------+------+-------------+---------------+-------------+------+ 230 |.set_parent | | | n | y | n | 231 +----------------+------+-------------+---------------+-------------+------+ 232 |.get_parent | | | n | y | n | 233 +----------------+------+-------------+---------------+-------------+------+ 234 +----------------+------+-------------+---------------+-------------+------+ 235 |.recalc_accuracy| | | | | | 236 +----------------+------+-------------+---------------+-------------+------+ 237 +----------------+------+-------------+---------------+-------------+------+ 238 |.init | | | | | | 239 +----------------+------+-------------+---------------+-------------+------+ 240 241.. [1] either one of round_rate or determine_rate is required. 242 243Finally, register your clock at run-time with a hardware-specific 244registration function. This function simply populates struct clk_foo's 245data and then passes the common struct clk parameters to the framework 246with a call to:: 247 248 clk_register(...) 249 250See the basic clock types in ``drivers/clk/clk-*.c`` for examples. 251 252Disabling clock gating of unused clocks 253======================================= 254 255Sometimes during development it can be useful to be able to bypass the 256default disabling of unused clocks. For example, if drivers aren't enabling 257clocks properly but rely on them being on from the bootloader, bypassing 258the disabling means that the driver will remain functional while the issues 259are sorted out. 260 261To bypass this disabling, include "clk_ignore_unused" in the bootargs to the 262kernel. 263 264Locking 265======= 266 267The common clock framework uses two global locks, the prepare lock and the 268enable lock. 269 270The enable lock is a spinlock and is held across calls to the .enable, 271.disable operations. Those operations are thus not allowed to sleep, 272and calls to the clk_enable(), clk_disable() API functions are allowed in 273atomic context. 274 275For clk_is_enabled() API, it is also designed to be allowed to be used in 276atomic context. However, it doesn't really make any sense to hold the enable 277lock in core, unless you want to do something else with the information of 278the enable state with that lock held. Otherwise, seeing if a clk is enabled is 279a one-shot read of the enabled state, which could just as easily change after 280the function returns because the lock is released. Thus the user of this API 281needs to handle synchronizing the read of the state with whatever they're 282using it for to make sure that the enable state doesn't change during that 283time. 284 285The prepare lock is a mutex and is held across calls to all other operations. 286All those operations are allowed to sleep, and calls to the corresponding API 287functions are not allowed in atomic context. 288 289This effectively divides operations in two groups from a locking perspective. 290 291Drivers don't need to manually protect resources shared between the operations 292of one group, regardless of whether those resources are shared by multiple 293clocks or not. However, access to resources that are shared between operations 294of the two groups needs to be protected by the drivers. An example of such a 295resource would be a register that controls both the clock rate and the clock 296enable/disable state. 297 298The clock framework is reentrant, in that a driver is allowed to call clock 299framework functions from within its implementation of clock operations. This 300can for instance cause a .set_rate operation of one clock being called from 301within the .set_rate operation of another clock. This case must be considered 302in the driver implementations, but the code flow is usually controlled by the 303driver in that case. 304 305Note that locking must also be considered when code outside of the common 306clock framework needs to access resources used by the clock operations. This 307is considered out of scope of this document. 308