Free Electrons

Embedded Linux Experts

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
===========================
Hardware Spinlock Framework
===========================

Introduction
============

Hardware spinlock modules provide hardware assistance for synchronization
and mutual exclusion between heterogeneous processors and those not operating
under a single, shared operating system.

For example, OMAP4 has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP,
each of which is running a different Operating System (the master, A9,
is usually running Linux and the slave processors, the M3 and the DSP,
are running some flavor of RTOS).

A generic hwspinlock framework allows platform-independent drivers to use
the hwspinlock device in order to access data structures that are shared
between remote processors, that otherwise have no alternative mechanism
to accomplish synchronization and mutual exclusion operations.

This is necessary, for example, for Inter-processor communications:
on OMAP4, cpu-intensive multimedia tasks are offloaded by the host to the
remote M3 and/or C64x+ slave processors (by an IPC subsystem called Syslink).

To achieve fast message-based communications, a minimal kernel support
is needed to deliver messages arriving from a remote processor to the
appropriate user process.

This communication is based on simple data structures that is shared between
the remote processors, and access to it is synchronized using the hwspinlock
module (remote processor directly places new messages in this shared data
structure).

A common hwspinlock interface makes it possible to have generic, platform-
independent, drivers.

User API
========

::

  struct hwspinlock *hwspin_lock_request(void);

Dynamically assign an hwspinlock and return its address, or NULL
in case an unused hwspinlock isn't available. Users of this
API will usually want to communicate the lock's id to the remote core
before it can be used to achieve synchronization.

Should be called from a process context (might sleep).

::

  struct hwspinlock *hwspin_lock_request_specific(unsigned int id);

Assign a specific hwspinlock id and return its address, or NULL
if that hwspinlock is already in use. Usually board code will
be calling this function in order to reserve specific hwspinlock
ids for predefined purposes.

Should be called from a process context (might sleep).

::

  int of_hwspin_lock_get_id(struct device_node *np, int index);

Retrieve the global lock id for an OF phandle-based specific lock.
This function provides a means for DT users of a hwspinlock module
to get the global lock id of a specific hwspinlock, so that it can
be requested using the normal hwspin_lock_request_specific() API.

The function returns a lock id number on success, -EPROBE_DEFER if
the hwspinlock device is not yet registered with the core, or other
error values.

Should be called from a process context (might sleep).

::

  int hwspin_lock_free(struct hwspinlock *hwlock);

Free a previously-assigned hwspinlock; returns 0 on success, or an
appropriate error code on failure (e.g. -EINVAL if the hwspinlock
is already free).

Should be called from a process context (might sleep).

::

  int hwspin_lock_timeout(struct hwspinlock *hwlock, unsigned int timeout);

Lock a previously-assigned hwspinlock with a timeout limit (specified in
msecs). If the hwspinlock is already taken, the function will busy loop
waiting for it to be released, but give up when the timeout elapses.
Upon a successful return from this function, preemption is disabled so
the caller must not sleep, and is advised to release the hwspinlock as
soon as possible, in order to minimize remote cores polling on the
hardware interconnect.

Returns 0 when successful and an appropriate error code otherwise (most
notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
The function will never sleep.

::

  int hwspin_lock_timeout_irq(struct hwspinlock *hwlock, unsigned int timeout);

Lock a previously-assigned hwspinlock with a timeout limit (specified in
msecs). If the hwspinlock is already taken, the function will busy loop
waiting for it to be released, but give up when the timeout elapses.
Upon a successful return from this function, preemption and the local
interrupts are disabled, so the caller must not sleep, and is advised to
release the hwspinlock as soon as possible.

Returns 0 when successful and an appropriate error code otherwise (most
notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
The function will never sleep.

::

  int hwspin_lock_timeout_irqsave(struct hwspinlock *hwlock, unsigned int to,
				  unsigned long *flags);

Lock a previously-assigned hwspinlock with a timeout limit (specified in
msecs). If the hwspinlock is already taken, the function will busy loop
waiting for it to be released, but give up when the timeout elapses.
Upon a successful return from this function, preemption is disabled,
local interrupts are disabled and their previous state is saved at the
given flags placeholder. The caller must not sleep, and is advised to
release the hwspinlock as soon as possible.

Returns 0 when successful and an appropriate error code otherwise (most
notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).

The function will never sleep.

::

  int hwspin_trylock(struct hwspinlock *hwlock);


Attempt to lock a previously-assigned hwspinlock, but immediately fail if
it is already taken.

Upon a successful return from this function, preemption is disabled so
caller must not sleep, and is advised to release the hwspinlock as soon as
possible, in order to minimize remote cores polling on the hardware
interconnect.

Returns 0 on success and an appropriate error code otherwise (most
notably -EBUSY if the hwspinlock was already taken).
The function will never sleep.

::

  int hwspin_trylock_irq(struct hwspinlock *hwlock);


Attempt to lock a previously-assigned hwspinlock, but immediately fail if
it is already taken.

Upon a successful return from this function, preemption and the local
interrupts are disabled so caller must not sleep, and is advised to
release the hwspinlock as soon as possible.

Returns 0 on success and an appropriate error code otherwise (most
notably -EBUSY if the hwspinlock was already taken).

The function will never sleep.

::

  int hwspin_trylock_irqsave(struct hwspinlock *hwlock, unsigned long *flags);

Attempt to lock a previously-assigned hwspinlock, but immediately fail if
it is already taken.

Upon a successful return from this function, preemption is disabled,
the local interrupts are disabled and their previous state is saved
at the given flags placeholder. The caller must not sleep, and is advised
to release the hwspinlock as soon as possible.

Returns 0 on success and an appropriate error code otherwise (most
notably -EBUSY if the hwspinlock was already taken).
The function will never sleep.

::

  void hwspin_unlock(struct hwspinlock *hwlock);

Unlock a previously-locked hwspinlock. Always succeed, and can be called
from any context (the function never sleeps).

.. note::

  code should **never** unlock an hwspinlock which is already unlocked
  (there is no protection against this).

::

  void hwspin_unlock_irq(struct hwspinlock *hwlock);

Unlock a previously-locked hwspinlock and enable local interrupts.
The caller should **never** unlock an hwspinlock which is already unlocked.

Doing so is considered a bug (there is no protection against this).
Upon a successful return from this function, preemption and local
interrupts are enabled. This function will never sleep.

::

  void
  hwspin_unlock_irqrestore(struct hwspinlock *hwlock, unsigned long *flags);

Unlock a previously-locked hwspinlock.

The caller should **never** unlock an hwspinlock which is already unlocked.
Doing so is considered a bug (there is no protection against this).
Upon a successful return from this function, preemption is reenabled,
and the state of the local interrupts is restored to the state saved at
the given flags. This function will never sleep.

::

  int hwspin_lock_get_id(struct hwspinlock *hwlock);

Retrieve id number of a given hwspinlock. This is needed when an
hwspinlock is dynamically assigned: before it can be used to achieve
mutual exclusion with a remote cpu, the id number should be communicated
to the remote task with which we want to synchronize.

Returns the hwspinlock id number, or -EINVAL if hwlock is null.

Typical usage
=============

::

	#include <linux/hwspinlock.h>
	#include <linux/err.h>

	int hwspinlock_example1(void)
	{
		struct hwspinlock *hwlock;
		int ret;

		/* dynamically assign a hwspinlock */
		hwlock = hwspin_lock_request();
		if (!hwlock)
			...

		id = hwspin_lock_get_id(hwlock);
		/* probably need to communicate id to a remote processor now */

		/* take the lock, spin for 1 sec if it's already taken */
		ret = hwspin_lock_timeout(hwlock, 1000);
		if (ret)
			...

		/*
		* we took the lock, do our thing now, but do NOT sleep
		*/

		/* release the lock */
		hwspin_unlock(hwlock);

		/* free the lock */
		ret = hwspin_lock_free(hwlock);
		if (ret)
			...

		return ret;
	}

	int hwspinlock_example2(void)
	{
		struct hwspinlock *hwlock;
		int ret;

		/*
		* assign a specific hwspinlock id - this should be called early
		* by board init code.
		*/
		hwlock = hwspin_lock_request_specific(PREDEFINED_LOCK_ID);
		if (!hwlock)
			...

		/* try to take it, but don't spin on it */
		ret = hwspin_trylock(hwlock);
		if (!ret) {
			pr_info("lock is already taken\n");
			return -EBUSY;
		}

		/*
		* we took the lock, do our thing now, but do NOT sleep
		*/

		/* release the lock */
		hwspin_unlock(hwlock);

		/* free the lock */
		ret = hwspin_lock_free(hwlock);
		if (ret)
			...

		return ret;
	}


API for implementors
====================

::

  int hwspin_lock_register(struct hwspinlock_device *bank, struct device *dev,
		const struct hwspinlock_ops *ops, int base_id, int num_locks);

To be called from the underlying platform-specific implementation, in
order to register a new hwspinlock device (which is usually a bank of
numerous locks). Should be called from a process context (this function
might sleep).

Returns 0 on success, or appropriate error code on failure.

::

  int hwspin_lock_unregister(struct hwspinlock_device *bank);

To be called from the underlying vendor-specific implementation, in order
to unregister an hwspinlock device (which is usually a bank of numerous
locks).

Should be called from a process context (this function might sleep).

Returns the address of hwspinlock on success, or NULL on error (e.g.
if the hwspinlock is still in use).

Important structs
=================

struct hwspinlock_device is a device which usually contains a bank
of hardware locks. It is registered by the underlying hwspinlock
implementation using the hwspin_lock_register() API.

::

	/**
	* struct hwspinlock_device - a device which usually spans numerous hwspinlocks
	* @dev: underlying device, will be used to invoke runtime PM api
	* @ops: platform-specific hwspinlock handlers
	* @base_id: id index of the first lock in this device
	* @num_locks: number of locks in this device
	* @lock: dynamically allocated array of 'struct hwspinlock'
	*/
	struct hwspinlock_device {
		struct device *dev;
		const struct hwspinlock_ops *ops;
		int base_id;
		int num_locks;
		struct hwspinlock lock[0];
	};

struct hwspinlock_device contains an array of hwspinlock structs, each
of which represents a single hardware lock::

	/**
	* struct hwspinlock - this struct represents a single hwspinlock instance
	* @bank: the hwspinlock_device structure which owns this lock
	* @lock: initialized and used by hwspinlock core
	* @priv: private data, owned by the underlying platform-specific hwspinlock drv
	*/
	struct hwspinlock {
		struct hwspinlock_device *bank;
		spinlock_t lock;
		void *priv;
	};

When registering a bank of locks, the hwspinlock driver only needs to
set the priv members of the locks. The rest of the members are set and
initialized by the hwspinlock core itself.

Implementation callbacks
========================

There are three possible callbacks defined in 'struct hwspinlock_ops'::

	struct hwspinlock_ops {
		int (*trylock)(struct hwspinlock *lock);
		void (*unlock)(struct hwspinlock *lock);
		void (*relax)(struct hwspinlock *lock);
	};

The first two callbacks are mandatory:

The ->trylock() callback should make a single attempt to take the lock, and
return 0 on failure and 1 on success. This callback may **not** sleep.

The ->unlock() callback releases the lock. It always succeed, and it, too,
may **not** sleep.

The ->relax() callback is optional. It is called by hwspinlock core while
spinning on a lock, and can be used by the underlying implementation to force
a delay between two successive invocations of ->trylock(). It may **not** sleep.