1==================================
2PMBus core driver and internal API
3==================================
4
5Introduction
6============
7
8[from pmbus.org] The Power Management Bus (PMBus) is an open standard
9power-management protocol with a fully defined command language that facilitates
10communication with power converters and other devices in a power system. The
11protocol is implemented over the industry-standard SMBus serial interface and
12enables programming, control, and real-time monitoring of compliant power
13conversion products. This flexible and highly versatile standard allows for
14communication between devices based on both analog and digital technologies, and
15provides true interoperability which will reduce design complexity and shorten
16time to market for power system designers. Pioneered by leading power supply and
17semiconductor companies, this open power system standard is maintained and
18promoted by the PMBus Implementers Forum (PMBus-IF), comprising 30+ adopters
19with the objective to provide support to, and facilitate adoption among, users.
20
21Unfortunately, while PMBus commands are standardized, there are no mandatory
22commands, and manufacturers can add as many non-standard commands as they like.
23Also, different PMBUs devices act differently if non-supported commands are
24executed. Some devices return an error, some devices return 0xff or 0xffff and
25set a status error flag, and some devices may simply hang up.
26
27Despite all those difficulties, a generic PMBus device driver is still useful
28and supported since kernel version 2.6.39. However, it was necessary to support
29device specific extensions in addition to the core PMBus driver, since it is
30simply unknown what new device specific functionality PMBus device developers
31come up with next.
32
33To make device specific extensions as scalable as possible, and to avoid having
34to modify the core PMBus driver repeatedly for new devices, the PMBus driver was
35split into core, generic, and device specific code. The core code (in
36pmbus_core.c) provides generic functionality. The generic code (in pmbus.c)
37provides support for generic PMBus devices. Device specific code is responsible
38for device specific initialization and, if needed, maps device specific
39functionality into generic functionality. This is to some degree comparable
40to PCI code, where generic code is augmented as needed with quirks for all kinds
41of devices.
42
43PMBus device capabilities auto-detection
44========================================
45
46For generic PMBus devices, code in pmbus.c attempts to auto-detect all supported
47PMBus commands. Auto-detection is somewhat limited, since there are simply too
48many variables to consider. For example, it is almost impossible to autodetect
49which PMBus commands are paged and which commands are replicated across all
50pages (see the PMBus specification for details on multi-page PMBus devices).
51
52For this reason, it often makes sense to provide a device specific driver if not
53all commands can be auto-detected. The data structures in this driver can be
54used to inform the core driver about functionality supported by individual
55chips.
56
57Some commands are always auto-detected. This applies to all limit commands
58(lcrit, min, max, and crit attributes) as well as associated alarm attributes.
59Limits and alarm attributes are auto-detected because there are simply too many
60possible combinations to provide a manual configuration interface.
61
62PMBus internal API
63==================
64
65The API between core and device specific PMBus code is defined in
66drivers/hwmon/pmbus/pmbus.h. In addition to the internal API, pmbus.h defines
67standard PMBus commands and virtual PMBus commands.
68
69Standard PMBus commands
70-----------------------
71
72Standard PMBus commands (commands values 0x00 to 0xff) are defined in the PMBUs
73specification.
74
75Virtual PMBus commands
76----------------------
77
78Virtual PMBus commands are provided to enable support for non-standard
79functionality which has been implemented by several chip vendors and is thus
80desirable to support.
81
82Virtual PMBus commands start with command value 0x100 and can thus easily be
83distinguished from standard PMBus commands (which can not have values larger
84than 0xff). Support for virtual PMBus commands is device specific and thus has
85to be implemented in device specific code.
86
87Virtual commands are named PMBUS_VIRT_xxx and start with PMBUS_VIRT_BASE. All
88virtual commands are word sized.
89
90There are currently two types of virtual commands.
91
92- READ commands are read-only; writes are either ignored or return an error.
93- RESET commands are read/write. Reading reset registers returns zero
94  (used for detection), writing any value causes the associated history to be
95  reset.
96
97Virtual commands have to be handled in device specific driver code. Chip driver
98code returns non-negative values if a virtual command is supported, or a
99negative error code if not. The chip driver may return -ENODATA or any other
100Linux error code in this case, though an error code other than -ENODATA is
101handled more efficiently and thus preferred. Either case, the calling PMBus
102core code will abort if the chip driver returns an error code when reading
103or writing virtual registers (in other words, the PMBus core code will never
104send a virtual command to a chip).
105
106PMBus driver information
107------------------------
108
109PMBus driver information, defined in struct pmbus_driver_info, is the main means
110for device specific drivers to pass information to the core PMBus driver.
111Specifically, it provides the following information.
112
113- For devices supporting its data in Direct Data Format, it provides coefficients
114  for converting register values into normalized data. This data is usually
115  provided by chip manufacturers in device datasheets.
116- Supported chip functionality can be provided to the core driver. This may be
117  necessary for chips which react badly if non-supported commands are executed,
118  and/or to speed up device detection and initialization.
119- Several function entry points are provided to support overriding and/or
120  augmenting generic command execution. This functionality can be used to map
121  non-standard PMBus commands to standard commands, or to augment standard
122  command return values with device specific information.
123
124PEC Support
125===========
126
127Many PMBus devices support SMBus PEC (Packet Error Checking). If supported
128by both the I2C adapter and by the PMBus chip, it is by default enabled.
129If PEC is supported, the PMBus core driver adds an attribute named 'pec' to
130the I2C device. This attribute can be used to control PEC support in the
131communication with the PMBus chip.
132
133API functions
134=============
135
136Functions provided by chip driver
137---------------------------------
138
139All functions return the command return value (read) or zero (write) if
140successful. A return value of -ENODATA indicates that there is no manufacturer
141specific command, but that a standard PMBus command may exist. Any other
142negative return value indicates that the commands does not exist for this
143chip, and that no attempt should be made to read or write the standard
144command.
145
146As mentioned above, an exception to this rule applies to virtual commands,
147which *must* be handled in driver specific code. See "Virtual PMBus Commands"
148above for more details.
149
150Command execution in the core PMBus driver code is as follows::
151
152	if (chip_access_function) {
153		status = chip_access_function();
154		if (status != -ENODATA)
155			return status;
156	}
157	if (command >= PMBUS_VIRT_BASE)	/* For word commands/registers only */
158		return -EINVAL;
159	return generic_access();
160
161Chip drivers may provide pointers to the following functions in struct
162pmbus_driver_info. All functions are optional.
163
164::
165
166  int (*read_byte_data)(struct i2c_client *client, int page, int reg);
167
168Read byte from page <page>, register <reg>.
169<page> may be -1, which means "current page".
170
171
172::
173
174  int (*read_word_data)(struct i2c_client *client, int page, int phase,
175                        int reg);
176
177Read word from page <page>, phase <phase>, register <reg>. If the chip does not
178support multiple phases, the phase parameter can be ignored. If the chip
179supports multiple phases, a phase value of 0xff indicates all phases.
180
181::
182
183  int (*write_word_data)(struct i2c_client *client, int page, int reg,
184			 u16 word);
185
186Write word to page <page>, register <reg>.
187
188::
189
190  int (*write_byte)(struct i2c_client *client, int page, u8 value);
191
192Write byte to page <page>, register <reg>.
193<page> may be -1, which means "current page".
194
195::
196
197  int (*identify)(struct i2c_client *client, struct pmbus_driver_info *info);
198
199Determine supported PMBus functionality. This function is only necessary
200if a chip driver supports multiple chips, and the chip functionality is not
201pre-determined. It is currently only used by the generic pmbus driver
202(pmbus.c).
203
204Functions exported by core driver
205---------------------------------
206
207Chip drivers are expected to use the following functions to read or write
208PMBus registers. Chip drivers may also use direct I2C commands. If direct I2C
209commands are used, the chip driver code must not directly modify the current
210page, since the selected page is cached in the core driver and the core driver
211will assume that it is selected. Using pmbus_set_page() to select a new page
212is mandatory.
213
214::
215
216  int pmbus_set_page(struct i2c_client *client, u8 page, u8 phase);
217
218Set PMBus page register to <page> and <phase> for subsequent commands.
219If the chip does not support multiple phases, the phase parameter is
220ignored. Otherwise, a phase value of 0xff selects all phases.
221
222::
223
224  int pmbus_read_word_data(struct i2c_client *client, u8 page, u8 phase,
225                           u8 reg);
226
227Read word data from <page>, <phase>, <reg>. Similar to
228i2c_smbus_read_word_data(), but selects page and phase first. If the chip does
229not support multiple phases, the phase parameter is ignored. Otherwise, a phase
230value of 0xff selects all phases.
231
232::
233
234  int pmbus_write_word_data(struct i2c_client *client, u8 page, u8 reg,
235			    u16 word);
236
237Write word data to <page>, <reg>. Similar to i2c_smbus_write_word_data(), but
238selects page first.
239
240::
241
242  int pmbus_read_byte_data(struct i2c_client *client, int page, u8 reg);
243
244Read byte data from <page>, <reg>. Similar to i2c_smbus_read_byte_data(), but
245selects page first. <page> may be -1, which means "current page".
246
247::
248
249  int pmbus_write_byte(struct i2c_client *client, int page, u8 value);
250
251Write byte data to <page>, <reg>. Similar to i2c_smbus_write_byte(), but
252selects page first. <page> may be -1, which means "current page".
253
254::
255
256  void pmbus_clear_faults(struct i2c_client *client);
257
258Execute PMBus "Clear Fault" command on all chip pages.
259This function calls the device specific write_byte function if defined.
260Therefore, it must _not_ be called from that function.
261
262::
263
264  bool pmbus_check_byte_register(struct i2c_client *client, int page, int reg);
265
266Check if byte register exists. Return true if the register exists, false
267otherwise.
268This function calls the device specific write_byte function if defined to
269obtain the chip status. Therefore, it must _not_ be called from that function.
270
271::
272
273  bool pmbus_check_word_register(struct i2c_client *client, int page, int reg);
274
275Check if word register exists. Return true if the register exists, false
276otherwise.
277This function calls the device specific write_byte function if defined to
278obtain the chip status. Therefore, it must _not_ be called from that function.
279
280::
281
282  int pmbus_do_probe(struct i2c_client *client, struct pmbus_driver_info *info);
283
284Execute probe function. Similar to standard probe function for other drivers,
285with the pointer to struct pmbus_driver_info as additional argument. Calls
286identify function if supported. Must only be called from device probe
287function.
288
289::
290
291  const struct pmbus_driver_info
292	*pmbus_get_driver_info(struct i2c_client *client);
293
294Return pointer to struct pmbus_driver_info as passed to pmbus_do_probe().
295
296
297PMBus driver platform data
298==========================
299
300PMBus platform data is defined in include/linux/pmbus.h. Platform data
301currently provides a flags field with four bits used::
302
303	#define PMBUS_SKIP_STATUS_CHECK			BIT(0)
304
305	#define PMBUS_WRITE_PROTECTED			BIT(1)
306
307	#define PMBUS_NO_CAPABILITY			BIT(2)
308
309	#define PMBUS_READ_STATUS_AFTER_FAILED_CHECK	BIT(3)
310
311	struct pmbus_platform_data {
312		u32 flags;              /* Device specific flags */
313
314		/* regulator support */
315		int num_regulators;
316		struct regulator_init_data *reg_init_data;
317	};
318
319
320Flags
321-----
322
323PMBUS_SKIP_STATUS_CHECK
324
325During register detection, skip checking the status register for
326communication or command errors.
327
328Some PMBus chips respond with valid data when trying to read an unsupported
329register. For such chips, checking the status register is mandatory when
330trying to determine if a chip register exists or not.
331Other PMBus chips don't support the STATUS_CML register, or report
332communication errors for no explicable reason. For such chips, checking the
333status register must be disabled.
334
335Some i2c controllers do not support single-byte commands (write commands with
336no data, i2c_smbus_write_byte()). With such controllers, clearing the status
337register is impossible, and the PMBUS_SKIP_STATUS_CHECK flag must be set.
338
339PMBUS_WRITE_PROTECTED
340
341Set if the chip is write protected and write protection is not determined
342by the standard WRITE_PROTECT command.
343
344PMBUS_NO_CAPABILITY
345
346Some PMBus chips don't respond with valid data when reading the CAPABILITY
347register. For such chips, this flag should be set so that the PMBus core
348driver doesn't use CAPABILITY to determine it's behavior.
349
350PMBUS_READ_STATUS_AFTER_FAILED_CHECK
351
352Read the STATUS register after each failed register check.
353
354Some PMBus chips end up in an undefined state when trying to read an
355unsupported register. For such chips, it is necessary to reset the
356chip pmbus controller to a known state after a failed register check.
357This can be done by reading a known register. By setting this flag the
358driver will try to read the STATUS register after each failed
359register check. This read may fail, but it will put the chip into a
360known state.
361