hwmon: (pmbus) Provide more documentation

Provide more documentation describing PMBus driver functionality and the API
between the PMBus core driver and PMBus chip drivers.
Signed-off-by: NGuenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: NRobert Coulson <robert.coulson@ericsson.com>

Documentation/hwmon/pmbus-core

+PMBus core driver and internal API
+==================================
+
+Introduction
+============
+
+[from pmbus.org] The Power Management Bus (PMBus) is an open standard
+power-management protocol with a fully defined command language that facilitates
+communication with power converters and other devices in a power system. The
+protocol is implemented over the industry-standard SMBus serial interface and
+enables programming, control, and real-time monitoring of compliant power
+conversion products. This flexible and highly versatile standard allows for
+communication between devices based on both analog and digital technologies, and
+provides true interoperability which will reduce design complexity and shorten
+time to market for power system designers. Pioneered by leading power supply and
+semiconductor companies, this open power system standard is maintained and
+promoted by the PMBus Implementers Forum (PMBus-IF), comprising 30+ adopters
+with the objective to provide support to, and facilitate adoption among, users.
+
+Unfortunately, while PMBus commands are standardized, there are no mandatory
+commands, and manufacturers can add as many non-standard commands as they like.
+Also, different PMBUs devices act differently if non-supported commands are
+executed. Some devices return an error, some devices return 0xff or 0xffff and
+set a status error flag, and some devices may simply hang up.
+
+Despite all those difficulties, a generic PMBus device driver is still useful
+and supported since kernel version 2.6.39. However, it was necessary to support
+device specific extensions in addition to the core PMBus driver, since it is
+simply unknown what new device specific functionality PMBus device developers
+come up with next.
+
+To make device specific extensions as scalable as possible, and to avoid having
+to modify the core PMBus driver repeatedly for new devices, the PMBus driver was
+split into core, generic, and device specific code. The core code (in
+pmbus_core.c) provides generic functionality. The generic code (in pmbus.c)
+provides support for generic PMBus devices. Device specific code is responsible
+for device specific initialization and, if needed, maps device specific
+functionality into generic functionality. This is to some degree comparable
+to PCI code, where generic code is augmented as needed with quirks for all kinds
+of devices.
+
+PMBus device capabilities auto-detection
+========================================
+
+For generic PMBus devices, code in pmbus.c attempts to auto-detect all supported
+PMBus commands. Auto-detection is somewhat limited, since there are simply too
+many variables to consider. For example, it is almost impossible to autodetect
+which PMBus commands are paged and which commands are replicated across all
+pages (see the PMBus specification for details on multi-page PMBus devices).
+
+For this reason, it often makes sense to provide a device specific driver if not
+all commands can be auto-detected. The data structures in this driver can be
+used to inform the core driver about functionality supported by individual
+chips.
+
+Some commands are always auto-detected. This applies to all limit commands
+(lcrit, min, max, and crit attributes) as well as associated alarm attributes.
+Limits and alarm attributes are auto-detected because there are simply too many
+possible combinations to provide a manual configuration interface.
+
+PMBus internal API
+==================
+
+The API between core and device specific PMBus code is defined in
+drivers/hwmon/pmbus/pmbus.h. In addition to the internal API, pmbus.h defines
+standard PMBus commands and virtual PMBus commands.
+
+Standard PMBus commands
+-----------------------
+
+Standard PMBus commands (commands values 0x00 to 0xff) are defined in the PMBUs
+specification.
+
+Virtual PMBus commands
+----------------------
+
+Virtual PMBus commands are provided to enable support for non-standard
+functionality which has been implemented by several chip vendors and is thus
+desirable to support.
+
+Virtual PMBus commands start with command value 0x100 and can thus easily be
+distinguished from standard PMBus commands (which can not have values larger
+than 0xff). Support for virtual PMBus commands is device specific and thus has
+to be implemented in device specific code.
+
+Virtual commands are named PMBUS_VIRT_xxx and start with PMBUS_VIRT_BASE. All
+virtual commands are word sized.
+
+There are currently two types of virtual commands.
+
+- READ commands are read-only; writes are either ignored or return an error.
+- RESET commands are read/write. Reading reset registers returns zero
+  (used for detection), writing any value causes the associated history to be
+  reset.
+
+Virtual commands have to be handled in device specific driver code. Chip driver
+code returns non-negative values if a virtual command is supported, or a
+negative error code if not. The chip driver may return -ENODATA or any other
+Linux error code in this case, though an error code other than -ENODATA is
+handled more efficiently and thus preferred. Either case, the calling PMBus
+core code will abort if the chip driver returns an error code when reading
+or writing virtual registers (in other words, the PMBus core code will never
+send a virtual command to a chip).
+
+PMBus driver information
+------------------------
+
+PMBus driver information, defined in struct pmbus_driver_info, is the main means
+for device specific drivers to pass information to the core PMBus driver.
+Specifically, it provides the following information.
+
+- For devices supporting its data in Direct Data Format, it provides coefficients
+  for converting register values into normalized data. This data is usually
+  provided by chip manufacturers in device datasheets.
+- Supported chip functionality can be provided to the core driver. This may be
+  necessary for chips which react badly if non-supported commands are executed,
+  and/or to speed up device detection and initialization.
+- Several function entry points are provided to support overriding and/or
+  augmenting generic command execution. This functionality can be used to map
+  non-standard PMBus commands to standard commands, or to augment standard
+  command return values with device specific information.
+
+  API functions
+  -------------
+
+  Functions provided by chip driver
+  ---------------------------------
+
+  All functions return the command return value (read) or zero (write) if
+  successful. A return value of -ENODATA indicates that there is no manufacturer
+  specific command, but that a standard PMBus command may exist. Any other
+  negative return value indicates that the commands does not exist for this
+  chip, and that no attempt should be made to read or write the standard
+  command.
+
+  As mentioned above, an exception to this rule applies to virtual commands,
+  which  _must_ be handled in driver specific code. See "Virtual PMBus Commands"
+  above for more details.
+
+  Command execution in the core PMBus driver code is as follows.
+
+	if (chip_access_function) {
+		status = chip_access_function();
+		if (status != -ENODATA)
+			return status;
+	}
+	if (command >= PMBUS_VIRT_BASE)	/* For word commands/registers only */
+		return -EINVAL;
+	return generic_access();
+
+  Chip drivers may provide pointers to the following functions in struct
+  pmbus_driver_info. All functions are optional.
+
+  int (*read_byte_data)(struct i2c_client *client, int page, int reg);
+
+  Read byte from page <page>, register <reg>.
+  <page> may be -1, which means "current page".
+
+  int (*read_word_data)(struct i2c_client *client, int page, int reg);
+
+  Read word from page <page>, register <reg>.
+
+  int (*write_word_data)(struct i2c_client *client, int page, int reg,
+		         u16 word);
+
+  Write word to page <page>, register <reg>.
+
+  int (*write_byte)(struct i2c_client *client, int page, u8 value);
+
+  Write byte to page <page>, register <reg>.
+  <page> may be -1, which means "current page".
+
+  int (*identify)(struct i2c_client *client, struct pmbus_driver_info *info);
+
+  Determine supported PMBus functionality. This function is only necessary
+  if a chip driver supports multiple chips, and the chip functionality is not
+  pre-determined. It is currently only used by the generic pmbus driver
+  (pmbus.c).
+
+  Functions exported by core driver
+  ---------------------------------
+
+  Chip drivers are expected to use the following functions to read or write
+  PMBus registers. Chip drivers may also use direct I2C commands. If direct I2C
+  commands are used, the chip driver code must not directly modify the current
+  page, since the selected page is cached in the core driver and the core driver
+  will assume that it is selected. Using pmbus_set_page() to select a new page
+  is mandatory.
+
+  int pmbus_set_page(struct i2c_client *client, u8 page);
+
+  Set PMBus page register to <page> for subsequent commands.
+
+  int pmbus_read_word_data(struct i2c_client *client, u8 page, u8 reg);
+
+  Read word data from <page>, <reg>. Similar to i2c_smbus_read_word_data(), but
+  selects page first.
+
+  int pmbus_write_word_data(struct i2c_client *client, u8 page, u8 reg,
+			    u16 word);
+
+  Write word data to <page>, <reg>. Similar to i2c_smbus_write_word_data(), but
+  selects page first.
+
+  int pmbus_read_byte_data(struct i2c_client *client, int page, u8 reg);
+
+  Read byte data from <page>, <reg>. Similar to i2c_smbus_read_byte_data(), but
+  selects page first. <page> may be -1, which means "current page".
+
+  int pmbus_write_byte(struct i2c_client *client, int page, u8 value);
+
+  Write byte data to <page>, <reg>. Similar to i2c_smbus_write_byte(), but
+  selects page first. <page> may be -1, which means "current page".
+
+  void pmbus_clear_faults(struct i2c_client *client);
+
+  Execute PMBus "Clear Fault" command on all chip pages.
+  This function calls the device specific write_byte function if defined.
+  Therefore, it must _not_ be called from that function.
+
+  bool pmbus_check_byte_register(struct i2c_client *client, int page, int reg);
+
+  Check if byte register exists. Return true if the register exists, false
+  otherwise.
+  This function calls the device specific write_byte function if defined to
+  obtain the chip status. Therefore, it must _not_ be called from that function.
+
+  bool pmbus_check_word_register(struct i2c_client *client, int page, int reg);
+
+  Check if word register exists. Return true if the register exists, false
+  otherwise.
+  This function calls the device specific write_byte function if defined to
+  obtain the chip status. Therefore, it must _not_ be called from that function.
+
+  int pmbus_do_probe(struct i2c_client *client, const struct i2c_device_id *id,
+                     struct pmbus_driver_info *info);
+
+  Execute probe function. Similar to standard probe function for other drivers,
+  with the pointer to struct pmbus_driver_info as additional argument. Calls
+  identify function if supported. Must only be called from device probe
+  function.
+
+  void pmbus_do_remove(struct i2c_client *client);
+
+  Execute driver remove function. Similar to standard driver remove function.
+
+  const struct pmbus_driver_info
+	*pmbus_get_driver_info(struct i2c_client *client);
+
+  Return pointer to struct pmbus_driver_info as passed to pmbus_do_probe().
+
+
+PMBus driver platform data
+==========================
+
+PMBus platform data is defined in include/linux/i2c/pmbus.h. Platform data
+currently only provides a flag field with a single bit used.
+
+#define PMBUS_SKIP_STATUS_CHECK (1 << 0)
+
+struct pmbus_platform_data {
+        u32 flags;              /* Device specific flags */
+};
+
+
+Flags
+-----
+
+PMBUS_SKIP_STATUS_CHECK
+
+During register detection, skip checking the status register for
+communication or command errors.
+
+Some PMBus chips respond with valid data when trying to read an unsupported
+register. For such chips, checking the status register is mandatory when
+trying to determine if a chip register exists or not.
+Other PMBus chips don't support the STATUS_CML register, or report
+communication errors for no explicable reason. For such chips, checking the
+status register must be disabled.
+
+Some i2c controllers do not support single-byte commands (write commands with
+no data, i2c_smbus_write_byte()). With such controllers, clearing the status
+register is impossible, and the PMBUS_SKIP_STATUS_CHECK flag must be set.