hwmon: (pmbus) Provide more documentation

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.