Documentation/hwmon/pmbus-core.rst - linux - Git at Google

 ==================================
 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.

 PEC Support
 ===========

 Many PMBus devices support SMBus PEC (Packet Error Checking). If supported
 by both the I2C adapter and by the PMBus chip, it is by default enabled.
 If PEC is supported, the PMBus core driver adds an attribute named 'pec' to
 the I2C device. This attribute can be used to control PEC support in the
 communication with the PMBus chip.

 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 phase,
                         int reg);

 Read word from page <page>, phase <phase>, register <reg>. If the chip does not
 support multiple phases, the phase parameter can be ignored. If the chip
 supports multiple phases, a phase value of 0xff indicates all phases.

 ::

   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, u8 phase);

 Set PMBus page register to <page> and <phase> for subsequent commands.
 If the chip does not support multiple phases, the phase parameter is
 ignored. Otherwise, a phase value of 0xff selects all phases.

 ::

   int pmbus_read_word_data(struct i2c_client *client, u8 page, u8 phase,
                            u8 reg);

 Read word data from <page>, <phase>, <reg>. Similar to
 i2c_smbus_read_word_data(), but selects page and phase first. If the chip does
 not support multiple phases, the phase parameter is ignored. Otherwise, a phase
 value of 0xff selects all phases.

 ::

   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, 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.

 ::

   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/pmbus.h. Platform data
 currently provides a flags field with four bits used::

 	#define PMBUS_SKIP_STATUS_CHECK			BIT(0)

 	#define PMBUS_WRITE_PROTECTED			BIT(1)

 	#define PMBUS_NO_CAPABILITY			BIT(2)

 	#define PMBUS_READ_STATUS_AFTER_FAILED_CHECK	BIT(3)

 	#define PMBUS_NO_WRITE_PROTECT			BIT(4)

 	#define PMBUS_USE_COEFFICIENTS_CMD		BIT(5)

 	struct pmbus_platform_data {
 		u32 flags;              /* Device specific flags */

 		/* regulator support */
 		int num_regulators;
 		struct regulator_init_data *reg_init_data;
 	};


 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.

 PMBUS_WRITE_PROTECTED

 Set if the chip is write protected and write protection is not determined
 by the standard WRITE_PROTECT command.

 PMBUS_NO_CAPABILITY

 Some PMBus chips don't respond with valid data when reading the CAPABILITY
 register. For such chips, this flag should be set so that the PMBus core
 driver doesn't use CAPABILITY to determine its behavior.

 PMBUS_READ_STATUS_AFTER_FAILED_CHECK

 Read the STATUS register after each failed register check.

 Some PMBus chips end up in an undefined state when trying to read an
 unsupported register. For such chips, it is necessary to reset the
 chip pmbus controller to a known state after a failed register check.
 This can be done by reading a known register. By setting this flag the
 driver will try to read the STATUS register after each failed
 register check. This read may fail, but it will put the chip into a
 known state.

 PMBUS_NO_WRITE_PROTECT

 Some PMBus chips respond with invalid data when reading the WRITE_PROTECT
 register. For such chips, this flag should be set so that the PMBus core
 driver doesn't use the WRITE_PROTECT command to determine its behavior.

 PMBUS_USE_COEFFICIENTS_CMD

 When this flag is set the PMBus core driver will use the COEFFICIENTS
 register to initialize the coefficients for the direct mode format.
	==================================
	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.

	PEC Support
	===========

	Many PMBus devices support SMBus PEC (Packet Error Checking). If supported
	by both the I2C adapter and by the PMBus chip, it is by default enabled.
	If PEC is supported, the PMBus core driver adds an attribute named 'pec' to
	the I2C device. This attribute can be used to control PEC support in the
	communication with the PMBus chip.

	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 phase,
	int reg);

	Read word from page <page>, phase <phase>, register <reg>. If the chip does not
	support multiple phases, the phase parameter can be ignored. If the chip
	supports multiple phases, a phase value of 0xff indicates all phases.

	::

	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, u8 phase);

	Set PMBus page register to <page> and <phase> for subsequent commands.
	If the chip does not support multiple phases, the phase parameter is
	ignored. Otherwise, a phase value of 0xff selects all phases.

	::

	int pmbus_read_word_data(struct i2c_client *client, u8 page, u8 phase,
	u8 reg);

	Read word data from <page>, <phase>, <reg>. Similar to
	i2c_smbus_read_word_data(), but selects page and phase first. If the chip does
	not support multiple phases, the phase parameter is ignored. Otherwise, a phase
	value of 0xff selects all phases.

	::

	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, 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.

	::

	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/pmbus.h. Platform data
	currently provides a flags field with four bits used::

	#define PMBUS_SKIP_STATUS_CHECK BIT(0)

	#define PMBUS_WRITE_PROTECTED BIT(1)

	#define PMBUS_NO_CAPABILITY BIT(2)

	#define PMBUS_READ_STATUS_AFTER_FAILED_CHECK BIT(3)

	#define PMBUS_NO_WRITE_PROTECT BIT(4)

	#define PMBUS_USE_COEFFICIENTS_CMD BIT(5)

	struct pmbus_platform_data {
	u32 flags; /* Device specific flags */

	/* regulator support */
	int num_regulators;
	struct regulator_init_data *reg_init_data;
	};


	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.

	PMBUS_WRITE_PROTECTED

	Set if the chip is write protected and write protection is not determined
	by the standard WRITE_PROTECT command.

	PMBUS_NO_CAPABILITY

	Some PMBus chips don't respond with valid data when reading the CAPABILITY
	register. For such chips, this flag should be set so that the PMBus core
	driver doesn't use CAPABILITY to determine its behavior.

	PMBUS_READ_STATUS_AFTER_FAILED_CHECK

	Read the STATUS register after each failed register check.

	Some PMBus chips end up in an undefined state when trying to read an
	unsupported register. For such chips, it is necessary to reset the
	chip pmbus controller to a known state after a failed register check.
	This can be done by reading a known register. By setting this flag the
	driver will try to read the STATUS register after each failed
	register check. This read may fail, but it will put the chip into a
	known state.

	PMBUS_NO_WRITE_PROTECT

	Some PMBus chips respond with invalid data when reading the WRITE_PROTECT
	register. For such chips, this flag should be set so that the PMBus core
	driver doesn't use the WRITE_PROTECT command to determine its behavior.

	PMBUS_USE_COEFFICIENTS_CMD

	When this flag is set the PMBus core driver will use the COEFFICIENTS
	register to initialize the coefficients for the direct mode format.