Provide more documentation describing PMBus driver functionality and the API
between the PMBus core driver and PMBus chip drivers.
Signed-off-by: Guenter Roeck <guenter.roeck@xxxxxxxxxxxx>
---
Documentation/hwmon/pmbus-core | 249 ++++++++++++++++++++++++++++++++++++++++
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+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.
+
+ int (*read_byte_data)(struct i2c_client *client, int page, int reg);
+
+ Read byte from page <page>, register <reg>.
+
+ 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>.
+
+ 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
+ ---------------------------------
+
+ 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.
+
+ 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.
+
+ void pmbus_clear_faults(struct i2c_client *client);
+
+ Execute PMBus "Clear Fault" command on all chip pages.
+
+ 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.
+
+ 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.
+
+ 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.
+
+ 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.
--
1.7.3.1
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