power: supply: core: Add kerneldoc to battery struct

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2071846
Tested: This is one of a series of patch sets to enable Arm SystemReady IR
 support in the kernel for NXP i.MX8 platforms.  This set updates the
 power subsystem.  This set has been tested via simple boot tests and
 the CI loop.

commit e0dbd7b
Author: Linus Walleij <linus.walleij@linaro.org>
Date:   Tue Nov 16 23:44:28 2021 +0100

    power: supply: core: Add kerneldoc to battery struct

    This complements the struct power_supply_battery_info with
    extensive kerneldoc explaining the different semantics of the
    fields, including an overview of the CC/CV charging concepts
    implicit in some of the struct members.

    This is done to first establish semantics before I can
    add more charging methods by breaking out the CC/CV parameters
    to its own struct.

    Tested-by: Randy Dunlap <rdunlap@infradead.org>
    Acked-by: Randy Dunlap <rdunlap@infradead.org>
    Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
    Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
    Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
    (cherry picked from commit e0dbd7b)

Signed-off-by: Al Stone <ahs3@redhat.com>

Author: Al Stone

include/linux/power_supply.h

@@ -351,37 +351,206 @@ struct power_supply_resistance_temp_table {
 
 #define POWER_SUPPLY_OCV_TEMP_MAX 20
 
-/*
+/**
+ * struct power_supply_battery_info - information about batteries
+ * @technology: from the POWER_SUPPLY_TECHNOLOGY_* enum
+ * @energy_full_design_uwh: energy content when fully charged in microwatt
+ *   hours
+ * @charge_full_design_uah: charge content when fully charged in microampere
+ *   hours
+ * @voltage_min_design_uv: minimum voltage across the poles when the battery
+ *   is at minimum voltage level in microvolts. If the voltage drops below this
+ *   level the battery will need precharging when using CC/CV charging.
+ * @voltage_max_design_uv: voltage across the poles when the battery is fully
+ *   charged in microvolts. This is the "nominal voltage" i.e. the voltage
+ *   printed on the label of the battery.
+ * @tricklecharge_current_ua: the tricklecharge current used when trickle
+ *   charging the battery in microamperes. This is the charging phase when the
+ *   battery is completely empty and we need to carefully trickle in some
+ *   charge until we reach the precharging voltage.
+ * @precharge_current_ua: current to use in the precharge phase in microamperes,
+ *   the precharge rate is limited by limiting the current to this value.
+ * @precharge_voltage_max_uv: the maximum voltage allowed when precharging in
+ *   microvolts. When we pass this voltage we will nominally switch over to the
+ *   CC (constant current) charging phase defined by constant_charge_current_ua
+ *   and constant_charge_voltage_max_uv.
+ * @charge_term_current_ua: when the current in the CV (constant voltage)
+ *   charging phase drops below this value in microamperes the charging will
+ *   terminate completely and not restart until the voltage over the battery
+ *   poles reach charge_restart_voltage_uv unless we use maintenance charging.
+ * @charge_restart_voltage_uv: when the battery has been fully charged by
+ *   CC/CV charging and charging has been disabled, and the voltage subsequently
+ *   drops below this value in microvolts, the charging will be restarted
+ *   (typically using CV charging).
+ * @overvoltage_limit_uv: If the voltage exceeds the nominal voltage
+ *   voltage_max_design_uv and we reach this voltage level, all charging must
+ *   stop and emergency procedures take place, such as shutting down the system
+ *   in some cases.
+ * @constant_charge_current_max_ua: current in microamperes to use in the CC
+ *   (constant current) charging phase. The charging rate is limited
+ *   by this current. This is the main charging phase and as the current is
+ *   constant into the battery the voltage slowly ascends to
+ *   constant_charge_voltage_max_uv.
+ * @constant_charge_voltage_max_uv: voltage in microvolts signifying the end of
+ *   the CC (constant current) charging phase and the beginning of the CV
+ *   (constant voltage) charging phase.
+ * @factory_internal_resistance_uohm: the internal resistance of the battery
+ *   at fabrication time, expressed in microohms. This resistance will vary
+ *   depending on the lifetime and charge of the battery, so this is just a
+ *   nominal ballpark figure.
+ * @ocv_temp: array indicating the open circuit voltage (OCV) capacity
+ *   temperature indices. This is an array of temperatures in degrees Celsius
+ *   indicating which capacity table to use for a certain temperature, since
+ *   the capacity for reasons of chemistry will be different at different
+ *   temperatures. Determining capacity is a multivariate problem and the
+ *   temperature is the first variable we determine.
+ * @temp_ambient_alert_min: the battery will go outside of operating conditions
+ *   when the ambient temperature goes below this temperature in degrees
+ *   Celsius.
+ * @temp_ambient_alert_max: the battery will go outside of operating conditions
+ *   when the ambient temperature goes above this temperature in degrees
+ *   Celsius.
+ * @temp_alert_min: the battery should issue an alert if the internal
+ *   temperature goes below this temperature in degrees Celsius.
+ * @temp_alert_max: the battery should issue an alert if the internal
+ *   temperature goes above this temperature in degrees Celsius.
+ * @temp_min: the battery will go outside of operating conditions when
+ *   the internal temperature goes below this temperature in degrees Celsius.
+ *   Normally this means the system should shut down.
+ * @temp_max: the battery will go outside of operating conditions when
+ *   the internal temperature goes above this temperature in degrees Celsius.
+ *   Normally this means the system should shut down.
+ * @ocv_table: for each entry in ocv_temp there is a corresponding entry in
+ *   ocv_table and a size for each entry in ocv_table_size. These arrays
+ *   determine the capacity in percent in relation to the voltage in microvolts
+ *   at the indexed temperature.
+ * @ocv_table_size: for each entry in ocv_temp this array is giving the size of
+ *   each entry in the array of capacity arrays in ocv_table.
+ * @resist_table: this is a table that correlates a battery temperature to the
+ *   expected internal resistance at this temperature. The resistance is given
+ *   as a percentage of factory_internal_resistance_uohm. Knowing the
+ *   resistance of the battery is usually necessary for calculating the open
+ *   circuit voltage (OCV) that is then used with the ocv_table to calculate
+ *   the capacity of the battery. The resist_table must be ordered descending
+ *   by temperature: highest temperature with lowest resistance first, lowest
+ *   temperature with highest resistance last.
+ * @resist_table_size: the number of items in the resist_table.
+ *
  * This is the recommended struct to manage static battery parameters,
  * populated by power_supply_get_battery_info(). Most platform drivers should
  * use these for consistency.
+ *
  * Its field names must correspond to elements in enum power_supply_property.
  * The default field value is -EINVAL.
- * Power supply class itself doesn't use this.
+ *
+ * The charging parameters here assume a CC/CV charging scheme. This method
+ * is most common with Lithium Ion batteries (other methods are possible) and
+ * looks as follows:
+ *
+ * ^ Battery voltage
+ * |                                               --- overvoltage_limit_uv
+ * |
+ * |                    ...................................................
+ * |                 .. constant_charge_voltage_max_uv
+ * |              ..
+ * |             .
+ * |            .
+ * |           .
+ * |          .
+ * |         .
+ * |     .. precharge_voltage_max_uv
+ * |  ..
+ * |. (trickle charging)
+ * +------------------------------------------------------------------> time
+ *
+ * ^ Current into the battery
+ * |
+ * |      ............. constant_charge_current_max_ua
+ * |      .            .
+ * |      .             .
+ * |      .              .
+ * |      .               .
+ * |      .                ..
+ * |      .                  ....
+ * |      .                       .....
+ * |    ... precharge_current_ua       .......  charge_term_current_ua
+ * |    .                                    .
+ * |    .                                    .
+ * |.... tricklecharge_current_ua            .
+ * |                                         .
+ * +-----------------------------------------------------------------> time
+ *
+ * These diagrams are synchronized on time and the voltage and current
+ * follow each other.
+ *
+ * With CC/CV charging commence over time like this for an empty battery:
+ *
+ * 1. When the battery is completely empty it may need to be charged with
+ *    an especially small current so that electrons just "trickle in",
+ *    this is the tricklecharge_current_ua.
+ *
+ * 2. Next a small initial pre-charge current (precharge_current_ua)
+ *    is applied if the voltage is below precharge_voltage_max_uv until we
+ *    reach precharge_voltage_max_uv. CAUTION: in some texts this is referred
+ *    to as "trickle charging" but the use in the Linux kernel is different
+ *    see below!
+ *
+ * 3. Then the main charging current is applied, which is called the constant
+ *    current (CC) phase. A current regulator is set up to allow
+ *    constant_charge_current_max_ua of current to flow into the battery.
+ *    The chemical reaction in the battery will make the voltage go up as
+ *    charge goes into the battery. This current is applied until we reach
+ *    the constant_charge_voltage_max_uv voltage.
+ *
+ * 4. At this voltage we switch over to the constant voltage (CV) phase. This
+ *    means we allow current to go into the battery, but we keep the voltage
+ *    fixed. This current will continue to charge the battery while keeping
+ *    the voltage the same. A chemical reaction in the battery goes on
+ *    storing energy without affecting the voltage. Over time the current
+ *    will slowly drop and when we reach charge_term_current_ua we will
+ *    end the constant voltage phase.
+ *
+ * After this the battery is fully charged, and if we do not support maintenance
+ * charging, the charging will not restart until power dissipation makes the
+ * voltage fall so that we reach charge_restart_voltage_uv and at this point
+ * we restart charging at the appropriate phase, usually this will be inside
+ * the CV phase.
+ *
+ * If we support maintenance charging the voltage is however kept high after
+ * the CV phase with a very low current. This is meant to let the same charge
+ * go in for usage while the charger is still connected, mainly for
+ * dissipation for the power consuming entity while connected to the
+ * charger.
+ *
+ * All charging MUST terminate if the overvoltage_limit_uv is ever reached.
+ * Overcharging Lithium Ion cells can be DANGEROUS and lead to fire or
+ * explosions.
+ *
+ * The power supply class itself doesn't use this struct as of now.
  */
 
 struct power_supply_battery_info {
-	unsigned int technology;	    /* from the enum above */
-	int energy_full_design_uwh;	    /* microWatt-hours */
-	int charge_full_design_uah;	    /* microAmp-hours */
-	int voltage_min_design_uv;	    /* microVolts */
-	int voltage_max_design_uv;	    /* microVolts */
-	int tricklecharge_current_ua;	    /* microAmps */
-	int precharge_current_ua;	    /* microAmps */
-	int precharge_voltage_max_uv;	    /* microVolts */
-	int charge_term_current_ua;	    /* microAmps */
-	int charge_restart_voltage_uv;	    /* microVolts */
-	int overvoltage_limit_uv;	    /* microVolts */
-	int constant_charge_current_max_ua; /* microAmps */
-	int constant_charge_voltage_max_uv; /* microVolts */
-	int factory_internal_resistance_uohm;   /* microOhms */
-	int ocv_temp[POWER_SUPPLY_OCV_TEMP_MAX];/* celsius */
-	int temp_ambient_alert_min;             /* celsius */
-	int temp_ambient_alert_max;             /* celsius */
-	int temp_alert_min;                     /* celsius */
-	int temp_alert_max;                     /* celsius */
-	int temp_min;                           /* celsius */
-	int temp_max;                           /* celsius */
+	unsigned int technology;
+	int energy_full_design_uwh;
+	int charge_full_design_uah;
+	int voltage_min_design_uv;
+	int voltage_max_design_uv;
+	int tricklecharge_current_ua;
+	int precharge_current_ua;
+	int precharge_voltage_max_uv;
+	int charge_term_current_ua;
+	int charge_restart_voltage_uv;
+	int overvoltage_limit_uv;
+	int constant_charge_current_max_ua;
+	int constant_charge_voltage_max_uv;
+	int factory_internal_resistance_uohm;
+	int ocv_temp[POWER_SUPPLY_OCV_TEMP_MAX];
+	int temp_ambient_alert_min;
+	int temp_ambient_alert_max;
+	int temp_alert_min;
+	int temp_alert_max;
+	int temp_min;
+	int temp_max;
 	struct power_supply_battery_ocv_table *ocv_table[POWER_SUPPLY_OCV_TEMP_MAX];
 	int ocv_table_size[POWER_SUPPLY_OCV_TEMP_MAX];
 	struct power_supply_resistance_temp_table *resist_table;