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Mon, 01 Apr 2019 23:30:42 -0700 (PDT) From: William Breathitt Gray To: gregkh@linuxfoundation.org Subject: [PATCH v10 03/18] docs: Add Generic Counter interface documentation Date: Tue, 2 Apr 2019 15:30:38 +0900 Message-Id: <1a232b5fdfe434b5e066f389f839243bcf151a33.1554184734.git.vilhelm.gray@gmail.com> X-Mailer: git-send-email 2.21.0 In-Reply-To: References: MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190401_233044_325799_7217CC1C X-CRM114-Status: GOOD ( 23.59 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mark.rutland@arm.com, benjamin.gaignard@st.com, linux-pwm@vger.kernel.org, linux-iio@vger.kernel.org, patrick.havelange@essensium.com, thierry.reding@gmail.com, pmeerw@pmeerw.net, lars@metafoo.de, daniel.lezcano@linaro.org, linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org, david@lechnology.com, William Breathitt Gray , robh+dt@kernel.org, Jonathan Cameron , tglx@linutronix.de, fabrice.gasnier@st.com, esben@haabendal.dk, shawnguo@kernel.org, linux-kernel@vger.kernel.org, leoyang.li@nxp.com, knaack.h@gmx.de, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org, jic23@kernel.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org X-Virus-Scanned: ClamAV using ClamSMTP This patch adds high-level documentation about the Generic Counter interface. Reviewed-by: Jonathan Cameron Signed-off-by: William Breathitt Gray --- Documentation/driver-api/generic-counter.rst | 342 +++++++++++++++++++ Documentation/driver-api/index.rst | 1 + MAINTAINERS | 1 + 3 files changed, 344 insertions(+) create mode 100644 Documentation/driver-api/generic-counter.rst diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst new file mode 100644 index 000000000000..f51db893f595 --- /dev/null +++ b/Documentation/driver-api/generic-counter.rst @@ -0,0 +1,342 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================= +Generic Counter Interface +========================= + +Introduction +============ + +Counter devices are prevalent within a diverse spectrum of industries. +The ubiquitous presence of these devices necessitates a common interface +and standard of interaction and exposure. This driver API attempts to +resolve the issue of duplicate code found among existing counter device +drivers by introducing a generic counter interface for consumption. The +Generic Counter interface enables drivers to support and expose a common +set of components and functionality present in counter devices. + +Theory +====== + +Counter devices can vary greatly in design, but regardless of whether +some devices are quadrature encoder counters or tally counters, all +counter devices consist of a core set of components. This core set of +components, shared by all counter devices, is what forms the essence of +the Generic Counter interface. + +There are three core components to a counter: + +* Count: + Count data for a set of Signals. + +* Signal: + Input data that is evaluated by the counter to determine the count + data. + +* Synapse: + The association of a Signal with a respective Count. + +COUNT +----- +A Count represents the count data for a set of Signals. The Generic +Counter interface provides the following available count data types: + +* COUNT_POSITION: + Unsigned integer value representing position. + +A Count has a count function mode which represents the update behavior +for the count data. The Generic Counter interface provides the following +available count function modes: + +* Increase: + Accumulated count is incremented. + +* Decrease: + Accumulated count is decremented. + +* Pulse-Direction: + Rising edges on signal A updates the respective count. The input level + of signal B determines direction. + +* Quadrature: + A pair of quadrature encoding signals are evaluated to determine + position and direction. The following Quadrature modes are available: + + - x1 A: + If direction is forward, rising edges on quadrature pair signal A + updates the respective count; if the direction is backward, falling + edges on quadrature pair signal A updates the respective count. + Quadrature encoding determines the direction. + + - x1 B: + If direction is forward, rising edges on quadrature pair signal B + updates the respective count; if the direction is backward, falling + edges on quadrature pair signal B updates the respective count. + Quadrature encoding determines the direction. + + - x2 A: + Any state transition on quadrature pair signal A updates the + respective count. Quadrature encoding determines the direction. + + - x2 B: + Any state transition on quadrature pair signal B updates the + respective count. Quadrature encoding determines the direction. + + - x4: + Any state transition on either quadrature pair signals updates the + respective count. Quadrature encoding determines the direction. + +A Count has a set of one or more associated Signals. + +SIGNAL +------ +A Signal represents a counter input data; this is the input data that is +evaluated by the counter to determine the count data; e.g. a quadrature +signal output line of a rotary encoder. Not all counter devices provide +user access to the Signal data. + +The Generic Counter interface provides the following available signal +data types for when the Signal data is available for user access: + +* SIGNAL_LEVEL: + Signal line state level. The following states are possible: + + - SIGNAL_LEVEL_LOW: + Signal line is in a low state. + + - SIGNAL_LEVEL_HIGH: + Signal line is in a high state. + +A Signal may be associated with one or more Counts. + +SYNAPSE +------- +A Synapse represents the association of a Signal with a respective +Count. Signal data affects respective Count data, and the Synapse +represents this relationship. + +The Synapse action mode specifies the Signal data condition which +triggers the respective Count's count function evaluation to update the +count data. The Generic Counter interface provides the following +available action modes: + +* None: + Signal does not trigger the count function. In Pulse-Direction count + function mode, this Signal is evaluated as Direction. + +* Rising Edge: + Low state transitions to high state. + +* Falling Edge: + High state transitions to low state. + +* Both Edges: + Any state transition. + +A counter is defined as a set of input signals associated with count +data that are generated by the evaluation of the state of the associated +input signals as defined by the respective count functions. Within the +context of the Generic Counter interface, a counter consists of Counts +each associated with a set of Signals, whose respective Synapse +instances represent the count function update conditions for the +associated Counts. + +Paradigm +======== + +The most basic counter device may be expressed as a single Count +associated with a single Signal via a single Synapse. Take for example +a counter device which simply accumulates a count of rising edges on a +source input line:: + + Count Synapse Signal + ----- ------- ------ + +---------------------+ + | Data: Count | Rising Edge ________ + | Function: Increase | <------------- / Source \ + | | ____________ + +---------------------+ + +In this example, the Signal is a source input line with a pulsing +voltage, while the Count is a persistent count value which is repeatedly +incremented. The Signal is associated with the respective Count via a +Synapse. The increase function is triggered by the Signal data condition +specified by the Synapse -- in this case a rising edge condition on the +voltage input line. In summary, the counter device existence and +behavior is aptly represented by respective Count, Signal, and Synapse +components: a rising edge condition triggers an increase function on an +accumulating count datum. + +A counter device is not limited to a single Signal; in fact, in theory +many Signals may be associated with even a single Count. For example, a +quadrature encoder counter device can keep track of position based on +the states of two input lines:: + + Count Synapse Signal + ----- ------- ------ + +-------------------------+ + | Data: Position | Both Edges ___ + | Function: Quadrature x4 | <------------ / A \ + | | _______ + | | + | | Both Edges ___ + | | <------------ / B \ + | | _______ + +-------------------------+ + +In this example, two Signals (quadrature encoder lines A and B) are +associated with a single Count: a rising or falling edge on either A or +B triggers the "Quadrature x4" function which determines the direction +of movement and updates the respective position data. The "Quadrature +x4" function is likely implemented in the hardware of the quadrature +encoder counter device; the Count, Signals, and Synapses simply +represent this hardware behavior and functionality. + +Signals associated with the same Count can have differing Synapse action +mode conditions. For example, a quadrature encoder counter device +operating in a non-quadrature Pulse-Direction mode could have one input +line dedicated for movement and a second input line dedicated for +direction:: + + Count Synapse Signal + ----- ------- ------ + +---------------------------+ + | Data: Position | Rising Edge ___ + | Function: Pulse-Direction | <------------- / A \ (Movement) + | | _______ + | | + | | None ___ + | | <------------- / B \ (Direction) + | | _______ + +---------------------------+ + +Only Signal A triggers the "Pulse-Direction" update function, but the +instantaneous state of Signal B is still required in order to know the +direction so that the position data may be properly updated. Ultimately, +both Signals are associated with the same Count via two respective +Synapses, but only one Synapse has an active action mode condition which +triggers the respective count function while the other is left with a +"None" condition action mode to indicate its respective Signal's +availability for state evaluation despite its non-triggering mode. + +Keep in mind that the Signal, Synapse, and Count are abstract +representations which do not need to be closely married to their +respective physical sources. This allows the user of a counter to +divorce themselves from the nuances of physical components (such as +whether an input line is differential or single-ended) and instead focus +on the core idea of what the data and process represent (e.g. position +as interpreted from quadrature encoding data). + +Userspace Interface +=================== + +Several sysfs attributes are generated by the Generic Counter interface, +and reside under the /sys/bus/counter/devices/counterX directory, where +counterX refers to the respective counter device. Please see +Documentation/ABI/testing/sys-bus-counter-generic-sysfs for detailed +information on each Generic Counter interface sysfs attribute. + +Through these sysfs attributes, programs and scripts may interact with +the Generic Counter paradigm Counts, Signals, and Synapses of respective +counter devices. + +Driver API +========== + +Driver authors may utilize the Generic Counter interface in their code +by including the include/linux/counter.h header file. This header file +provides several core data structures, function prototypes, and macros +for defining a counter device. + +.. kernel-doc:: include/linux/counter.h + :internal: + +.. kernel-doc:: drivers/counter/generic-counter.c + :export: + +Implementation +============== + +To support a counter device, a driver must first allocate the available +Counter Signals via counter_signal structures. These Signals should +be stored as an array and set to the signals array member of an +allocated counter_device structure before the Counter is registered to +the system. + +Counter Counts may be allocated via counter_count structures, and +respective Counter Signal associations (Synapses) made via +counter_synapse structures. Associated counter_synapse structures are +stored as an array and set to the the synapses array member of the +respective counter_count structure. These counter_count structures are +set to the counts array member of an allocated counter_device structure +before the Counter is registered to the system. + +Driver callbacks should be provided to the counter_device structure via +a constant counter_ops structure in order to communicate with the +device: to read and write various Signals and Counts, and to set and get +the "action mode" and "function mode" for various Synapses and Counts +respectively. + +A defined counter_device structure may be registered to the system by +passing it to the counter_register function, and unregistered by passing +it to the counter_unregister function. Similarly, the +devm_counter_register and devm_counter_unregister functions may be used +if device memory-managed registration is desired. + +Extension sysfs attributes can be created for auxiliary functionality +and data by passing in defined counter_device_ext, counter_count_ext, +and counter_signal_ext structures. In these cases, the +counter_device_ext structure is used for global configuration of the +respective Counter device, while the counter_count_ext and +counter_signal_ext structures allow for auxiliary exposure and +configuration of a specific Count or Signal respectively. + +Architecture +============ + +When the Generic Counter interface counter module is loaded, the +counter_init function is called which registers a bus_type named +"counter" to the system. Subsequently, when the module is unloaded, the +counter_exit function is called which unregisters the bus_type named +"counter" from the system. + +Counter devices are registered to the system via the counter_register +function, and later removed via the counter_unregister function. The +counter_register function establishes a unique ID for the Counter +device and creates a respective sysfs directory, where X is the +mentioned unique ID: + + /sys/bus/counter/devices/counterX + +Sysfs attributes are created within the counterX directory to expose +functionality, configurations, and data relating to the Counts, Signals, +and Synapses of the Counter device, as well as options and information +for the Counter device itself. + +Each Signal has a directory created to house its relevant sysfs +attributes, where Y is the unique ID of the respective Signal: + + /sys/bus/counter/devices/counterX/signalY + +Similarly, each Count has a directory created to house its relevant +sysfs attributes, where Y is the unique ID of the respective Count: + + /sys/bus/counter/devices/counterX/countY + +For a more detailed breakdown of the available Generic Counter interface +sysfs attributes, please refer to the +Documentation/ABI/testing/sys-bus-counter file. + +The Signals and Counts associated with the Counter device are registered +to the system as well by the counter_register function. The +signal_read/signal_write driver callbacks are associated with their +respective Signal attributes, while the count_read/count_write and +function_get/function_set driver callbacks are associated with their +respective Count attributes; similarly, the same is true for the +action_get/action_set driver callbacks and their respective Synapse +attributes. If a driver callback is left undefined, then the respective +read/write permission is left disabled for the relevant attributes. + +Similarly, extension sysfs attributes are created for the defined +counter_device_ext, counter_count_ext, and counter_signal_ext +structures that are passed in. diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst index ab38ced66a44..37dce988c7fd 100644 --- a/Documentation/driver-api/index.rst +++ b/Documentation/driver-api/index.rst @@ -55,6 +55,7 @@ available subsections can be seen below. slimbus soundwire/index fpga/index + generic-counter .. only:: subproject and html diff --git a/MAINTAINERS b/MAINTAINERS index af701863a249..208c92ff4801 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -3971,6 +3971,7 @@ M: William Breathitt Gray L: linux-iio@vger.kernel.org S: Maintained F: Documentation/ABI/testing/sysfs-bus-counter* +F: Documentation/driver-api/generic-counter.rst F: drivers/counter/ F: include/linux/counter.h F: include/linux/counter_enum.h