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[72.189.64.225]) by smtp.gmail.com with ESMTPSA id g19sm3160409qke.32.2020.05.16.12.20.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 16 May 2020 12:20:36 -0700 (PDT) From: William Breathitt Gray To: jic23@kernel.org Cc: kamel.bouhara@bootlin.com, gwendal@chromium.org, alexandre.belloni@bootlin.com, david@lechnology.com, linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, syednwaris@gmail.com, patrick.havelange@essensium.com, fabrice.gasnier@st.com, mcoquelin.stm32@gmail.com, alexandre.torgue@st.com, William Breathitt Gray Subject: [PATCH v2 0/4] Introduce the Counter character device interface Date: Sat, 16 May 2020 15:19:58 -0400 Message-Id: X-Mailer: git-send-email 2.26.2 MIME-Version: 1.0 Sender: linux-iio-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-iio@vger.kernel.org Changes in v2: - Use fixed-width data types to represent Counter data types - Use union of function prototypes to store read/write callbacks - Eliminate the counter-strings and counter-sysfs-callback files by inlining relevant code - Reimplement chrdev code to handle read/write calls instead of ioctl - Remove struct counter_enum (I'm postponing this development until I get the core functionality solid) - Remove devm_counter_unregister as unnecessary Over the past couple years we have noticed some shortcomings with the Counter sysfs interface. Although useful in the majority of situations, there are certain use-cases where interacting through sysfs attributes can become cumbersome and inefficient. A desire to support more advanced functionality such as timestamps, multi-axis positioning tables, and other such latency-sensitive applications, has motivated a reevaluation of the Counter subsystem. I believe a character device interface will be helpful for this more niche area of counter device use. To quell any concerns from the offset: this patchset makes no changes to the existing Counter sysfs userspace interface -- existing userspace applications will continue to work with no modifications necessary. I request that driver maintainers please test their applications to verify that this is true, and report any discrepancies if they arise. However, this patchset does contain a major reimplementation of the Counter subsystem core and driver API. A reimplementation was necessary in order to separate the sysfs code from the counter device drivers and internalize it as a dedicated component of the core Counter subsystem module. A minor benefit from all of this is that the sysfs interface is now ensured a certain amount of consistency because the translation is performed outside of individual counter device drivers. Essentially, the reimplementation has enabled counter device drivers to pass and handle data as native C datatypes now rather than the sysfs strings from before. A high-level view of how a count value is passed down from a counter device driver can be exemplified by the following: ---------------------- / Counter device \ +----------------------+ | Count register: 0x28 | +----------------------+ | ----------------- / raw count data / ----------------- | V +----------------------------+ | Counter device driver |----------+ +----------------------------+ | | Processes data from device | ------------------- |----------------------------| / driver callbacks / | Type: u64 | ------------------- | Value: 42 | | +----------------------------+ | | | ---------- | / u64 / | ---------- | | | | V | +----------------------+ | | Counter core | | +----------------------+ | | Routes device driver | | | callbacks to the | | | userspace interfaces | | +----------------------+ | | | ------------------- | / driver callbacks / | ------------------- | | +-------+---------------+ | | | | | +-------|-------+ | | | V | V +--------------------+ | +---------------------+ | Counter sysfs |<-+->| Counter chrdev | +--------------------+ +---------------------+ | Translates to the | | Translates to the | | standard Counter | | standard Counter | | sysfs output | | character device | |--------------------| |---------------------+ | Type: const char * | | Type: u64 | | Value: "42" | | Value: 42 | +--------------------+ +---------------------+ | | --------------- ---------- / const char * / / u64 / --------------- ---------- | | | V | +-----------+ | | read | | +-----------+ | \ Count: 42 / | ----------- | V +--------------------------------------------------+ | `/sys/bus/counter/devices/counterX/countY/count` | +--------------------------------------------------+ \ Count: "42" / -------------------------------------------------- I am aware that an in-kernel API can simplify the data transfer between counter device drivers and the userspace interfaces, but I want to postpone that development until after the new Counter character device interface is solidified. A userspace ABI is effectively immutable so I want to make sure we get that right before working on an in-kernel API that is more flexible to change. However, when we do develop an in-kernel API, it will likely be housed as part of the Counter core component, through which the userspace interfaces will then communicate. Interaction with Counter character devices occurs via standard character device read/write operations. This allows userspace applications to access and set counter data using native C datatypes rather than working through string translations. The following are some questions I have about this patchset: 1. Should the data format of the character device be configured via a sysfs attribute? In this patchset, the first 196095 bytes of the character device are dedicated as a selection area to choose which Counter components or extensions should be exposed; the subsequent bytes are the actual data for the Counter components and extensions that were selected. Moving this selection to a sysfs attribute and dedicating the character device to just data transfer might be a better design. If such a design is chosen, should the selection attribute be human-readable or binary? 2. How much space should allotted for strings? Each Counter component and extension has a respective size allotted for its data (u8 data is allotted 1 byte, u64 data is allotted 8 bytes, etc.); I have arbitrarily chosen to allot 64 bytes for strings. Is this an apt size, or should string data be allotted more or less space? 3. Should the owning component of an extension be handled by the device driver or Counter subsystem? The Counter subsystem figures out the owner (enum counter_owner_type) for each component/extension in the counter-sysfs and counter-chrdev code. When a callback must be executed, there are various switch statements throughout the code to check whether the respective Device, Signal, or Count version of the callback should be executed; similarly, the appropriate owner type must match for the struct counter_data macros such as COUNTER_DATA_DEVICE_U64, COUNTER_DATA_SIGNAL_U64, COUNTER_DATA_COUNT_U64, etc. All this complexity in the Counter subsystem code can be eliminated if a single callback type with a `void *owner` parameter is defined for use with all three owner types (Device, Signal, and Count). The device driver would then be responsible for casting the callback argument to the appropriate owner type; but in theory, this should not be much of a problem since the device driver is responsible for assigning the callbacks to the owning component anyway. William Breathitt Gray (4): counter: Internalize sysfs interface code docs: counter: Update to reflect sysfs internalization counter: Add character device interface docs: counter: Document character device interface Documentation/driver-api/generic-counter.rst | 275 +++- MAINTAINERS | 3 +- drivers/counter/104-quad-8.c | 547 +++---- drivers/counter/Makefile | 1 + drivers/counter/counter-chrdev.c | 656 ++++++++ drivers/counter/counter-chrdev.h | 16 + drivers/counter/counter-core.c | 187 +++ drivers/counter/counter-sysfs.c | 881 +++++++++++ drivers/counter/counter-sysfs.h | 14 + drivers/counter/counter.c | 1496 ------------------ drivers/counter/ftm-quaddec.c | 89 +- drivers/counter/stm32-lptimer-cnt.c | 161 +- drivers/counter/stm32-timer-cnt.c | 139 +- drivers/counter/ti-eqep.c | 211 +-- include/linux/counter.h | 626 ++++---- include/linux/counter_enum.h | 45 - include/uapi/linux/counter-types.h | 45 + 17 files changed, 2826 insertions(+), 2566 deletions(-) create mode 100644 drivers/counter/counter-chrdev.c create mode 100644 drivers/counter/counter-chrdev.h create mode 100644 drivers/counter/counter-core.c create mode 100644 drivers/counter/counter-sysfs.c create mode 100644 drivers/counter/counter-sysfs.h delete mode 100644 drivers/counter/counter.c delete mode 100644 include/linux/counter_enum.h create mode 100644 include/uapi/linux/counter-types.h