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[72.189.64.225]) by smtp.gmail.com with ESMTPSA id f12sm5276906qti.70.2020.09.26.19.18.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 26 Sep 2020 19:18:33 -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 v5 0/5] Introduce the Counter character device interface Date: Sat, 26 Sep 2020 22:18:13 -0400 Message-Id: X-Mailer: git-send-email 2.28.0 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-iio@vger.kernel.org Changes in v5: - Fixed typographical errors in documentation and comments - Updated flow charts in documentation for clarity - Moved uapi header to be part of the character device intro patch - Fix git squash mistake in 104-quad-8.c; remove redundant changes - Fix git merge mistake in 104-quad-8.c; fix locking race condition - Minor code cleanup for clarity; adjust whitespace/flow - Use put_device if device_add fails - Document sysfs structures - Rename "owner" symbols to "scope"; more apt name - Use resource-managed devm_* allocation functions - Rename *_free functions to *_remove; following common convention - Rename COUNTER_DATA* to COUNTER_COMP*; more obvious meaning - Rename various symbol and define names for clarity - Bring back static ops function; more secure to have static const - Rename counter_available union members to "enums" and "strs" - Implement COUNTER_EVENT* constants; event types are now standard - Implement atomic Counter watches swap; no more racy event config 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-axes 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 driver is exemplified by the following. The driver callbacks are first registered to the Counter core component for use by the Counter userspace interface components: +----------------------------+ | Counter device driver | +----------------------------+ | Processes data from device | +----------------------------+ | ------------------- / driver callbacks / ------------------- | 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 | +--------------------+ +---------------------+ Thereafter, data can be transferred directly between the Counter device driver and Counter userspace interface: ---------------------- / Counter device \ +----------------------+ | Count register: 0x28 | +----------------------+ | ----------------- / raw count data / ----------------- | V +----------------------------+ | Counter device driver | +----------------------------+ | Processes data from device | |----------------------------| | Type: u64 | | Value: 42 | +----------------------------+ | ---------- / u64 / ---------- | +---------------+---------------+ | | 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 * / / struct counter_event / --------------- ----------------------- | | | V | +-----------+ | | read | | +-----------+ | \ Count: 42 / | ----------- | V +--------------------------------------------------+ | `/sys/bus/counter/devices/counterX/countY/count` | +--------------------------------------------------+ \ Count: "42" / -------------------------------------------------- Counter device data is exposed through standard character device read operations. Device data is gathered when a Counter event is pushed by the respective Counter device driver. Configuration is handled via ioctl operations on the respective Counter character device node. The following are some questions I have about this patchset: 1. Should standard Counter component data types be defined as u8 or u32? Many standard Counter component types such COUNTER_COMP_SIGNAL_LEVEL have standard values defined (e.g. COUNTER_SIGNAL_LEVEL_LOW and COUNTER_SIGNAL_LEVEL_HIGH). These values are currently handled by the Counter subsystem code as u8 data types. If u32 is used for these values instead, C enum structures could be used by driver authors to implicit cast these values via the driver callback parameters; userspace would still use u32 with no issue. In theory this can work because GCC will treat enums are having a 32-bit size; but I worry about the possibility of build targets that have -fshort-enums enabled, resulting in enums having a size less than 32 bits. Would this be a problem? 2. Should I have reserved members in the userspace structures? The structures in include/uapi/linux/counter.h are available to userspace applications. Should I reserve space in these structures for future additions and usage? Will endianess and packing be a concern here? William Breathitt Gray (5): counter: Internalize sysfs interface code docs: counter: Update to reflect sysfs internalization counter: Add character device interface docs: counter: Document character device interface counter: 104-quad-8: Add IRQ support for the ACCES 104-QUAD-8 Documentation/ABI/testing/sysfs-bus-counter | 18 + .../ABI/testing/sysfs-bus-counter-104-quad-8 | 32 + Documentation/driver-api/generic-counter.rst | 408 ++++- .../userspace-api/ioctl/ioctl-number.rst | 1 + MAINTAINERS | 2 +- drivers/counter/104-quad-8.c | 775 +++++---- drivers/counter/Kconfig | 6 +- drivers/counter/Makefile | 1 + drivers/counter/counter-chrdev.c | 451 +++++ drivers/counter/counter-chrdev.h | 16 + drivers/counter/counter-core.c | 190 +++ drivers/counter/counter-sysfs.c | 862 ++++++++++ drivers/counter/counter-sysfs.h | 13 + drivers/counter/counter.c | 1496 ----------------- drivers/counter/ftm-quaddec.c | 60 +- drivers/counter/microchip-tcb-capture.c | 100 +- drivers/counter/stm32-lptimer-cnt.c | 175 +- drivers/counter/stm32-timer-cnt.c | 145 +- drivers/counter/ti-eqep.c | 226 +-- include/linux/counter.h | 618 +++---- include/linux/counter_enum.h | 45 - include/uapi/linux/counter.h | 99 ++ 22 files changed, 3053 insertions(+), 2686 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.h