Subject: [PATCH v5 0/5] Introduce the Counter character device interface

From: William Breathitt Gray <[email-masked]>
Date: Sat, 26 Sep 2020 22:18:13 -0400

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

-- 
2.28.0

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