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

William Breathitt Gray
2020-09-27T02:20:09Z

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