Re: [PATCH v5 0/5] Introduce the Counter character device interface
On Mon, Oct 12, 2020 at 07:35:11PM -0500, David Lechner wrote: > On 9/26/20 9:18 PM, William Breathitt Gray wrote: > > 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? > > We shouldn't have to worry about userspace programs using -fshort-enums > since that would break all kernel interfaces that use enums, not just > these - so no one should be using that compiler flag. > > So I am in favor of using strongly typed enums with u32 as the > "generic" enum member type. That reasoning pacifies my worries. I'll test out strongly typed enums in the next revision of this patchset. > > > > 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? > > > Since there doesn't seem to be a large number of counter devices > this probably isn't critical. Are there any aspects of counter > devices in general that couldn't be described with the proposed > structures? For example, could there be components more than two > levels deep (i.e. it would need id, parent id and grandparent id > to describe fully)? I can't think of any devices that would require more depth, so we probably don't need any additional members added to the userspace structures. The current structure should be flexible enough for future expansion, and any additional functionality we come across can be handled by implementing new extension types as we have for the sysfs attributes. William Breathitt Gray signature.asc Description: PGP signature
Re: [PATCH v5 0/5] Introduce the Counter character device interface
On 9/26/20 9:18 PM, William Breathitt Gray wrote: 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? We shouldn't have to worry about userspace programs using -fshort-enums since that would break all kernel interfaces that use enums, not just these - so no one should be using that compiler flag. So I am in favor of using strongly typed enums with u32 as the "generic" enum member type. 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? Since there doesn't seem to be a large number of counter devices this probably isn't critical. Are there any aspects of counter devices in general that couldn't be described with the proposed structures? For example, could there be components more than two levels deep (i.e. it would need id, parent id and grandparent id to describe fully)?
[PATCH v5 0/5] Introduce the Counter character device interface
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 / -
