Hi, William Breathitt Gray <vilhelm.gray@xxxxxxxxx> writes: > On Tue, Sep 24, 2019 at 04:46:57PM -0500, David Lechner wrote: >> On 9/22/19 6:35 PM, William Breathitt Gray wrote: >> > On Thu, Sep 19, 2019 at 11:03:05AM +0300, Felipe Balbi wrote: >> >> Add support for Intel PSE Quadrature Encoder >> >> >> >> Signed-off-by: Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx> >> >> --- >> >> >> >> Changes since v1: >> >> - Many more private sysfs files converted over to counter interface >> >> >> >> >> >> How do you want me to model this device's Capture Compare Mode (see >> >> below)? >> > >> > Hi Felipe, >> > >> > I'm CCing Fabien and David as they may be interested in the timestamps >> > discussion. See below for some ideas I have on implementing this. >> > >> >> Could be an interesting read (thread from my first counter driver): >> >> https://lore.kernel.org/linux-iio/1b913919-beb9-34e7-d915-6bcc40eeee1d@xxxxxxxxxxxxxx/ >> >> What would be useful to me is something like the buffer feature in iio >> where a timestamp is associated with a count and stored in a buffer so that >> we can look at a window of all values recorded in the last 20ms. Being able >> to access this via mmap would be very helpful for performance (running on >> 300MHz ARM). Anything to do with timestamps in sysfs is probably not useful >> unless it is a rare event, like a watchdog timeout. > > I'm CCing Jonathan Cameron since I'm not familiar with how IIO handles > timestamps and buffers. I don't want to reinvent something that is > working well, so hopefully we can reuse the IIO timestamp design for the > Counter subsystem. > > I would argue that a human-readable timestamps printout is useful for > certain applications (e.g. a tally counter attached to a fault line: a > human administrator will be able to review previous fault times). > However as you point out, a low latency operation is necessary for > performance critical applications. > > Although you are correct that mmap is a good low latency operation to > get access to a timestamp buffer, I'm afraid giving direct access to > memory like that will lead to many incompatible representations of > timestamp data (e.g. variations in endianness, signedness, data size, > etc.). I would like a standardized representation for this data that > userspace applications can expect to receive and interpret, especially > when time is widely represented as an unsigned integer. > > Felipe suggested the creation of a counter_event structure so that users > can poll on an attribute. This kind of behavior is useful for notifying > users of interrupts and other events, but I think we should restrict the > use of the read call on these sysfs attributes to just human-readable > data. Instead, perhaps ioctl calls can be used to facilitate binary data > transfers. > > For example, we can define a COUNTER_GET_TIMESTAMPS_IOCTL ioctl request > that returns a counter_timestamps structure with a timestamps array > populated: > > struct counter_timestamps{ > size_t num_timestamps; > unsigned int *timestamps; > } > > That would allow quick access to the timestamps data, while also > restricting it to a standard representation that all userspace > applications can follow and interpret. In addition, this won't interfer > with polling, so users can still wait for an interrupt and then decide > whether they want to use the slower human-readable printout (via read) > or the faster binary data access (via ioctl). Seems like we're starting to build the need for a /dev/counter[0123...] representation of the subsystem. If that's the case, then it may very well be that sysfs becomes somewhat optional. I think is makes sense to rely more on character devices specially since I know of devices running linux with so little memory that sysfs (and a bunch of other features) are removed from the kernel. Having a character device representation would allow counter subsystem to be used on such devices. cheers -- balbi
