On Mon, Sep 30, 2019 at 08:22:39AM +0300, Felipe Balbi wrote: > > 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 A character device node for a counter might be a good idea. If a performance critical application can't depend on parsing a sysfs printout for timestamps, then it probably doesn't want to do so for the other attributes either. I think you are right that certain systems would have sysfs disabled for that very reason. I think latency concerns are the same reason the GPIO subsystem started providing character device nodes as well. We can do similar with the Counter subsystem: provide character device nodes by default, and optionally provide the human-readable sysfs interface as well. This would allow applications with latency concerns to use a standard interface for the Counter subsystem, while optionally providing a simpler sysfs interface for other users. William Breathitt Gray
