Re: [PATCH v2 0/4] Introduce the Counter character device interface
On Tue, Jun 02, 2020 at 10:18:07AM -0500, David Lechner wrote: > On 5/31/20 12:14 PM, William Breathitt Gray wrote: > > Yielding the following /dev/counter0 memory layout: > > > > ++-++---+ > > | Byte 0 | Byte 1 - Byte 8 | Byte 9 | Byte 10 - Byte 17 | > > ++-++---+ > > | Boundary 0 | Count 0 | Boundary 1 | Count 1 | > > ++-++---+ > > A potential pitfall with this sort of packing is that some platforms > do not support unaligned access, so data would have to be "unpacked" > before it could be used. Since the user defines the format of this data, they could reorganize it to a more streamline alignment; for example: # echo "C0 C1 C0E0 C1E0" > counter0/chrdev_format Yielding the following /dev/counter0 memory layout instead: +-+--+++ | Byte 0 - Byte 7 | Byte 8 - Byte 15 | Byte 16| Byte 17| +-+--+++ | Count 0 | Count 1 | Boundary 0 | Boundary 1 | +-+--+++ In the future, we could also define a padding argument to give users more control over the exact offsets: # echo "C0E0 P7 C0 C1E0 P7 C1" > counter0/chrdev_format Yielding the following /dev/counter0 memory layout instead: ++-+--++ | Byte 0 | Byte 1 - Byte 7 | Byte 8 - Byte 15 | Byte 16| ++-+--++ | Boundary 0 | Padding | Count 0 | Boundary 1 | ++-+--++ +---+---+ | Byte 17 - Byte 23 | Byte 24 - Byte 31 | +---+---+ | Padding | Count 1 | +---+---+ I not sure it's best to introduce padding support with this patchset given how much is already changing, but I don't anticipate packing alignment to be something difficult to support in the future with this interface. William Breathitt Gray signature.asc Description: PGP signature
Re: [PATCH v2 0/4] Introduce the Counter character device interface
On 5/31/20 12:14 PM, William Breathitt Gray wrote: Yielding the following /dev/counter0 memory layout: ++-++---+ | Byte 0 | Byte 1 - Byte 8 | Byte 9 | Byte 10 - Byte 17 | ++-++---+ | Boundary 0 | Count 0 | Boundary 1 | Count 1 | ++-++---+ A potential pitfall with this sort of packing is that some platforms do not support unaligned access, so data would have to be "unpacked" before it could be used.
Re: [PATCH v2 0/4] Introduce the Counter character device interface
On Sun, 31 May 2020 13:14:06 -0400 William Breathitt Gray wrote: > On Sun, May 31, 2020 at 04:18:13PM +0100, Jonathan Cameron wrote: > > On Sun, 24 May 2020 13:54:39 -0400 > > William Breathitt Gray wrote: > > > After giving this some more thought, I believe human-readable sysfs > > > attributes are the way to go to support configuration of the character > > > device. I am thinking of a system like this: > > > > > > * Users configure the counter character device via a sysfs attribute > > > such as /sys/bus/counter/devices/counterX/chrdev_format or similar. > > > > > > * Users may write to this sysfs attribute to select the components they > > > want to interface -- the layout can be determined as well from the > > > order. For example: > > > > > > # echo "C0 C3 C2" > /sys/bus/counter/devices/counter0/chrdev_format > > > > I guess that 'just' meets the sysfs requirement of one file => one thing. > > We can massage this further to make it more apt, but the main idea here > is that configuration should be separate from our data; and that > configuration should be performed via sysfs. > > > > This would select Counts 0, 3, and 2 (in that order) to be available > > > in the /dev/counter0 node as a contiguous memory region. > > > > > > You can select extensions in a similar fashion: > > > > > > # echo "C4E2 S1E0" > /sys/bus/counter/devices/counter0/chrdev_format > > > > > > This would select extension 2 from Count 4, and extension 0 from > > > Signal 1. > > > > I'm not totally clear why we'd want to have a chrdev access to extensions. > > To be honest I'm not totally sure what an extension is today as it's been > > a week ;) > > In the context of the Counter subsystem, an extension is data/control > that is not one of the core components of the Counter paradigm (i.e. not > a Counter, Signal, nor Synapse). Extensions essentially represent > configuration options for the counter device and auxiliary > functionality. > > The "Implementation" section of the Generic Counter documentation > Documentation/driver-api/generic-counter.rst file gives some good > examples of extensions, but I'll provide an example here for the sake of > this new character device interface. > > Suppose we have a robot controlling a laser on a dual-axes positioning > table. A counter device is used to track the position of the laser: > Count 0 represents position on the X axis, while Count 1 represents > position on the Y axis. Because this machine is moving across two axes > at the same time, we want to grab both counts together via the > character device subsystem (grabbing them separately via sysfs would be > imprecise due to the inherent latency). > > The motors are physically able the robot out of the work area, which is > not something we want to happen. A common setup in systems like this is > to set soft boundaries on the counter device to represent the edge of > the work area; when the boundary is passed, a flag is set high on the > device to indicate the position is out-of-bounds. > > On the Counter subsystem side, this counter device would appear as > having four sysfs attributes: count0/count, count0/boundary, > count1/count, and count1/boundary. In terms of the character device > interface, we could perform a setup like this: > > # echo "C0E0 C0 C1E1 C1" > counter0/chrdev_format > > Yielding the following /dev/counter0 memory layout: > > ++-++---+ > | Byte 0 | Byte 1 - Byte 8 | Byte 9 | Byte 10 - Byte 17 | > ++-++---+ > | Boundary 0 | Count 0 | Boundary 1 | Count 1 | > ++-++---+ > > Now a single read() operation can grab the counts together as well as > their respective boundary flags to verify whether the current counts are > valid. This is a scenario where using sysfs wouldn't be viable to use: > we could check the count0/boundary sysfs attribute first, but by the > time we read the count0/count sysfs attribute second, the robot has > already moved to a new (possibly invalid) position. Ok. So typically something like a condition flag. Data that indeed makes sense to be associated with a set of counter values. > > > Perhaps an example? I see timestamp below. What is that attached to? > > If we gave multiple counters, do they each have a timestamp? > > Some counter devices feature "timestamp" functionality. I haven't yet > implemented this in the Counter subsystem because it's new functionality > and I want to keep this patchset limited to the existing Counter > subsystem functionality support. > > However, to briefly go into the topic (we'll need to discuss this more > in-depth before committing to a Counter subsystem design), some counter > devices can keep track of historic counts based on various events; we > call these "timestamps", although they may not necessary be tied
Re: [PATCH v2 0/4] Introduce the Counter character device interface
On Sun, May 31, 2020 at 04:18:13PM +0100, Jonathan Cameron wrote: > On Sun, 24 May 2020 13:54:39 -0400 > William Breathitt Gray wrote: > > After giving this some more thought, I believe human-readable sysfs > > attributes are the way to go to support configuration of the character > > device. I am thinking of a system like this: > > > > * Users configure the counter character device via a sysfs attribute > > such as /sys/bus/counter/devices/counterX/chrdev_format or similar. > > > > * Users may write to this sysfs attribute to select the components they > > want to interface -- the layout can be determined as well from the > > order. For example: > > > > # echo "C0 C3 C2" > /sys/bus/counter/devices/counter0/chrdev_format > > I guess that 'just' meets the sysfs requirement of one file => one thing. We can massage this further to make it more apt, but the main idea here is that configuration should be separate from our data; and that configuration should be performed via sysfs. > > This would select Counts 0, 3, and 2 (in that order) to be available > > in the /dev/counter0 node as a contiguous memory region. > > > > You can select extensions in a similar fashion: > > > > # echo "C4E2 S1E0" > /sys/bus/counter/devices/counter0/chrdev_format > > > > This would select extension 2 from Count 4, and extension 0 from > > Signal 1. > > I'm not totally clear why we'd want to have a chrdev access to extensions. > To be honest I'm not totally sure what an extension is today as it's been > a week ;) In the context of the Counter subsystem, an extension is data/control that is not one of the core components of the Counter paradigm (i.e. not a Counter, Signal, nor Synapse). Extensions essentially represent configuration options for the counter device and auxiliary functionality. The "Implementation" section of the Generic Counter documentation Documentation/driver-api/generic-counter.rst file gives some good examples of extensions, but I'll provide an example here for the sake of this new character device interface. Suppose we have a robot controlling a laser on a dual-axes positioning table. A counter device is used to track the position of the laser: Count 0 represents position on the X axis, while Count 1 represents position on the Y axis. Because this machine is moving across two axes at the same time, we want to grab both counts together via the character device subsystem (grabbing them separately via sysfs would be imprecise due to the inherent latency). The motors are physically able the robot out of the work area, which is not something we want to happen. A common setup in systems like this is to set soft boundaries on the counter device to represent the edge of the work area; when the boundary is passed, a flag is set high on the device to indicate the position is out-of-bounds. On the Counter subsystem side, this counter device would appear as having four sysfs attributes: count0/count, count0/boundary, count1/count, and count1/boundary. In terms of the character device interface, we could perform a setup like this: # echo "C0E0 C0 C1E1 C1" > counter0/chrdev_format Yielding the following /dev/counter0 memory layout: ++-++---+ | Byte 0 | Byte 1 - Byte 8 | Byte 9 | Byte 10 - Byte 17 | ++-++---+ | Boundary 0 | Count 0 | Boundary 1 | Count 1 | ++-++---+ Now a single read() operation can grab the counts together as well as their respective boundary flags to verify whether the current counts are valid. This is a scenario where using sysfs wouldn't be viable to use: we could check the count0/boundary sysfs attribute first, but by the time we read the count0/count sysfs attribute second, the robot has already moved to a new (possibly invalid) position. > Perhaps an example? I see timestamp below. What is that attached to? > If we gave multiple counters, do they each have a timestamp? Some counter devices feature "timestamp" functionality. I haven't yet implemented this in the Counter subsystem because it's new functionality and I want to keep this patchset limited to the existing Counter subsystem functionality support. However, to briefly go into the topic (we'll need to discuss this more in-depth before committing to a Counter subsystem design), some counter devices can keep track of historic counts based on various events; we call these "timestamps", although they may not necessary be tied to a wall-clock time. For example, quadrature encoders often have an "index" signal in addition to the quadrature A and B lines. This index signal may be used to home a positioning device, or perhaps to indicate that a full revolution -- or some other event -- has occurred. It's common for quadrature counter devices to provide a FIFO buffer that logs these "index" events by saving the current
Re: [PATCH v2 0/4] Introduce the Counter character device interface
On Sun, 24 May 2020 13:54:39 -0400 William Breathitt Gray wrote: > On Sun, May 24, 2020 at 05:25:42PM +0100, Jonathan Cameron wrote: > > > > ... > > > > > The following are some questions I have about this patchset: > > > > > > 1. Should the data format of the character device be configured via a > > >sysfs attribute? > > > > > >In this patchset, the first 196095 bytes of the character device are > > >dedicated as a selection area to choose which Counter components or > > >extensions should be exposed; the subsequent bytes are the actual > > >data for the Counter components and extensions that were selected. > > > > That sounds like the worst of all possible worlds. Reality is you need > > to do some magic library so at that point you might as well have ioctl > > options to configure it. I wonder if you can keep the data flow > > to be a simple 'read' from the chardev but move the control away from > > that. Either control via some chrdevs but keep them to the 'set / get' > > if this element is going to turn up in the read or not. You rapidly > > run into problems though, such as now to see how large a given element > > is going to be etc. Plus ioctls are rather messier to extend than > > simply adding a new sysfs file. Various subsystems do complex > > 'descriptor' type approaches to get around this, or you could do > > self describing records rather than raw data - like an input > > ev_dev event. > > Yes I agree, I don't think combining nondata with data is good design -- > it's better if users are able to simply perform read/write on the > character device without having to keep track of valid offsets and > controls. > > After giving this some more thought, I believe human-readable sysfs > attributes are the way to go to support configuration of the character > device. I am thinking of a system like this: > > * Users configure the counter character device via a sysfs attribute > such as /sys/bus/counter/devices/counterX/chrdev_format or similar. > > * Users may write to this sysfs attribute to select the components they > want to interface -- the layout can be determined as well from the > order. For example: > > # echo "C0 C3 C2" > /sys/bus/counter/devices/counter0/chrdev_format I guess that 'just' meets the sysfs requirement of one file => one thing. > > This would select Counts 0, 3, and 2 (in that order) to be available > in the /dev/counter0 node as a contiguous memory region. > > You can select extensions in a similar fashion: > > # echo "C4E2 S1E0" > /sys/bus/counter/devices/counter0/chrdev_format > > This would select extension 2 from Count 4, and extension 0 from > Signal 1. I'm not totally clear why we'd want to have a chrdev access to extensions. To be honest I'm not totally sure what an extension is today as it's been a week ;) Perhaps an example? I see timestamp below. What is that attached to? If we gave multiple counters, do they each have a timestamp? > > * Users may read from this chrdev_format sysfs attribute in order to see > the currently configured format of the character device. > > * Users may perform read/write operations on the /dev/counterX node > directly; the layout of the data is what they user has configured via > the chrdev_format sysfs attribute. For example: > > # echo "C0 C1 S0 S1" > /sys/bus/counter/devices/counter0/chrdev_format > > Yields the following /dev/counter0 memory layout: > > +-+--+--+--+ > | Byte 0 - Byte 7 | Byte 8 - Byte 15 | Byte 16 | Byte 17 | > +-+--+--+--+ > | Count 0 | Count 1 | Signal 0 | Signal 2 | > +-+--+--+--+ > > * Users may perform select/poll operations on the /dev/counterX node. > Users can be notified if data is available or events have occurred. One thing to think about early if watermarks. We bolted them on late in IIO and maybe we could have done it better from the start. I'd almost guarantee someone will want one fairly soon - particularly as it's more than possible you'll have a counter device with a hardware fifo. I have some vague recollection that ti-ecap stuff could be presented as a short fifo for starters. > > The benefit of this design is that the format is robust so users can > choose the components they want to interface and in the layout they > want. For example, if I am writing a userspace application to control a > dual-axis positioning table, I can select the two counts I care about > for the position axes. This allows me to read just those two values > directly from /dev/counterX with a simple read() call, without having to > fumble around seeking to an offset and parsing the layout. I wonder if I'm over thinking things for counters, but you may run into the complexity of different counters having different sampling frequencies. Here you are suggesting
Re: [PATCH v2 0/4] Introduce the Counter character device interface
On Sun, May 24, 2020 at 05:25:42PM +0100, Jonathan Cameron wrote: > > ... > > > The following are some questions I have about this patchset: > > > > 1. Should the data format of the character device be configured via a > >sysfs attribute? > > > >In this patchset, the first 196095 bytes of the character device are > >dedicated as a selection area to choose which Counter components or > >extensions should be exposed; the subsequent bytes are the actual > >data for the Counter components and extensions that were selected. > > That sounds like the worst of all possible worlds. Reality is you need > to do some magic library so at that point you might as well have ioctl > options to configure it. I wonder if you can keep the data flow > to be a simple 'read' from the chardev but move the control away from > that. Either control via some chrdevs but keep them to the 'set / get' > if this element is going to turn up in the read or not. You rapidly > run into problems though, such as now to see how large a given element > is going to be etc. Plus ioctls are rather messier to extend than > simply adding a new sysfs file. Various subsystems do complex > 'descriptor' type approaches to get around this, or you could do > self describing records rather than raw data - like an input > ev_dev event. Yes I agree, I don't think combining nondata with data is good design -- it's better if users are able to simply perform read/write on the character device without having to keep track of valid offsets and controls. After giving this some more thought, I believe human-readable sysfs attributes are the way to go to support configuration of the character device. I am thinking of a system like this: * Users configure the counter character device via a sysfs attribute such as /sys/bus/counter/devices/counterX/chrdev_format or similar. * Users may write to this sysfs attribute to select the components they want to interface -- the layout can be determined as well from the order. For example: # echo "C0 C3 C2" > /sys/bus/counter/devices/counter0/chrdev_format This would select Counts 0, 3, and 2 (in that order) to be available in the /dev/counter0 node as a contiguous memory region. You can select extensions in a similar fashion: # echo "C4E2 S1E0" > /sys/bus/counter/devices/counter0/chrdev_format This would select extension 2 from Count 4, and extension 0 from Signal 1. * Users may read from this chrdev_format sysfs attribute in order to see the currently configured format of the character device. * Users may perform read/write operations on the /dev/counterX node directly; the layout of the data is what they user has configured via the chrdev_format sysfs attribute. For example: # echo "C0 C1 S0 S1" > /sys/bus/counter/devices/counter0/chrdev_format Yields the following /dev/counter0 memory layout: +-+--+--+--+ | Byte 0 - Byte 7 | Byte 8 - Byte 15 | Byte 16 | Byte 17 | +-+--+--+--+ | Count 0 | Count 1 | Signal 0 | Signal 2 | +-+--+--+--+ * Users may perform select/poll operations on the /dev/counterX node. Users can be notified if data is available or events have occurred. The benefit of this design is that the format is robust so users can choose the components they want to interface and in the layout they want. For example, if I am writing a userspace application to control a dual-axis positioning table, I can select the two counts I care about for the position axes. This allows me to read just those two values directly from /dev/counterX with a simple read() call, without having to fumble around seeking to an offset and parsing the layout. Similarly, support for future extensions is simple to implement. When timestamp support is implemented, users can just select the desired timestamp extension and read it directly from the /dev/counterX node; they should also be able to perform a select()/poll() call to be notified on new timestamps. So what do you think of this sort of design? I think there is a useful robustness to the simplicity of performing a single read/write call on /dev/counterX. > > > >Moving this selection to a sysfs attribute and dedicating the > >character device to just data transfer might be a better design. If > >such a design is chosen, should the selection attribute be > >human-readable or binary? > > Sysfs basically requires things are more or less human readable. > So if you go that way I think it needs to be. > > > > > 2. How much space should allotted for strings? > > > >Each Counter component and extension has a respective size allotted > >for its data (u8 data is allotted 1 byte, u64 data is allotted 8 > >bytes, etc.); I have arbitrarily chosen to allot 64 bytes for > >strings. Is this an apt size, or should string
Re: [PATCH v2 0/4] Introduce the Counter character device interface
... > The following are some questions I have about this patchset: > > 1. Should the data format of the character device be configured via a >sysfs attribute? > >In this patchset, the first 196095 bytes of the character device are >dedicated as a selection area to choose which Counter components or >extensions should be exposed; the subsequent bytes are the actual >data for the Counter components and extensions that were selected. That sounds like the worst of all possible worlds. Reality is you need to do some magic library so at that point you might as well have ioctl options to configure it. I wonder if you can keep the data flow to be a simple 'read' from the chardev but move the control away from that. Either control via some chrdevs but keep them to the 'set / get' if this element is going to turn up in the read or not. You rapidly run into problems though, such as now to see how large a given element is going to be etc. Plus ioctls are rather messier to extend than simply adding a new sysfs file. Various subsystems do complex 'descriptor' type approaches to get around this, or you could do self describing records rather than raw data - like an input ev_dev event. > >Moving this selection to a sysfs attribute and dedicating the >character device to just data transfer might be a better design. If >such a design is chosen, should the selection attribute be >human-readable or binary? Sysfs basically requires things are more or less human readable. So if you go that way I think it needs to be. > > 2. How much space should allotted for strings? > >Each Counter component and extension has a respective size allotted >for its data (u8 data is allotted 1 byte, u64 data is allotted 8 >bytes, etc.); I have arbitrarily chosen to allot 64 bytes for >strings. Is this an apt size, or should string data be allotted more >or less space? I'd go with that being big enough, but try to keep the expose interface such that the size can change it it needs to the in the future. > > 3. Should the owning component of an extension be handled by the device >driver or Counter subsystem? > >The Counter subsystem figures out the owner (enum counter_owner_type) >for each component/extension in the counter-sysfs and counter-chrdev >code. When a callback must be executed, there are various switch >statements throughout the code to check whether the respective >Device, Signal, or Count version of the callback should be executed; >similarly, the appropriate owner type must match for the struct >counter_data macros such as COUNTER_DATA_DEVICE_U64, >COUNTER_DATA_SIGNAL_U64, COUNTER_DATA_COUNT_U64, etc. > >All this complexity in the Counter subsystem code can be eliminated >if a single callback type with a `void *owner` parameter is defined >for use with all three owner types (Device, Signal, and Count). The >device driver would then be responsible for casting the callback >argument to the appropriate owner type; but in theory, this should >not be much of a problem since the device driver is responsible for >assigning the callbacks to the owning component anyway. Whilst its more complex for subsytem I think it's better to keep everything typed if we possibly can. Always a trade off though, so use your discretion. Jonathan > > William Breathitt Gray (4): > counter: Internalize sysfs interface code > docs: counter: Update to reflect sysfs internalization > counter: Add character device interface > docs: counter: Document character device interface > > Documentation/driver-api/generic-counter.rst | 275 +++- > MAINTAINERS |3 +- > drivers/counter/104-quad-8.c | 547 +++ > drivers/counter/Makefile |1 + > drivers/counter/counter-chrdev.c | 656 > drivers/counter/counter-chrdev.h | 16 + > drivers/counter/counter-core.c | 187 +++ > drivers/counter/counter-sysfs.c | 881 +++ > drivers/counter/counter-sysfs.h | 14 + > drivers/counter/counter.c| 1496 -- > drivers/counter/ftm-quaddec.c| 89 +- > drivers/counter/stm32-lptimer-cnt.c | 161 +- > drivers/counter/stm32-timer-cnt.c| 139 +- > drivers/counter/ti-eqep.c| 211 +-- > include/linux/counter.h | 626 > include/linux/counter_enum.h | 45 - > include/uapi/linux/counter-types.h | 45 + > 17 files changed, 2826 insertions(+), 2566 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 >
