diff mbox series

[v8,17/22] counter: Add character device interface

Message ID 720278e3aaf3f249657ec18d158eca3f962baf8e.1613131238.git.vilhelm.gray@gmail.com (mailing list archive)
State New
Headers show
Series Introduce the Counter character device interface | expand

Commit Message

William Breathitt Gray Feb. 12, 2021, 12:13 p.m. UTC
This patch introduces a character device interface for the Counter
subsystem. 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.

Cc: David Lechner <david@lechnology.com>
Cc: Gwendal Grignou <gwendal@chromium.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
---
 drivers/counter/Makefile         |   2 +-
 drivers/counter/counter-chrdev.c | 496 +++++++++++++++++++++++++++++++
 drivers/counter/counter-chrdev.h |  16 +
 drivers/counter/counter-core.c   |  37 ++-
 include/linux/counter.h          |  45 +++
 include/uapi/linux/counter.h     |  70 +++++
 6 files changed, 661 insertions(+), 5 deletions(-)
 create mode 100644 drivers/counter/counter-chrdev.c
 create mode 100644 drivers/counter/counter-chrdev.h

Comments

Jonathan Cameron Feb. 14, 2021, 6:06 p.m. UTC | #1
On Fri, 12 Feb 2021 21:13:41 +0900
William Breathitt Gray <vilhelm.gray@gmail.com> wrote:

> This patch introduces a character device interface for the Counter
> subsystem. 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.
> 
> Cc: David Lechner <david@lechnology.com>
> Cc: Gwendal Grignou <gwendal@chromium.org>
> Cc: Dan Carpenter <dan.carpenter@oracle.com>
> Cc: Oleksij Rempel <o.rempel@pengutronix.de>
> Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>

Hi William,

A few minor comments.  Mostly seems to have come together well and
makes sense to me.

Jonathan

> ---
>  drivers/counter/Makefile         |   2 +-
>  drivers/counter/counter-chrdev.c | 496 +++++++++++++++++++++++++++++++
>  drivers/counter/counter-chrdev.h |  16 +
>  drivers/counter/counter-core.c   |  37 ++-
>  include/linux/counter.h          |  45 +++
>  include/uapi/linux/counter.h     |  70 +++++
>  6 files changed, 661 insertions(+), 5 deletions(-)
>  create mode 100644 drivers/counter/counter-chrdev.c
>  create mode 100644 drivers/counter/counter-chrdev.h
> 

...

> diff --git a/drivers/counter/counter-core.c b/drivers/counter/counter-core.c
> index bcf672e1fc0d..c137fcb97d9c 100644
> --- a/drivers/counter/counter-core.c
> +++ b/drivers/counter/counter-core.c
> @@ -5,12 +5,16 @@
>   */
>  #include <linux/counter.h>
>  #include <linux/device.h>
> +#include <linux/device/bus.h>
>  #include <linux/export.h>
> +#include <linux/fs.h>
>  #include <linux/gfp.h>
>  #include <linux/idr.h>
>  #include <linux/init.h>
>  #include <linux/module.h>
> +#include <linux/types.h>
>  
> +#include "counter-chrdev.h"
>  #include "counter-sysfs.h"
>  
>  /* Provides a unique ID for each counter device */
> @@ -33,6 +37,8 @@ static struct bus_type counter_bus_type = {
>  	.name = "counter"
>  };
>  
> +static dev_t counter_devt;
> +
>  /**
>   * counter_register - register Counter to the system
>   * @counter:	pointer to Counter to register
> @@ -54,7 +60,6 @@ int counter_register(struct counter_device *const counter)
>  	if (counter->id < 0)
>  		return counter->id;
>  
> -	/* Configure device structure for Counter */

Not sure why this comment gets removed here.

>  	dev->type = &counter_device_type;
>  	dev->bus = &counter_bus_type;
>  	if (counter->parent) {
> @@ -65,18 +70,25 @@ int counter_register(struct counter_device *const counter)
>  	device_initialize(dev);
>  	dev_set_drvdata(dev, counter);
>  
> +	/* Add Counter character device */
> +	err = counter_chrdev_add(counter, counter_devt);
> +	if (err < 0)
> +		goto err_free_id;
> +
>  	/* Add Counter sysfs attributes */
>  	err = counter_sysfs_add(counter);
>  	if (err < 0)
> -		goto err_free_id;
> +		goto err_remove_chrdev;
>  
>  	/* Add device to system */
>  	err = device_add(dev);
>  	if (err < 0)
> -		goto err_free_id;
> +		goto err_remove_chrdev;

It might be worth thinking about using cdev_device_add()
though will require a slightly different order of adding.


>  
>  	return 0;
>  
> +err_remove_chrdev:
> +	counter_chrdev_remove(counter);
>  err_free_id:
>  	put_device(dev);
>  	return err;
> @@ -138,13 +150,30 @@ int devm_counter_register(struct device *dev,
>  }
>  EXPORT_SYMBOL_GPL(devm_counter_register);
>  
> +#define COUNTER_DEV_MAX 256
> +
>  static int __init counter_init(void)
>  {
> -	return bus_register(&counter_bus_type);
> +	int err;
> +
> +	err = bus_register(&counter_bus_type);
> +	if (err < 0)
> +		return err;
> +
> +	err = alloc_chrdev_region(&counter_devt, 0, COUNTER_DEV_MAX, "counter");
> +	if (err < 0)
> +		goto err_unregister_bus;
> +
> +	return 0;
> +
> +err_unregister_bus:
> +	bus_unregister(&counter_bus_type);
> +	return err;
>  }
>  
>  static void __exit counter_exit(void)
>  {
> +	unregister_chrdev_region(counter_devt, COUNTER_DEV_MAX);
>  	bus_unregister(&counter_bus_type);
>  }
>  

...

> diff --git a/include/uapi/linux/counter.h b/include/uapi/linux/counter.h
> index 6113938a6044..3d647a5383b8 100644
> --- a/include/uapi/linux/counter.h
> +++ b/include/uapi/linux/counter.h
> @@ -6,6 +6,19 @@
>  #ifndef _UAPI_COUNTER_H_
>  #define _UAPI_COUNTER_H_
>  
> +#include <linux/ioctl.h>
> +#include <linux/types.h>
> +
> +/* Component type definitions */
> +enum counter_component_type {
> +	COUNTER_COMPONENT_NONE,
> +	COUNTER_COMPONENT_SIGNAL,
> +	COUNTER_COMPONENT_COUNT,
> +	COUNTER_COMPONENT_FUNCTION,
> +	COUNTER_COMPONENT_SYNAPSE_ACTION,
> +	COUNTER_COMPONENT_EXTENSION,
> +};
> +
>  /* Component scope definitions */
>  enum counter_scope {
>  	COUNTER_SCOPE_DEVICE,
> @@ -13,6 +26,63 @@ enum counter_scope {
>  	COUNTER_SCOPE_COUNT,
>  };
>  
> +/**
> + * struct counter_component - Counter component identification
> + * @type: component type (one of enum counter_component_type)
> + * @scope: component scope (one of enum counter_scope)
> + * @parent: parent component ID (matching the Y/Z suffix of the respective sysfs
> + *	    path as described in Documentation/ABI/testing/sysfs-bus-counter)

Probably good to give an example here as well as the cross reference.

> + * @id: component ID (matching the Y/Z suffix of the respective sysfs path as
> + *	described in Documentation/ABI/testing/sysfs-bus-counter)
> + */
> +struct counter_component {
> +	__u8 type;
> +	__u8 scope;
> +	__u8 parent;
> +	__u8 id;
> +};
> +
> +/* Event type definitions */
> +enum counter_event_type {
> +	COUNTER_EVENT_OVERFLOW,
> +	COUNTER_EVENT_UNDERFLOW,
> +	COUNTER_EVENT_OVERFLOW_UNDERFLOW,
> +	COUNTER_EVENT_THRESHOLD,
> +	COUNTER_EVENT_INDEX,
> +};
> +
> +/**
> + * struct counter_watch - Counter component watch configuration
> + * @component: component to watch when event triggers
> + * @event: event that triggers (one of enum counter_event_type)
> + * @channel: event channel (typically 0 unless the device supports concurrent
> + *	     events of the same type)
> + */
> +struct counter_watch {
> +	struct counter_component component;
> +	__u8 event;
> +	__u8 channel;
> +};
> +
> +/* ioctl commands */
> +#define COUNTER_ADD_WATCH_IOCTL _IOW(0x3E, 0x00, struct counter_watch)
> +#define COUNTER_ENABLE_EVENTS_IOCTL _IO(0x3E, 0x01)
> +#define COUNTER_DISABLE_EVENTS_IOCTL _IO(0x3E, 0x02)
> +
> +/**
> + * struct counter_event - Counter event data
> + * @timestamp: best estimate of time of event occurrence, in nanoseconds
> + * @value: component value
> + * @watch: component watch configuration
> + * @status: return status (system error number)
> + */
> +struct counter_event {
> +	__aligned_u64 timestamp;
> +	__aligned_u64 value;
> +	struct counter_watch watch;
> +	__u8 status;
> +};
> +
>  /* Count direction values */
>  enum counter_count_direction {
>  	COUNTER_COUNT_DIRECTION_FORWARD,
Oleksij Rempel Feb. 15, 2021, 9:24 a.m. UTC | #2
Hi William,

On Fri, Feb 12, 2021 at 09:13:41PM +0900, William Breathitt Gray wrote:
> This patch introduces a character device interface for the Counter
> subsystem. 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.

.....

> +/**
> + * counter_push_event - queue event for userspace reading
> + * @counter:	pointer to Counter structure
> + * @event:	triggered event
> + * @channel:	event channel
> + *
> + * Note: If no one is watching for the respective event, it is silently
> + * discarded.
> + */
> +void counter_push_event(struct counter_device *const counter, const u8 event,
> +			const u8 channel)
> +{
> +	struct counter_event ev = {0};
> +	unsigned int copied = 0;
> +	unsigned long flags;
> +	struct counter_event_node *event_node;
> +	struct counter_comp_node *comp_node;
> +
> +	ev.timestamp = ktime_get_ns();
> +	ev.watch.event = event;
> +	ev.watch.channel = channel;
> +
> +	/* Could be in an interrupt context, so use a raw spin lock */
> +	raw_spin_lock_irqsave(&counter->events_list_lock, flags);
> +
> +	/* Search for event in the list */
> +	list_for_each_entry(event_node, &counter->events_list, l)
> +		if (event_node->event == event &&
> +		    event_node->channel == channel)
> +			break;
> +
> +	/* If event is not in the list */
> +	if (&event_node->l == &counter->events_list)
> +		goto exit_early;

To cover my "interrupt-counter" use case, I added following line here to
convert the tail drop fifo to the head drop fifo:

+       list_for_each_entry(comp_node, &event_node->comp_list, l)
+               to_copy += sizeof(ev);
+
+       while (kfifo_avail(&counter->events) < to_copy)
+               kfifo_skip(&counter->events);

May be it make sense to make it optional and integrate in to you patches
before it goes mainline?

Regards,
Oleksij
William Breathitt Gray Feb. 16, 2021, 1:53 a.m. UTC | #3
On Mon, Feb 15, 2021 at 10:24:12AM +0100, Oleksij Rempel wrote:
> Hi William,
> 
> On Fri, Feb 12, 2021 at 09:13:41PM +0900, William Breathitt Gray wrote:
> > This patch introduces a character device interface for the Counter
> > subsystem. 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.
> 
> .....
> 
> > +/**
> > + * counter_push_event - queue event for userspace reading
> > + * @counter:	pointer to Counter structure
> > + * @event:	triggered event
> > + * @channel:	event channel
> > + *
> > + * Note: If no one is watching for the respective event, it is silently
> > + * discarded.
> > + */
> > +void counter_push_event(struct counter_device *const counter, const u8 event,
> > +			const u8 channel)
> > +{
> > +	struct counter_event ev = {0};
> > +	unsigned int copied = 0;
> > +	unsigned long flags;
> > +	struct counter_event_node *event_node;
> > +	struct counter_comp_node *comp_node;
> > +
> > +	ev.timestamp = ktime_get_ns();
> > +	ev.watch.event = event;
> > +	ev.watch.channel = channel;
> > +
> > +	/* Could be in an interrupt context, so use a raw spin lock */
> > +	raw_spin_lock_irqsave(&counter->events_list_lock, flags);
> > +
> > +	/* Search for event in the list */
> > +	list_for_each_entry(event_node, &counter->events_list, l)
> > +		if (event_node->event == event &&
> > +		    event_node->channel == channel)
> > +			break;
> > +
> > +	/* If event is not in the list */
> > +	if (&event_node->l == &counter->events_list)
> > +		goto exit_early;
> 
> To cover my "interrupt-counter" use case, I added following line here to
> convert the tail drop fifo to the head drop fifo:
> 
> +       list_for_each_entry(comp_node, &event_node->comp_list, l)
> +               to_copy += sizeof(ev);
> +
> +       while (kfifo_avail(&counter->events) < to_copy)
> +               kfifo_skip(&counter->events);
> 
> May be it make sense to make it optional and integrate in to you patches
> before it goes mainline?
> 
> Regards,
> Oleksij
> -- 
> Pengutronix e.K.                           |                             |
> Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
> 31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
> Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |

Hello Oleksij,

I appreciate your suggestion, and I'm excited about your
interrupt-counter driver -- it looks like the Counter character device
interface will be central for that driver. However, I want to hold off
on adding further functionality to this patchset so that we can stablize
it enough to merge.

Once the Counter character device interface is merged, please send your
changes again as a patch and we can review it further then.

Thank you,

William Breathitt Gray
William Breathitt Gray Feb. 24, 2021, 5:34 a.m. UTC | #4
On Sun, Feb 14, 2021 at 06:06:12PM +0000, Jonathan Cameron wrote:
> On Fri, 12 Feb 2021 21:13:41 +0900
> William Breathitt Gray <vilhelm.gray@gmail.com> wrote:
> 
> > This patch introduces a character device interface for the Counter
> > subsystem. 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.
> > 
> > Cc: David Lechner <david@lechnology.com>
> > Cc: Gwendal Grignou <gwendal@chromium.org>
> > Cc: Dan Carpenter <dan.carpenter@oracle.com>
> > Cc: Oleksij Rempel <o.rempel@pengutronix.de>
> > Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
> 
> Hi William,
> 
> A few minor comments.  Mostly seems to have come together well and
> makes sense to me.
> 
> Jonathan
> 
> > ---
> >  drivers/counter/Makefile         |   2 +-
> >  drivers/counter/counter-chrdev.c | 496 +++++++++++++++++++++++++++++++
> >  drivers/counter/counter-chrdev.h |  16 +
> >  drivers/counter/counter-core.c   |  37 ++-
> >  include/linux/counter.h          |  45 +++
> >  include/uapi/linux/counter.h     |  70 +++++
> >  6 files changed, 661 insertions(+), 5 deletions(-)
> >  create mode 100644 drivers/counter/counter-chrdev.c
> >  create mode 100644 drivers/counter/counter-chrdev.h
> > 
> 
> ...
> 
> > diff --git a/drivers/counter/counter-core.c b/drivers/counter/counter-core.c
> > index bcf672e1fc0d..c137fcb97d9c 100644
> > --- a/drivers/counter/counter-core.c
> > +++ b/drivers/counter/counter-core.c
> > @@ -5,12 +5,16 @@
> >   */
> >  #include <linux/counter.h>
> >  #include <linux/device.h>
> > +#include <linux/device/bus.h>
> >  #include <linux/export.h>
> > +#include <linux/fs.h>
> >  #include <linux/gfp.h>
> >  #include <linux/idr.h>
> >  #include <linux/init.h>
> >  #include <linux/module.h>
> > +#include <linux/types.h>
> >  
> > +#include "counter-chrdev.h"
> >  #include "counter-sysfs.h"
> >  
> >  /* Provides a unique ID for each counter device */
> > @@ -33,6 +37,8 @@ static struct bus_type counter_bus_type = {
> >  	.name = "counter"
> >  };
> >  
> > +static dev_t counter_devt;
> > +
> >  /**
> >   * counter_register - register Counter to the system
> >   * @counter:	pointer to Counter to register
> > @@ -54,7 +60,6 @@ int counter_register(struct counter_device *const counter)
> >  	if (counter->id < 0)
> >  		return counter->id;
> >  
> > -	/* Configure device structure for Counter */
> 
> Not sure why this comment gets removed here.

This comment wasn't suppose to be removed. I'll revert this.

> >  	dev->type = &counter_device_type;
> >  	dev->bus = &counter_bus_type;
> >  	if (counter->parent) {
> > @@ -65,18 +70,25 @@ int counter_register(struct counter_device *const counter)
> >  	device_initialize(dev);
> >  	dev_set_drvdata(dev, counter);
> >  
> > +	/* Add Counter character device */
> > +	err = counter_chrdev_add(counter, counter_devt);
> > +	if (err < 0)
> > +		goto err_free_id;
> > +
> >  	/* Add Counter sysfs attributes */
> >  	err = counter_sysfs_add(counter);
> >  	if (err < 0)
> > -		goto err_free_id;
> > +		goto err_remove_chrdev;
> >  
> >  	/* Add device to system */
> >  	err = device_add(dev);
> >  	if (err < 0)
> > -		goto err_free_id;
> > +		goto err_remove_chrdev;
> 
> It might be worth thinking about using cdev_device_add()
> though will require a slightly different order of adding.

I think using cdev_device_add() should be possible. I'll adjust
counter_chrdev_add() accordingly to account for this.
 
> >  
> >  	return 0;
> >  
> > +err_remove_chrdev:
> > +	counter_chrdev_remove(counter);
> >  err_free_id:
> >  	put_device(dev);
> >  	return err;
> > @@ -138,13 +150,30 @@ int devm_counter_register(struct device *dev,
> >  }
> >  EXPORT_SYMBOL_GPL(devm_counter_register);
> >  
> > +#define COUNTER_DEV_MAX 256
> > +
> >  static int __init counter_init(void)
> >  {
> > -	return bus_register(&counter_bus_type);
> > +	int err;
> > +
> > +	err = bus_register(&counter_bus_type);
> > +	if (err < 0)
> > +		return err;
> > +
> > +	err = alloc_chrdev_region(&counter_devt, 0, COUNTER_DEV_MAX, "counter");
> > +	if (err < 0)
> > +		goto err_unregister_bus;
> > +
> > +	return 0;
> > +
> > +err_unregister_bus:
> > +	bus_unregister(&counter_bus_type);
> > +	return err;
> >  }
> >  
> >  static void __exit counter_exit(void)
> >  {
> > +	unregister_chrdev_region(counter_devt, COUNTER_DEV_MAX);
> >  	bus_unregister(&counter_bus_type);
> >  }
> >  
> 
> ...
> 
> > diff --git a/include/uapi/linux/counter.h b/include/uapi/linux/counter.h
> > index 6113938a6044..3d647a5383b8 100644
> > --- a/include/uapi/linux/counter.h
> > +++ b/include/uapi/linux/counter.h
> > @@ -6,6 +6,19 @@
> >  #ifndef _UAPI_COUNTER_H_
> >  #define _UAPI_COUNTER_H_
> >  
> > +#include <linux/ioctl.h>
> > +#include <linux/types.h>
> > +
> > +/* Component type definitions */
> > +enum counter_component_type {
> > +	COUNTER_COMPONENT_NONE,
> > +	COUNTER_COMPONENT_SIGNAL,
> > +	COUNTER_COMPONENT_COUNT,
> > +	COUNTER_COMPONENT_FUNCTION,
> > +	COUNTER_COMPONENT_SYNAPSE_ACTION,
> > +	COUNTER_COMPONENT_EXTENSION,
> > +};
> > +
> >  /* Component scope definitions */
> >  enum counter_scope {
> >  	COUNTER_SCOPE_DEVICE,
> > @@ -13,6 +26,63 @@ enum counter_scope {
> >  	COUNTER_SCOPE_COUNT,
> >  };
> >  
> > +/**
> > + * struct counter_component - Counter component identification
> > + * @type: component type (one of enum counter_component_type)
> > + * @scope: component scope (one of enum counter_scope)
> > + * @parent: parent component ID (matching the Y/Z suffix of the respective sysfs
> > + *	    path as described in Documentation/ABI/testing/sysfs-bus-counter)
> 
> Probably good to give an example here as well as the cross reference.

Ack.

William Breathitt Gray
diff mbox series

Patch

diff --git a/drivers/counter/Makefile b/drivers/counter/Makefile
index cbe1d06af6a9..c4870eb5b1dd 100644
--- a/drivers/counter/Makefile
+++ b/drivers/counter/Makefile
@@ -4,7 +4,7 @@ 
 #
 
 obj-$(CONFIG_COUNTER) += counter.o
-counter-y := counter-core.o counter-sysfs.o
+counter-y := counter-core.o counter-sysfs.o counter-chrdev.o
 
 obj-$(CONFIG_104_QUAD_8)	+= 104-quad-8.o
 obj-$(CONFIG_STM32_TIMER_CNT)	+= stm32-timer-cnt.o
diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
new file mode 100644
index 000000000000..16f02df7f73d
--- /dev/null
+++ b/drivers/counter/counter-chrdev.c
@@ -0,0 +1,496 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Generic Counter character device interface
+ * Copyright (C) 2020 William Breathitt Gray
+ */
+
+#include <linux/cdev.h>
+#include <linux/counter.h>
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/export.h>
+#include <linux/fs.h>
+#include <linux/kdev_t.h>
+#include <linux/kfifo.h>
+#include <linux/list.h>
+#include <linux/mutex.h>
+#include <linux/nospec.h>
+#include <linux/poll.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/timekeeping.h>
+#include <linux/types.h>
+#include <linux/wait.h>
+#include <linux/uaccess.h>
+
+#include "counter-chrdev.h"
+
+struct counter_comp_node {
+	struct list_head l;
+	struct counter_component component;
+	struct counter_comp comp;
+	void *parent;
+};
+
+static ssize_t counter_chrdev_read(struct file *filp, char __user *buf,
+				   size_t len, loff_t *f_ps)
+{
+	struct counter_device *const counter = filp->private_data;
+	int err;
+	unsigned int copied;
+
+	if (len < sizeof(struct counter_event))
+		return -EINVAL;
+
+	do {
+		if (kfifo_is_empty(&counter->events)) {
+			if (filp->f_flags & O_NONBLOCK)
+				return -EAGAIN;
+
+			err = wait_event_interruptible(counter->events_wait,
+					!kfifo_is_empty(&counter->events));
+			if (err < 0)
+				return err;
+		}
+
+		if (mutex_lock_interruptible(&counter->events_lock))
+			return -ERESTARTSYS;
+		err = kfifo_to_user(&counter->events, buf, len, &copied);
+		mutex_unlock(&counter->events_lock);
+		if (err < 0)
+			return err;
+	} while (!copied);
+
+	return copied;
+}
+
+static __poll_t counter_chrdev_poll(struct file *filp,
+				    struct poll_table_struct *pollt)
+{
+	struct counter_device *const counter = filp->private_data;
+	__poll_t events = 0;
+
+	poll_wait(filp, &counter->events_wait, pollt);
+
+	if (!kfifo_is_empty(&counter->events))
+		events = EPOLLIN | EPOLLRDNORM;
+
+	return events;
+}
+
+static void counter_events_list_free(struct list_head *const events_list)
+{
+	struct counter_event_node *p, *n;
+	struct counter_comp_node *q, *o;
+
+	list_for_each_entry_safe(p, n, events_list, l) {
+		/* Free associated component nodes */
+		list_for_each_entry_safe(q, o, &p->comp_list, l) {
+			list_del(&q->l);
+			kfree(q);
+		}
+
+		/* Free event node */
+		list_del(&p->l);
+		kfree(p);
+	}
+}
+
+static int counter_set_event_node(struct counter_device *const counter,
+				  struct counter_watch *const watch,
+				  const struct counter_comp_node *const cfg)
+{
+	struct counter_event_node *event_node;
+	struct counter_comp_node *comp_node;
+
+	/* Search for event in the list */
+	list_for_each_entry(event_node, &counter->next_events_list, l)
+		if (event_node->event == watch->event &&
+		    event_node->channel == watch->channel)
+			break;
+
+	/* If event is not already in the list */
+	if (&event_node->l == &counter->next_events_list) {
+		/* Allocate new event node */
+		event_node = kmalloc(sizeof(*event_node), GFP_ATOMIC);
+		if (!event_node)
+			return -ENOMEM;
+
+		/* Configure event node and add to the list */
+		event_node->event = watch->event;
+		event_node->channel = watch->channel;
+		INIT_LIST_HEAD(&event_node->comp_list);
+		list_add(&event_node->l, &counter->next_events_list);
+	}
+
+	/* Check if component watch has already been set before */
+	list_for_each_entry(comp_node, &event_node->comp_list, l)
+		if (comp_node->parent == cfg->parent &&
+		    comp_node->comp.count_u8_read == cfg->comp.count_u8_read)
+			return -EINVAL;
+
+	/* Allocate component node */
+	comp_node = kmalloc(sizeof(*comp_node), GFP_ATOMIC);
+	if (!comp_node) {
+		/* Free event node if no one else is watching */
+		if (list_empty(&event_node->comp_list)) {
+			list_del(&event_node->l);
+			kfree(event_node);
+		}
+		return -ENOMEM;
+	}
+	*comp_node = *cfg;
+
+	/* Add component node to event node */
+	list_add_tail(&comp_node->l, &event_node->comp_list);
+
+	return 0;
+}
+
+static int counter_disable_events(struct counter_device *const counter)
+{
+	unsigned long flags;
+	int err = 0;
+
+	raw_spin_lock_irqsave(&counter->events_list_lock, flags);
+
+	counter_events_list_free(&counter->events_list);
+
+	if (counter->ops->events_configure)
+		err = counter->ops->events_configure(counter);
+
+	raw_spin_unlock_irqrestore(&counter->events_list_lock, flags);
+
+	counter_events_list_free(&counter->next_events_list);
+
+	return err;
+}
+
+static int counter_add_watch(struct counter_device *const counter,
+			     const unsigned long arg)
+{
+	void __user *const uwatch = (void __user *)arg;
+	struct counter_watch watch;
+	struct counter_comp_node comp_node = {0};
+	size_t parent, id;
+	struct counter_comp *ext;
+	size_t num_ext;
+	int err;
+
+	if (copy_from_user(&watch, uwatch, sizeof(watch)))
+		return -EFAULT;
+
+	if (watch.component.type == COUNTER_COMPONENT_NONE)
+		goto no_component;
+
+	parent = watch.component.parent;
+
+	/* Configure parent component info for comp node */
+	switch (watch.component.scope) {
+	case COUNTER_SCOPE_DEVICE:
+		ext = counter->ext;
+		num_ext = counter->num_ext;
+		break;
+	case COUNTER_SCOPE_SIGNAL:
+		if (parent >= counter->num_signals)
+			return -EINVAL;
+		parent = array_index_nospec(parent, counter->num_signals);
+
+		comp_node.parent = counter->signals + parent;
+
+		ext = counter->signals[parent].ext;
+		num_ext = counter->signals[parent].num_ext;
+		break;
+	case COUNTER_SCOPE_COUNT:
+		if (parent >= counter->num_counts)
+			return -EINVAL;
+		parent = array_index_nospec(parent, counter->num_counts);
+
+		comp_node.parent = counter->counts + parent;
+
+		ext = counter->counts[parent].ext;
+		num_ext = counter->counts[parent].num_ext;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	id = watch.component.id;
+
+	/* Configure component info for comp node */
+	switch (watch.component.type) {
+	case COUNTER_COMPONENT_SIGNAL:
+		if (watch.component.scope != COUNTER_SCOPE_SIGNAL)
+			return -EINVAL;
+
+		comp_node.comp.type = COUNTER_COMP_SIGNAL_LEVEL;
+		comp_node.comp.signal_u32_read = counter->ops->signal_read;
+		break;
+	case COUNTER_COMPONENT_COUNT:
+		if (watch.component.scope != COUNTER_SCOPE_COUNT)
+			return -EINVAL;
+
+		comp_node.comp.type = COUNTER_COMP_U64;
+		comp_node.comp.count_u64_read = counter->ops->count_read;
+		break;
+	case COUNTER_COMPONENT_FUNCTION:
+		if (watch.component.scope != COUNTER_SCOPE_COUNT)
+			return -EINVAL;
+
+		comp_node.comp.type = COUNTER_COMP_FUNCTION;
+		comp_node.comp.count_u32_read = counter->ops->function_read;
+		break;
+	case COUNTER_COMPONENT_SYNAPSE_ACTION:
+		if (watch.component.scope != COUNTER_SCOPE_COUNT)
+			return -EINVAL;
+		if (id >= counter->counts[parent].num_synapses)
+			return -EINVAL;
+		id = array_index_nospec(id, counter->counts[parent].num_synapses);
+
+		comp_node.comp.type = COUNTER_COMP_SYNAPSE_ACTION;
+		comp_node.comp.action_read = counter->ops->action_read;
+		comp_node.comp.priv = counter->counts[parent].synapses + id;
+		break;
+	case COUNTER_COMPONENT_EXTENSION:
+		if (id >= num_ext)
+			return -EINVAL;
+		id = array_index_nospec(id, num_ext);
+
+		comp_node.comp = ext[id];
+		break;
+	default:
+		return -EINVAL;
+	}
+	/* Check if any read callback is set; this is part of a union */
+	if (!comp_node.comp.count_u8_read)
+		return -EOPNOTSUPP;
+
+no_component:
+	if (counter->ops->watch_validate) {
+		err = counter->ops->watch_validate(counter, &watch);
+		if (err < 0)
+			return err;
+	}
+
+	comp_node.component = watch.component;
+
+	return counter_set_event_node(counter, &watch, &comp_node);
+}
+
+static long counter_chrdev_ioctl(struct file *filp, unsigned int cmd,
+				 unsigned long arg)
+{
+	struct counter_device *const counter = filp->private_data;
+	unsigned long flags;
+	int err = 0;
+
+	switch (cmd) {
+	case COUNTER_ADD_WATCH_IOCTL:
+		return counter_add_watch(counter, arg);
+	case COUNTER_ENABLE_EVENTS_IOCTL:
+		raw_spin_lock_irqsave(&counter->events_list_lock, flags);
+
+		counter_events_list_free(&counter->events_list);
+		list_replace_init(&counter->next_events_list,
+				  &counter->events_list);
+
+		if (counter->ops->events_configure)
+			err = counter->ops->events_configure(counter);
+
+		raw_spin_unlock_irqrestore(&counter->events_list_lock, flags);
+		return err;
+	case COUNTER_DISABLE_EVENTS_IOCTL:
+		return counter_disable_events(counter);
+	default:
+		return -ENOIOCTLCMD;
+	}
+}
+
+static int counter_chrdev_open(struct inode *inode, struct file *filp)
+{
+	struct counter_device *const counter = container_of(inode->i_cdev,
+							    typeof(*counter),
+							    chrdev);
+
+	get_device(&counter->dev);
+	filp->private_data = counter;
+
+	return nonseekable_open(inode, filp);
+}
+
+static int counter_chrdev_release(struct inode *inode, struct file *filp)
+{
+	struct counter_device *const counter = filp->private_data;
+	int err;
+
+	err = counter_disable_events(counter);
+	if (err < 0)
+		return err;
+
+	put_device(&counter->dev);
+
+	return 0;
+}
+
+static const struct file_operations counter_fops = {
+	.llseek = no_llseek,
+	.read = counter_chrdev_read,
+	.poll = counter_chrdev_poll,
+	.unlocked_ioctl = counter_chrdev_ioctl,
+	.open = counter_chrdev_open,
+	.release = counter_chrdev_release,
+};
+
+int counter_chrdev_add(struct counter_device *const counter,
+		       const dev_t counter_devt)
+{
+	struct device *const dev = &counter->dev;
+	struct cdev *const chrdev = &counter->chrdev;
+	int err;
+
+	/* Initialize Counter events lists */
+	INIT_LIST_HEAD(&counter->events_list);
+	INIT_LIST_HEAD(&counter->next_events_list);
+	raw_spin_lock_init(&counter->events_list_lock);
+
+	/* Initialize Counter events queue */
+	err = kfifo_alloc(&counter->events, 64, GFP_ATOMIC);
+	if (err)
+		return err;
+	init_waitqueue_head(&counter->events_wait);
+	mutex_init(&counter->events_lock);
+
+	/* Initialize character device */
+	cdev_init(chrdev, &counter_fops);
+	dev->devt = MKDEV(MAJOR(counter_devt), counter->id);
+	cdev_set_parent(chrdev, &dev->kobj);
+
+	return cdev_add(chrdev, dev->devt, 1);
+}
+
+void counter_chrdev_remove(struct counter_device *const counter)
+{
+	kfifo_free(&counter->events);
+	cdev_del(&counter->chrdev);
+}
+
+static int counter_get_data(struct counter_device *const counter,
+			    const struct counter_comp_node *const comp_node,
+			    u64 *const value)
+{
+	const struct counter_comp *const comp = &comp_node->comp;
+	void *const parent = comp_node->parent;
+	u8 value_u8 = 0;
+	u32 value_u32 = 0;
+	int ret;
+
+	if (comp_node->component.type == COUNTER_COMPONENT_NONE)
+		return 0;
+
+	switch (comp->type) {
+	case COUNTER_COMP_U8:
+	case COUNTER_COMP_BOOL:
+		switch (comp_node->component.scope) {
+		case COUNTER_SCOPE_DEVICE:
+			ret = comp->device_u8_read(counter, &value_u8);
+			break;
+		case COUNTER_SCOPE_SIGNAL:
+			ret = comp->signal_u8_read(counter, parent, &value_u8);
+			break;
+		case COUNTER_SCOPE_COUNT:
+			ret = comp->count_u8_read(counter, parent, &value_u8);
+			break;
+		}
+		*value = value_u8;
+		return ret;
+	case COUNTER_COMP_SIGNAL_LEVEL:
+	case COUNTER_COMP_FUNCTION:
+	case COUNTER_COMP_ENUM:
+	case COUNTER_COMP_COUNT_DIRECTION:
+	case COUNTER_COMP_COUNT_MODE:
+		switch (comp_node->component.scope) {
+		case COUNTER_SCOPE_DEVICE:
+			ret = comp->device_u32_read(counter, &value_u32);
+			break;
+		case COUNTER_SCOPE_SIGNAL:
+			ret = comp->signal_u32_read(counter, parent,
+						    &value_u32);
+			break;
+		case COUNTER_SCOPE_COUNT:
+			ret = comp->count_u32_read(counter, parent, &value_u32);
+			break;
+		}
+		*value = value_u32;
+		return ret;
+	case COUNTER_COMP_U64:
+		switch (comp_node->component.scope) {
+		case COUNTER_SCOPE_DEVICE:
+			return comp->device_u64_read(counter, value);
+		case COUNTER_SCOPE_SIGNAL:
+			return comp->signal_u64_read(counter, parent, value);
+		case COUNTER_SCOPE_COUNT:
+			return comp->count_u64_read(counter, parent, value);
+		default:
+			return -EINVAL;
+		}
+	case COUNTER_COMP_SYNAPSE_ACTION:
+		ret = comp->action_read(counter, parent, comp->priv,
+					&value_u32);
+		*value = value_u32;
+		return ret;
+	default:
+		return -EINVAL;
+	}
+}
+
+/**
+ * counter_push_event - queue event for userspace reading
+ * @counter:	pointer to Counter structure
+ * @event:	triggered event
+ * @channel:	event channel
+ *
+ * Note: If no one is watching for the respective event, it is silently
+ * discarded.
+ */
+void counter_push_event(struct counter_device *const counter, const u8 event,
+			const u8 channel)
+{
+	struct counter_event ev = {0};
+	unsigned int copied = 0;
+	unsigned long flags;
+	struct counter_event_node *event_node;
+	struct counter_comp_node *comp_node;
+
+	ev.timestamp = ktime_get_ns();
+	ev.watch.event = event;
+	ev.watch.channel = channel;
+
+	/* Could be in an interrupt context, so use a raw spin lock */
+	raw_spin_lock_irqsave(&counter->events_list_lock, flags);
+
+	/* Search for event in the list */
+	list_for_each_entry(event_node, &counter->events_list, l)
+		if (event_node->event == event &&
+		    event_node->channel == channel)
+			break;
+
+	/* If event is not in the list */
+	if (&event_node->l == &counter->events_list)
+		goto exit_early;
+
+	/* Read and queue relevant comp for userspace */
+	list_for_each_entry(comp_node, &event_node->comp_list, l) {
+		ev.watch.component = comp_node->component;
+		ev.status = -counter_get_data(counter, comp_node, &ev.value);
+
+		copied += kfifo_in(&counter->events, &ev, 1);
+	}
+
+	if (copied)
+		wake_up_poll(&counter->events_wait, EPOLLIN);
+
+exit_early:
+	raw_spin_unlock_irqrestore(&counter->events_list_lock, flags);
+}
+EXPORT_SYMBOL_GPL(counter_push_event);
diff --git a/drivers/counter/counter-chrdev.h b/drivers/counter/counter-chrdev.h
new file mode 100644
index 000000000000..cf5a318fe540
--- /dev/null
+++ b/drivers/counter/counter-chrdev.h
@@ -0,0 +1,16 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Counter character device interface
+ * Copyright (C) 2020 William Breathitt Gray
+ */
+#ifndef _COUNTER_CHRDEV_H_
+#define _COUNTER_CHRDEV_H_
+
+#include <linux/counter.h>
+#include <linux/types.h>
+
+int counter_chrdev_add(struct counter_device *const counter,
+		       const dev_t counter_devt);
+void counter_chrdev_remove(struct counter_device *const counter);
+
+#endif /* _COUNTER_CHRDEV_H_ */
diff --git a/drivers/counter/counter-core.c b/drivers/counter/counter-core.c
index bcf672e1fc0d..c137fcb97d9c 100644
--- a/drivers/counter/counter-core.c
+++ b/drivers/counter/counter-core.c
@@ -5,12 +5,16 @@ 
  */
 #include <linux/counter.h>
 #include <linux/device.h>
+#include <linux/device/bus.h>
 #include <linux/export.h>
+#include <linux/fs.h>
 #include <linux/gfp.h>
 #include <linux/idr.h>
 #include <linux/init.h>
 #include <linux/module.h>
+#include <linux/types.h>
 
+#include "counter-chrdev.h"
 #include "counter-sysfs.h"
 
 /* Provides a unique ID for each counter device */
@@ -33,6 +37,8 @@  static struct bus_type counter_bus_type = {
 	.name = "counter"
 };
 
+static dev_t counter_devt;
+
 /**
  * counter_register - register Counter to the system
  * @counter:	pointer to Counter to register
@@ -54,7 +60,6 @@  int counter_register(struct counter_device *const counter)
 	if (counter->id < 0)
 		return counter->id;
 
-	/* Configure device structure for Counter */
 	dev->type = &counter_device_type;
 	dev->bus = &counter_bus_type;
 	if (counter->parent) {
@@ -65,18 +70,25 @@  int counter_register(struct counter_device *const counter)
 	device_initialize(dev);
 	dev_set_drvdata(dev, counter);
 
+	/* Add Counter character device */
+	err = counter_chrdev_add(counter, counter_devt);
+	if (err < 0)
+		goto err_free_id;
+
 	/* Add Counter sysfs attributes */
 	err = counter_sysfs_add(counter);
 	if (err < 0)
-		goto err_free_id;
+		goto err_remove_chrdev;
 
 	/* Add device to system */
 	err = device_add(dev);
 	if (err < 0)
-		goto err_free_id;
+		goto err_remove_chrdev;
 
 	return 0;
 
+err_remove_chrdev:
+	counter_chrdev_remove(counter);
 err_free_id:
 	put_device(dev);
 	return err;
@@ -138,13 +150,30 @@  int devm_counter_register(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(devm_counter_register);
 
+#define COUNTER_DEV_MAX 256
+
 static int __init counter_init(void)
 {
-	return bus_register(&counter_bus_type);
+	int err;
+
+	err = bus_register(&counter_bus_type);
+	if (err < 0)
+		return err;
+
+	err = alloc_chrdev_region(&counter_devt, 0, COUNTER_DEV_MAX, "counter");
+	if (err < 0)
+		goto err_unregister_bus;
+
+	return 0;
+
+err_unregister_bus:
+	bus_unregister(&counter_bus_type);
+	return err;
 }
 
 static void __exit counter_exit(void)
 {
+	unregister_chrdev_region(counter_devt, COUNTER_DEV_MAX);
 	bus_unregister(&counter_bus_type);
 }
 
diff --git a/include/linux/counter.h b/include/linux/counter.h
index 2d544f58a79c..af896143d533 100644
--- a/include/linux/counter.h
+++ b/include/linux/counter.h
@@ -6,9 +6,14 @@ 
 #ifndef _COUNTER_H_
 #define _COUNTER_H_
 
+#include <linux/cdev.h>
 #include <linux/device.h>
 #include <linux/kernel.h>
+#include <linux/kfifo.h>
+#include <linux/mutex.h>
+#include <linux/spinlock_types.h>
 #include <linux/types.h>
+#include <linux/wait.h>
 #include <uapi/linux/counter.h>
 
 struct counter_device;
@@ -199,6 +204,20 @@  struct counter_count {
 	size_t num_ext;
 };
 
+/**
+ * struct counter_event_node - Counter Event node
+ * @l:		list of current watching Counter events
+ * @event:	event that triggers
+ * @channel:	event channel
+ * @comp_list:	list of components to watch when event triggers
+ */
+struct counter_event_node {
+	struct list_head l;
+	u8 event;
+	u8 channel;
+	struct list_head comp_list;
+};
+
 /**
  * struct counter_ops - Callbacks from driver
  * @signal_read:	read callback for Signals. The read level of the
@@ -221,6 +240,13 @@  struct counter_count {
  * @action_write:	write callback for Synapse action modes. The action mode
  *			to write for the respective Synapse is passed in via the
  *			action parameter.
+ * @events_configure:	write callback to configure events. The list of struct
+ *			counter_event_node may be accessed via the events_list
+ *			member of the counter parameter.
+ * @watch_validate:	callback to validate a watch. The Counter component
+ *			watch configuration is passed in via the watch
+ *			parameter. A return value of 0 indicates a valid Counter
+ *			component watch configuration.
  */
 struct counter_ops {
 	int (*signal_read)(struct counter_device *counter,
@@ -244,6 +270,9 @@  struct counter_ops {
 			    struct counter_count *count,
 			    struct counter_synapse *synapse,
 			    enum counter_synapse_action action);
+	int (*events_configure)(struct counter_device *counter);
+	int (*watch_validate)(struct counter_device *counter,
+			      const struct counter_watch *watch);
 };
 
 /**
@@ -260,6 +289,13 @@  struct counter_ops {
  * @priv:		optional private data supplied by driver
  * @id:			unique ID used to identify the Counter
  * @dev:		internal device structure
+ * @chrdev:		internal character device structure
+ * @events_list:	list of current watching Counter events
+ * @events_list_lock:	lock to protect Counter events list operations
+ * @next_events_list:	list of next watching Counter events
+ * @events:		queue of detected Counter events
+ * @events_wait:	wait queue to allow blocking reads of Counter events
+ * @events_lock:	lock to protect Counter events queue read operations
  */
 struct counter_device {
 	const char *name;
@@ -279,12 +315,21 @@  struct counter_device {
 
 	int id;
 	struct device dev;
+	struct cdev chrdev;
+	struct list_head events_list;
+	raw_spinlock_t events_list_lock;
+	struct list_head next_events_list;
+	DECLARE_KFIFO_PTR(events, struct counter_event);
+	wait_queue_head_t events_wait;
+	struct mutex events_lock;
 };
 
 int counter_register(struct counter_device *const counter);
 void counter_unregister(struct counter_device *const counter);
 int devm_counter_register(struct device *dev,
 			  struct counter_device *const counter);
+void counter_push_event(struct counter_device *const counter, const u8 event,
+			const u8 channel);
 
 #define COUNTER_COMP_DEVICE_U8(_name, _read, _write) \
 { \
diff --git a/include/uapi/linux/counter.h b/include/uapi/linux/counter.h
index 6113938a6044..3d647a5383b8 100644
--- a/include/uapi/linux/counter.h
+++ b/include/uapi/linux/counter.h
@@ -6,6 +6,19 @@ 
 #ifndef _UAPI_COUNTER_H_
 #define _UAPI_COUNTER_H_
 
+#include <linux/ioctl.h>
+#include <linux/types.h>
+
+/* Component type definitions */
+enum counter_component_type {
+	COUNTER_COMPONENT_NONE,
+	COUNTER_COMPONENT_SIGNAL,
+	COUNTER_COMPONENT_COUNT,
+	COUNTER_COMPONENT_FUNCTION,
+	COUNTER_COMPONENT_SYNAPSE_ACTION,
+	COUNTER_COMPONENT_EXTENSION,
+};
+
 /* Component scope definitions */
 enum counter_scope {
 	COUNTER_SCOPE_DEVICE,
@@ -13,6 +26,63 @@  enum counter_scope {
 	COUNTER_SCOPE_COUNT,
 };
 
+/**
+ * struct counter_component - Counter component identification
+ * @type: component type (one of enum counter_component_type)
+ * @scope: component scope (one of enum counter_scope)
+ * @parent: parent component ID (matching the Y/Z suffix of the respective sysfs
+ *	    path as described in Documentation/ABI/testing/sysfs-bus-counter)
+ * @id: component ID (matching the Y/Z suffix of the respective sysfs path as
+ *	described in Documentation/ABI/testing/sysfs-bus-counter)
+ */
+struct counter_component {
+	__u8 type;
+	__u8 scope;
+	__u8 parent;
+	__u8 id;
+};
+
+/* Event type definitions */
+enum counter_event_type {
+	COUNTER_EVENT_OVERFLOW,
+	COUNTER_EVENT_UNDERFLOW,
+	COUNTER_EVENT_OVERFLOW_UNDERFLOW,
+	COUNTER_EVENT_THRESHOLD,
+	COUNTER_EVENT_INDEX,
+};
+
+/**
+ * struct counter_watch - Counter component watch configuration
+ * @component: component to watch when event triggers
+ * @event: event that triggers (one of enum counter_event_type)
+ * @channel: event channel (typically 0 unless the device supports concurrent
+ *	     events of the same type)
+ */
+struct counter_watch {
+	struct counter_component component;
+	__u8 event;
+	__u8 channel;
+};
+
+/* ioctl commands */
+#define COUNTER_ADD_WATCH_IOCTL _IOW(0x3E, 0x00, struct counter_watch)
+#define COUNTER_ENABLE_EVENTS_IOCTL _IO(0x3E, 0x01)
+#define COUNTER_DISABLE_EVENTS_IOCTL _IO(0x3E, 0x02)
+
+/**
+ * struct counter_event - Counter event data
+ * @timestamp: best estimate of time of event occurrence, in nanoseconds
+ * @value: component value
+ * @watch: component watch configuration
+ * @status: return status (system error number)
+ */
+struct counter_event {
+	__aligned_u64 timestamp;
+	__aligned_u64 value;
+	struct counter_watch watch;
+	__u8 status;
+};
+
 /* Count direction values */
 enum counter_count_direction {
 	COUNTER_COUNT_DIRECTION_FORWARD,