Re: [PATCH v7 3/5] counter: Add character device interface

On Wed, Dec 30, 2020 at 03:36:35PM -0600, David Lechner wrote:

On 12/25/20 6:15 PM, William Breathitt Gray wrote:

quoted

diff --git a/include/uapi/linux/counter.h b/include/uapi/linux/counter.h
new file mode 100644
index 000000000000..7585dc9db19d
--- /dev/null
+++ b/include/uapi/linux/counter.h

@@ -0,0 +1,123 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * Userspace ABI for Counter character devices
+ * Copyright (C) 2020 William Breathitt Gray
+ */
+#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 {

Do we need COUNTER_SCOPE_NONE to go with COUNTER_COMPONENT_NONE?

COUNTER_COMPONENT_NONE alone should be fine because it already indicates
that the 'component' member of the struct counter_watch is to be ignored
(i.e. type, scope, etc. will not be evaluated and that section of code
is bypassed).

quoted

+	COUNTER_SCOPE_DEVICE,
+	COUNTER_SCOPE_SIGNAL,
+	COUNTER_SCOPE_COUNT,
+};
+
+/**
+ * struct counter_component - Counter component identification
+ * @type: component type (Count, extension, etc.)

Instead of "Count, extension, etc.", it could be more helpful
to say one of enum counter_component_type.

Ack.

quoted

+ * @scope: component scope (Device, Count, or Signal)

Same here. @scope must be one of enum counter_scope.

Ack.

quoted

+ * @parent: parent component identification number
+ * @id: component identification number

It could be helpful to say that these id numbers match
the X/Y/Z in the sysfs paths as described in the sysfs
ABI.

Ack.

quoted

+ */
+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

It would be helpful to say that @event must be one of
enum counter_event_type.

Ack.

quoted

+ * @channel: event channel

It would be useful to say that @channel should be 0 unless
a component has more than one event of the same type.

I'll make this clearer.

quoted

+ */
+struct counter_watch {
+	struct counter_component component;
+	__u8 event;
+	__u8 channel;
+};
+
+/* ioctl commands */
+#define COUNTER_CLEAR_WATCHES_IOCTL _IO(0x3E, 0x00)
+#define COUNTER_ADD_WATCH_IOCTL _IOW(0x3E, 0x01, struct counter_watch)
+#define COUNTER_LOAD_WATCHES_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
+ * @errno: system error number
+ */
+struct counter_event {
+	__aligned_u64 timestamp;
+	__aligned_u64 value;
+	struct counter_watch watch;
+	__u8 errno;

There are error codes larger than 255. Probably better
make this __u32.

Are error codes larger than 255 actually useful in this case? I noticed
the exit() function will only return the least significant byte of
status to the parent: https://man7.org/linux/man-pages/man3/exit.3.html

William Breathitt Gray