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

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Date:   Sat, 30 Jan 2021 13:59:31 +0900
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     David Lechner <david@...hnology.com>
Cc:     jic23@...nel.org, kernel@...gutronix.de,
        linux-stm32@...md-mailman.stormreply.com, a.fatoum@...gutronix.de,
        kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        syednwaris@...il.com, patrick.havelange@...ensium.com,
        fabrice.gasnier@...com, mcoquelin.stm32@...il.com,
        alexandre.torgue@...com, Dan Carpenter <dan.carpenter@...cle.com>
Subject: 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:
> 
> > 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).

> > +	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.

> > + * @scope: component scope (Device, Count, or Signal)
> 
> Same here. @scope must be one of enum counter_scope.

Ack.

> > + * @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.

> > + */
> > +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.

> > + * @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.

> > + */
> > +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

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