linux-kernel - Re: [PATCH v6 3/5] counter: Add character device interface

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Date:   Thu, 21 Jan 2021 17:03:11 +0900
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     jic23@...nel.org, kamel.bouhara@...tlin.com, gwendal@...omium.org,
        a.fatoum@...gutronix.de, david@...hnology.com,
        linux-iio@...r.kernel.org, patrick.havelange@...ensium.com,
        alexandre.belloni@...tlin.com, mcoquelin.stm32@...il.com,
        linux-kernel@...r.kernel.org,
        Dan Carpenter <dan.carpenter@...cle.com>,
        kernel@...gutronix.de, fabrice.gasnier@...com,
        syednwaris@...il.com, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, alexandre.torgue@...com,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH v6 3/5] counter: Add character device interface

On Tue, Jan 19, 2021 at 10:20:22AM +0100, Oleksij Rempel wrote:
> On Sun, Nov 22, 2020 at 03:29:54PM -0500, 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.
> > 
> > Cc: David Lechner <david@...hnology.com>
> > Cc: Gwendal Grignou <gwendal@...omium.org>
> > Cc: Dan Carpenter <dan.carpenter@...cle.com>
> > Signed-off-by: William Breathitt Gray <vilhelm.gray@...il.com>
> > ---
> 
> Hello William,
> 
> the series looks quite interesting, we have some thoughts... see below:
> 
> [...]
> > +/**
> > + * 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.
> > + *
> > + * RETURNS:
> > + * 0 on success, negative error number on failure.
> > + */
> > +int 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;
> > +	int err = 0;
> > +
> > +	ev.timestamp = ktime_get_ns();
> > +	ev.watch.event = event;
> > +	ev.watch.channel = channel;
> > +
> > +	raw_spin_lock_irqsave(&counter->events_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) {
> > +		err = counter_get_data(counter, comp_node, &ev.value);
> > +		if (err)
> > +			goto exit_early;
> > +
> > +		ev.watch.component = comp_node->component;
> > +
> > +		copied += kfifo_put(&counter->events, ev);
> 
> We want to calculate the frequency of some IRQ pulses in user space and
> counter values with time stamps really fits well here. As the pulses are
> from a physical system (rotating wheel), they will only change at a
> certain rate. We want to have the possibility to read from the counter
> device less often, we intentionally want to skip (meaning miss)
> events.
> 
> When reading we're interested in the newest events. The kfifo implements
> a "tail" drop FIFO, which means new values are added at the end, and if
> the FIFO is full, they are dropped. We need a "head" drop FIFO which
> discards the oldest events, keeping only the recent ones.
> 
> As far as we know, kfifo doesn't offer a head drop mode, but I think
> this can be added.

I'm not sure if kfifo has this kind of mode, but it seems like something
that should be there if it is not already -- I imagine this kind of
operation mode would be pretty common. Perhaps someone knows how to
achieve this and will share here.

> 
> [...]
> 
> >  struct counter_device {
> >  	const char *name;
> > @@ -270,12 +270,20 @@ struct counter_device {
> >  
> >  	int id;
> >  	struct device dev;
> > +	struct cdev chrdev;
> > +	raw_spinlock_t events_lock;
> > +	struct list_head events_list;
> > +	struct list_head next_events_list;
> > +	DECLARE_KFIFO(events, struct counter_event, 64);
> 
> Do you plan to make the size of the FIFO configurable?
> 
> regards,
> Oleksij & Marc

I suppose it wouldn't be a problem to make this a configurable setting;
I think I can implement this via kfifo_alloc() and kfifo_free(). How
would users control this -- maybe using a sysfs attribute?

William Breathitt Gray

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