linux-kernel - Re: [PATCH v8 17/22] counter: Add character device interface

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Date:   Mon, 15 Feb 2021 10:24:12 +0100
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
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
Subject: Re: [PATCH v8 17/22] counter: Add character device interface

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
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