linux-kernel - Re: [PATCH 3/4] ALSA: timer: Introduce virtual userspace-driven timers

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Message-ID: <0576f5dd-656b-4085-8c8d-b0f845875f0f@wanadoo.fr>
Date: Sun, 28 Jul 2024 08:59:06 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Ivan Orlov <ivan.orlov0322@...il.com>, perex@...ex.cz, tiwai@...e.com,
 corbet@....net, broonie@...nel.org, shuah@...nel.org
Cc: linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-sound@...r.kernel.org, linux-kernel@...r.kernel.org, aholzinger@....de
Subject: Re: [PATCH 3/4] ALSA: timer: Introduce virtual userspace-driven
 timers

Le 26/07/2024 à 09:47, Ivan Orlov a écrit :
> Implement two ioctl calls in order to support virtual userspace-driven
> ALSA timers.
> 
> The first ioctl is SNDRV_TIMER_IOCTL_CREATE, which gets the
> snd_utimer_info struct as a parameter and returns a file descriptor of
> a virtual timer. It also updates the `id` field of the snd_utimer_info
> struct, which provides a unique identifier for the timer (basically,
> the subdevice number which can be used when creating timer instances).
> 
> This patch also introduces a tiny id allocator for the userspace-driven
> timers, which guarantees that we don't have more than 128 of them in the
> system.
> 
> Another ioctl is SNDRV_TIMER_IOCTL_TRIGGER, which allows us to trigger
> the virtual timer (and calls snd_timer_interrupt for the timer under
> the hood), causing all of the timer instances binded to this timer to
> execute their callbacks.
> 
> The maximum amount of ticks available for the timer is 1 for the sake of
> simplification of the userspace API. 'start', 'stop', 'open' and 'close'
> callbacks for the userspace-driven timers are empty since we don't
> really do any hardware initialization here.
> 
> Suggested-by: Axel Holzinger <aholzinger@....de>
> Signed-off-by: Ivan Orlov <ivan.orlov0322@...il.com>
> ---

...

> +#ifdef CONFIG_SND_UTIMER
> +/*
> + * Since userspace-driven timers are passed to userspace, we need to have an identifier
> + * which will allow us to use them (basically, the subdevice number of udriven timer).
> + *
> + * We have a pool of SNDRV_UTIMERS_MAX_COUNT ids from 0 to (SNDRV_UTIMERS_MAX_COUNT - 1).
> + * When we take one of them, the corresponding entry in snd_utimer_ids becomes true.
> + */
> +static bool snd_utimer_ids[SNDRV_UTIMERS_MAX_COUNT];
> +
> +static void snd_utimer_put_id(struct snd_utimer *utimer)
> +{
> +	int timer_id = utimer->id;
> +
> +	snd_BUG_ON(timer_id < 0 || timer_id >= SNDRV_UTIMERS_MAX_COUNT);
> +	snd_utimer_ids[timer_id] = false;
> +}
> +
> +static int snd_utimer_take_id(void)
> +{
> +	size_t i;
> +
> +	for (i = 0; i < SNDRV_UTIMERS_MAX_COUNT; i++) {
> +		if (!snd_utimer_ids[i]) {
> +			snd_utimer_ids[i] = true;
> +			return i;
> +		}
> +	}
> +
> +	return -EBUSY;
> +}

Also the bitmap API could be useful here.

CJ