linux-kernel - Re: [PATCH v2] ALSA: control: add ioctl to retrieve full card components

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Message-ID: <fce36281-1aa4-4a83-afa1-2470df4453a0@perex.cz>
Date: Thu, 22 Jan 2026 14:30:04 +0100
From: Jaroslav Kysela <perex@...ex.cz>
To: Maciej Strozek <mstrozek@...nsource.cirrus.com>,
 Takashi Iwai <tiwai@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-sound@...r.kernel.org,
 patches@...nsource.cirrus.com, alsa-devel@...a-project.org
Subject: Re: [PATCH v2] ALSA: control: add ioctl to retrieve full card
 components

On 1/22/26 12:12, Maciej Strozek wrote:
> The fixed-size components field in SNDRV_CTL_IOCTL_CARD_INFO can be too
> small on systems with many audio devices.
> 
> Keep the existing struct snd_ctl_card_info ABI intact and add a new ioctl
> to retrieve the full components string.
> 
> When the legacy components field is truncated, append '>' to indicate
> that the full string is available via the new ioctl.

Thanks for the patch. Comments bellow.

> @@ -88,7 +89,8 @@ struct snd_card {
>   	char irq_descr[32];		/* Interrupt description */
>   	char mixername[80];		/* mixer name */
>   	char components[128];		/* card components delimited with
> -								space */
> +								space, truncated to 127 chars */
> +	char components_extended[SNDRV_CTL_COMPONENTS_LEN];	/* full card components string */

I would remove the original components[128] string and replace it
with dynamically allocated 'char *components_ptr' with 'unsigned int 
components_ptr_alloc' to store allocated size. Then the snd_component_add() 
function may allocate (and resize) dynamically the components_ptr on demand 
(using e.g. 32 byte step). This will also catch all drivers using the original 
struct member when compiled and make the memory usage a little bit happier.

> +/*
> + * Card components can exceed the fixed 128 bytes in snd_ctl_card_info.
> + * Use SNDRV_CTL_IOCTL_CARD_COMPONENTS to retrieve the full string.
> + */
> +#define SNDRV_CTL_COMPONENTS_LEN	512

This define seems not much useful. The sizeof() on the structure member works 
here.

> +struct snd_ctl_card_components {
> +	int card;			/* card number */
> +	unsigned int length;		/* returned length of components string */

... not used ... Zero terminated string is enough IMHO.

> +	unsigned char components[SNDRV_CTL_COMPONENTS_LEN];
> +};

					Jaroslav

-- 
Jaroslav Kysela <perex@...ex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.