ALSA: control: add ioctl to retrieve full card components

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 SNDRV_CTL_IOCTL_CARD_BYTES that carries a variable-length payload
selected by a type discriminator. The first defined type
SND_CTL_CARD_BTYPE_COMPONENTS returns the full components string. The
ioctl is designed to be reused for other variable-length card payloads
in the future.

The user-space caller may set data_allocated == 0 (or data == NULL) to
query the required length; otherwise the kernel copies the payload into
the user buffer and writes back the actual length in data_len.

When the legacy components field in struct snd_ctl_card_info is
truncated, '>' is written just before the NUL terminator to signal to
user-space that the full string is available via the new ioctl.

card->components is now dynamically allocated and grown in 32 byte
increments via krealloc(), capped at 512 bytes.

Link: https://github.com/alsa-project/alsa-lib/pull/494
Suggested-by: Jaroslav Kysela <perex@perex.cz>
Suggested-by: Takashi Iwai <tiwai@suse.com>
Signed-off-by: Maciej Strozek <mstrozek@opensource.cirrus.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260720103505.1860399-2-mstrozek@opensource.cirrus.com

3 files changed, 26 insertions, 3 deletions

diff --git a/include/sound/control.h b/include/sound/control.h
index e07f6b960641..909db0d0485d 100644
--- a/include/sound/control.h
+++ b/include/sound/control.h
@@ -7,6 +7,7 @@
  *  Copyright (c) by Jaroslav Kysela <perex@perex.cz>
  */
 
+#include <linux/rwsem.h>
 #include <linux/wait.h>
 #include <linux/nospec.h>
 #include <sound/asound.h>
@@ -167,6 +168,8 @@ snd_ctl_find_id_mixer(struct snd_card *card, const char *name)
 
 int snd_ctl_create(struct snd_card *card);
 
+extern struct rw_semaphore snd_ioctl_rwsem;
+
 int snd_ctl_register_ioctl(snd_kctl_ioctl_func_t fcn);
 int snd_ctl_unregister_ioctl(snd_kctl_ioctl_func_t fcn);
 #ifdef CONFIG_COMPAT
diff --git a/include/sound/core.h b/include/sound/core.h
index 404785b7d885..2ca24ac7e37f 100644
--- a/include/sound/core.h
+++ b/include/sound/core.h
@@ -108,8 +108,8 @@ struct snd_card {
 	char longname[80];		/* name of this soundcard */
 	char irq_descr[32];		/* Interrupt description */
 	char mixername[80];		/* mixer name */
-	char components[128];		/* card components delimited with
-								space */
+	char *components;		/* card components, space-delimited */
+	unsigned int components_alloc_size;	/* current allocation size of components */
 	struct module *module;		/* top-level module */
 
 	void *private_data;		/* private data for soundcard */
diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
index d3ce75ba938a..500599213f93 100644
--- a/include/uapi/sound/asound.h
+++ b/include/uapi/sound/asound.h
@@ -1058,7 +1058,7 @@ struct snd_timer_tread {
  *                                                                          *
  ****************************************************************************/
 
-#define SNDRV_CTL_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 9)
+#define SNDRV_CTL_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 10)
 
 struct snd_ctl_card_info {
 	int card;			/* card number */
@@ -1072,6 +1072,25 @@ struct snd_ctl_card_info {
 	unsigned char components[128];	/* card components / fine identification, delimited with one space (AC97 etc..) */
 };
 
+/*
+ * Card components can exceed the fixed 128 bytes in snd_ctl_card_info.
+ * Use SNDRV_CTL_IOCTL_CARD_BYTES with type SND_CTL_CARD_BTYPE_COMPONENTS
+ * to retrieve the full string.
+ */
+
+/* Type values for struct snd_ctl_card_bytes::type */
+enum {
+	SND_CTL_CARD_BTYPE_COMPONENTS	= 1,	/* full card components string */
+};
+
+struct snd_ctl_card_bytes {
+	__u32 type;				/* SND_CTL_CARD_BTYPE_* */
+	__u32 data_allocated;	/* size of @data buffer in bytes */
+	__u32 data_len;			/* in/out: actual data length in bytes */
+	__u32 reserved;			/* explicit pad */
+	__u64 data;				/* user buffer (pointer stored as __u64) */
+};
+
 typedef int __bitwise snd_ctl_elem_type_t;
 #define	SNDRV_CTL_ELEM_TYPE_NONE	((__force snd_ctl_elem_type_t) 0) /* invalid */
 #define	SNDRV_CTL_ELEM_TYPE_BOOLEAN	((__force snd_ctl_elem_type_t) 1) /* boolean type */
@@ -1198,6 +1217,7 @@ struct snd_ctl_tlv {
 
 #define SNDRV_CTL_IOCTL_PVERSION	_IOR('U', 0x00, int)
 #define SNDRV_CTL_IOCTL_CARD_INFO	_IOR('U', 0x01, struct snd_ctl_card_info)
+#define SNDRV_CTL_IOCTL_CARD_BYTES	_IOWR('U', 0x02, struct snd_ctl_card_bytes)
 #define SNDRV_CTL_IOCTL_ELEM_LIST	_IOWR('U', 0x10, struct snd_ctl_elem_list)
 #define SNDRV_CTL_IOCTL_ELEM_INFO	_IOWR('U', 0x11, struct snd_ctl_elem_info)
 #define SNDRV_CTL_IOCTL_ELEM_READ	_IOWR('U', 0x12, struct snd_ctl_elem_value)