ALSA: control: Verify put() result when in debug mode

The put() operation is expected to return:
1) 0 on success if no changes were made
2) 1 on success if changes were made
3) error code otherwise

Currently 2) is usually ignored when writing control-operations. While
forcing compliance is not an option right now, make it easier for
developers to adhere to the expectations and notice problems by logging
them when CONFIG_SND_CTL_DEBUG is enabled.

Due to large size of struct snd_ctl_elem_value, 'value_buf' is provided
as a reusable buffer for kctl->put() verification. This prevents
exhausting the stack when verifying the operation.

>From user perspective, patch introduces a new trace/events category
'snd_ctl' containing a single 'snd_ctl_put' event type. Log sample:

  amixer-1086    [003] .....    8.035939: snd_ctl_put: success: expected=0, actual=0 for ctl numid=1, iface=MIXER, name='Master Playback Volume', index=0, device=0, subdevice=0, card=0
  amixer-1087    [003] .....    8.938721: snd_ctl_put: success: expected=1, actual=1 for ctl numid=1, iface=MIXER, name='Master Playback Volume', index=0, device=0, subdevice=0, card=0
  amixer-1088    [003] .....    9.631470: snd_ctl_put: success: expected=1, actual=1 for ctl numid=1, iface=MIXER, name='Master Playback Volume', index=0, device=0, subdevice=0, card=0
  amixer-1089    [000] .....    9.636786: snd_ctl_put: fail: expected=1, actual=0 for ctl numid=5, iface=MIXER, name='Loopback Mute', index=0, device=0, subdevice=0, card=0

Signed-off-by: Cezary Rojewski <cezary.rojewski@intel.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260224205619.584795-1-cezary.rojewski@intel.com

include/sound/core.h

@@ -133,6 +133,9 @@ struct snd_card {
 #ifdef CONFIG_SND_DEBUG
 	struct dentry *debugfs_root;    /* debugfs root for card */
 #endif
+#ifdef CONFIG_SND_CTL_DEBUG
+	struct snd_ctl_elem_value *value_buf; /* buffer for kctl->put() verification */
+#endif
 
 #ifdef CONFIG_PM
 	unsigned int power_state;	/* power state */

sound/core/Makefile

@@ -23,6 +23,7 @@ snd-pcm-$(CONFIG_SND_PCM_IEC958) += pcm_iec958.o
 # for trace-points
 CFLAGS_pcm_lib.o := -I$(src)
 CFLAGS_pcm_native.o := -I$(src)
+CFLAGS_control.o := -I$(src)
 
 snd-pcm-dmaengine-y := pcm_dmaengine.o
 

sound/core/control.c

@@ -19,6 +19,13 @@
 #include <sound/info.h>
 #include <sound/control.h>
 
+#ifdef CONFIG_SND_CTL_DEBUG
+#define CREATE_TRACE_POINTS
+#include "control_trace.h"
+#else
+#define trace_snd_ctl_put(card, kctl, iname, expected, actual)
+#endif
+
 // Max allocation size for user controls.
 static int max_user_ctl_alloc_size = 8 * 1024 * 1024;
 module_param_named(max_user_ctl_alloc_size, max_user_ctl_alloc_size, int, 0444);
@@ -1264,6 +1271,72 @@ static int snd_ctl_elem_read_user(struct snd_card *card,
 	return result;
 }
 
+#if IS_ENABLED(CONFIG_SND_CTL_DEBUG)
+
+static const char *const snd_ctl_elem_iface_names[] = {
+	[SNDRV_CTL_ELEM_IFACE_CARD]		= "CARD",
+	[SNDRV_CTL_ELEM_IFACE_HWDEP]		= "HWDEP",
+	[SNDRV_CTL_ELEM_IFACE_MIXER]		= "MIXER",
+	[SNDRV_CTL_ELEM_IFACE_PCM]		= "PCM",
+	[SNDRV_CTL_ELEM_IFACE_RAWMIDI]		= "RAWMIDI",
+	[SNDRV_CTL_ELEM_IFACE_TIMER]		= "TIMER",
+	[SNDRV_CTL_ELEM_IFACE_SEQUENCER]	= "SEQUENCER",
+};
+
+static int snd_ctl_put_verify(struct snd_card *card, struct snd_kcontrol *kctl,
+			      struct snd_ctl_elem_value *control)
+{
+	struct snd_ctl_elem_value *original = card->value_buf;
+	struct snd_ctl_elem_info info;
+	const char *iname;
+	int ret, retcmp;
+
+	memset(original, 0, sizeof(*original));
+	memset(&info, 0, sizeof(info));
+
+	ret = kctl->info(kctl, &info);
+	if (ret)
+		return ret;
+
+	ret = kctl->get(kctl, original);
+	if (ret)
+		return ret;
+
+	ret = kctl->put(kctl, control);
+	if (ret < 0)
+		return ret;
+
+	/* Sanitize the new value (control->value) before comparing. */
+	fill_remaining_elem_value(control, &info, 0);
+
+	/* With known state for both new and original, do the comparison. */
+	retcmp = memcmp(&original->value, &control->value, sizeof(original->value));
+	if (retcmp)
+		retcmp = 1;
+
+	iname = snd_ctl_elem_iface_names[kctl->id.iface];
+	trace_snd_ctl_put(&kctl->id, iname, card->number, ret, retcmp);
+
+	return ret;
+}
+
+static int snd_ctl_put(struct snd_card *card, struct snd_kcontrol *kctl,
+		       struct snd_ctl_elem_value *control, unsigned int access)
+{
+	if ((access & SNDRV_CTL_ELEM_ACCESS_SKIP_CHECK) ||
+	    (access & SNDRV_CTL_ELEM_ACCESS_VOLATILE))
+		return kctl->put(kctl, control);
+
+	return snd_ctl_put_verify(card, kctl, control);
+}
+#else
+static inline int snd_ctl_put(struct snd_card *card, struct snd_kcontrol *kctl,
+			      struct snd_ctl_elem_value *control, unsigned int access)
+{
+	return kctl->put(kctl, control);
+}
+#endif
+
 static int snd_ctl_elem_write(struct snd_card *card, struct snd_ctl_file *file,
 			      struct snd_ctl_elem_value *control)
 {
@@ -1300,7 +1373,8 @@ static int snd_ctl_elem_write(struct snd_card *card, struct snd_ctl_file *file,
 							   false);
 	}
 	if (!result)
-		result = kctl->put(kctl, control);
+		result = snd_ctl_put(card, kctl, control, vd->access);
+
 	if (result < 0) {
 		up_write(&card->controls_rwsem);
 		return result;

sound/core/control_trace.h

@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM snd_ctl
+
+#if !defined(_TRACE_SND_CTL_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_SND_CTL_H
+
+#include <linux/tracepoint.h>
+#include <uapi/sound/asound.h>
+
+TRACE_EVENT(snd_ctl_put,
+
+	TP_PROTO(struct snd_ctl_elem_id *id, const char *iname, unsigned int card,
+		 int expected, int actual),
+
+	TP_ARGS(id, iname, card, expected, actual),
+
+	TP_STRUCT__entry(
+		__field(unsigned int,	numid)
+		__string(iname,		iname)
+		__string(kname,		id->name)
+		__field(unsigned int,	index)
+		__field(unsigned int,	device)
+		__field(unsigned int,	subdevice)
+		__field(unsigned int,	card)
+		__field(int,		expected)
+		__field(int,		actual)
+	),
+
+	TP_fast_assign(
+		__entry->numid = id->numid;
+		__assign_str(iname);
+		__assign_str(kname);
+		__entry->index = id->index;
+		__entry->device = id->device;
+		__entry->subdevice = id->subdevice;
+		__entry->card = card;
+		__entry->expected = expected;
+		__entry->actual = actual;
+	),
+
+	TP_printk("%s: expected=%d, actual=%d for ctl numid=%d, iface=%s, name='%s', index=%d, device=%d, subdevice=%d, card=%d\n",
+		  __entry->expected == __entry->actual ? "success" : "fail",
+		  __entry->expected, __entry->actual, __entry->numid,
+		  __get_str(iname), __get_str(kname), __entry->index,
+		  __entry->device, __entry->subdevice, __entry->card)
+);
+
+#endif /* _TRACE_SND_CTL_H */
+
+/* This part must be outside protection */
+#undef TRACE_INCLUDE_PATH
+#define TRACE_INCLUDE_PATH .
+#define TRACE_INCLUDE_FILE control_trace
+#include <trace/define_trace.h>

sound/core/init.c

@@ -362,6 +362,11 @@ static int snd_card_init(struct snd_card *card, struct device *parent,
 #ifdef CONFIG_SND_DEBUG
 	card->debugfs_root = debugfs_create_dir(dev_name(&card->card_dev),
 						sound_debugfs_root);
+#endif
+#ifdef CONFIG_SND_CTL_DEBUG
+	card->value_buf = kmalloc(sizeof(*card->value_buf), GFP_KERNEL);
+	if (!card->value_buf)
+		return -ENOMEM;
 #endif
 	return 0;
 
@@ -587,6 +592,9 @@ static int snd_card_do_free(struct snd_card *card)
 	snd_device_free_all(card);
 	if (card->private_free)
 		card->private_free(card);
+#ifdef CONFIG_SND_CTL_DEBUG
+	kfree(card->value_buf);
+#endif
 	if (snd_info_card_free(card) < 0) {
 		dev_warn(card->dev, "unable to free card info\n");
 		/* Not fatal error */