[PATCH AUTOSEL 6.19-5.10] ASoC: fsl_easrc: Fix event generation in fsl_easrc_iec958_put_bits()

[Sasha Levin <sashal@kernel.org>]

From: Mark Brown <broonie@kernel.org>

[ Upstream commit 54a86cf48eaa6d1ab5130d756b718775e81e1748 ]

ALSA controls should return 1 if the value in the control changed but the
control put operation fsl_easrc_iec958_put_bits() unconditionally returns
0, causing ALSA to not generate any change events. This is detected by
mixer-test with large numbers of messages in the form:

    No event generated for Context 3 IEC958 CS5
    Context 3 IEC958 CS5.0 orig 5224 read 5225, is_volatile 0

Add a suitable check.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://patch.msgid.link/20260205-asoc-fsl-easrc-fix-events-v1-1-39d4c766918b@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

The driver has been present since v5.8 (2020), so it exists in all
current stable trees.

## Analysis

**What the commit fixes:**
The `fsl_easrc_iec958_put_bits()` function is an ALSA control put
callback. Per the ALSA API contract, put callbacks must return 1 when
the control value changes and 0 when it doesn't. This function
unconditionally returned 0, which meant ALSA never generated change
events for IEC958 controls on this hardware. The fix adds a simple
comparison before the assignment and returns whether the value changed.

**Bug category:** Incorrect return value / missing event generation — a
real functional bug.

**Scope and risk:**
- 3 lines changed (adding `int ret`, comparing old vs new value,
  returning `ret` instead of `0`)
- Purely local to one function in one driver
- The fix is obviously correct — standard ALSA pattern used across
  hundreds of drivers
- Zero risk of regression — the only behavioral change is that change
  events are now properly generated

**Author:** Mark Brown, the ASoC subsystem maintainer. He is both the
author and committer, demonstrating high confidence in the fix.

**Stable criteria:**
- Obviously correct: Yes — trivial pattern
- Fixes real bug: Yes — missing change events mean userspace
  applications (mixers, audio daemons) don't get notified of control
  changes
- Small and contained: Yes — 3 lines in one function
- No new features: Correct — just fixes existing behavior to match API
  contract
- Tested: Detected by mixer-test, fix validated by same

**User impact:** Any system using the NXP/Freescale EASRC (Enhanced
Asynchronous Sample Rate Converter) where userspace monitors IEC958
control changes would silently miss updates. This affects audio
applications relying on ALSA event notifications.

## Verification

- Read the current code at `sound/soc/fsl/fsl_easrc.c:46-62` — confirmed
  the fix is applied and is the standard pattern (compare before assign,
  return difference)
- `git log master --reverse` confirmed driver was introduced in commit
  955ac624058f9 (2020-04-16, v5.8 cycle), so it exists in all maintained
  stable trees
- The function is referenced at line 125 as `.put =
  fsl_easrc_iec958_put_bits` in a macro, confirming it's used as an ALSA
  control callback
- Author is Mark Brown (broonie@kernel.org), the ASoC maintainer —
  verified from commit metadata
- The fix pattern (checking old != new before returning from put
  callback) is a well-established ALSA convention — no novel logic

**YES**

 sound/soc/fsl/fsl_easrc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/sound/soc/fsl/fsl_easrc.c b/sound/soc/fsl/fsl_easrc.c
index 733374121196e..6c56134c60cc8 100644
--- a/sound/soc/fsl/fsl_easrc.c
+++ b/sound/soc/fsl/fsl_easrc.c
@@ -52,10 +52,13 @@ static int fsl_easrc_iec958_put_bits(struct snd_kcontrol *kcontrol,
 	struct soc_mreg_control *mc =
 		(struct soc_mreg_control *)kcontrol->private_value;
 	unsigned int regval = ucontrol->value.integer.value[0];
+	int ret;
+
+	ret = (easrc_priv->bps_iec958[mc->regbase] != regval);
 
 	easrc_priv->bps_iec958[mc->regbase] = regval;
 
-	return 0;
+	return ret;
 }
 
 static int fsl_easrc_iec958_get_bits(struct snd_kcontrol *kcontrol,
-- 
2.51.0