[alsa-devel] [PATCH] ASoC: Fix overflow bug in SOC_DOUBLE_R_SX_TLV

[Stuart Longland]

When SX_TLV widgets are read, if the gain is set to a value below 0dB,
the mixer control is erroniously read as being at maximum volume.

The value read out of the CODEC register is never sign-extended, and
when the minimum value is subtracted (read; added, since the minimum is
negative) the result is a number greater than the maximum allowed value
for the control, and hence it saturates.

Solution: Mask the result so that it "wraps around", emulating
sign-extension.

Signed-off-by: Stuart Longland <redhatter at gentoo.org>
---
 sound/soc/soc-core.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/sound/soc/soc-core.c b/sound/soc/soc-core.c
index a82a797..0470288 100644
--- a/sound/soc/soc-core.c
+++ b/sound/soc/soc-core.c
@@ -2400,8 +2400,8 @@ int snd_soc_get_volsw_2r_sx(struct snd_kcontrol *kcontrol,
 	int val = snd_soc_read(codec, mc->reg) & mask;
 	int valr = snd_soc_read(codec, mc->rreg) & mask;
 
-	ucontrol->value.integer.value[0] = ((val & 0xff)-min);
-	ucontrol->value.integer.value[1] = ((valr & 0xff)-min);
+	ucontrol->value.integer.value[0] = ((val & 0xff)-min) & mask;
+	ucontrol->value.integer.value[1] = ((valr & 0xff)-min) & mask;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(snd_soc_get_volsw_2r_sx);
-- 
1.6.4.4