[alsa-devel] [alsa-lib][PATCH] ASoC: hdmi-codec: use unsigned type to structure members with bit-field instead of signed type

[Takashi Sakamoto]

This is a fix for Linux 4.10-rc1.

In C language specification, a bit-field is interpreted as a signed or
unsigned integer type consisting of the specified number of bits.

In GCC manual, the range of a signed bit field of N bits is from
-(2^N) / 2 to ((2^N) / 2) - 1
https://www.gnu.org/software/gnu-c-manual/gnu-c-manual.html#Bit-Fields

Therefore, when defined as 1 bit-field with signed type, variables can
represents -1 and 0.

The snd-soc-hdmi-codec module includes a structure which has signed type
members with bit-fields. Codes of this module assign 0 and 1 to the
members. This seems to result in implementation-dependent behaviours.

As of v4.10-rc1 merge window, outside of sound subsystem, this structure
is referred by below GPU modules.
 - tda998x
 - sti-drm
 - mediatek-drm-hdmi
 - msm

As long as I review their codes relevant to the structure, the structure
members are used just for condition statements and printk formats.
My proposal of change is a bit intrusive to the printk formats but this
may be acceptable.

Totally, it's reasonable to use unsigned type for the structure members.
This bug is detected by Sparse, static code analyzer with below warnings.

./include/sound/hdmi-codec.h:39:26: error: dubious one-bit signed bitfield
./include/sound/hdmi-codec.h:40:28: error: dubious one-bit signed bitfield
./include/sound/hdmi-codec.h:41:29: error: dubious one-bit signed bitfield
./include/sound/hdmi-codec.h:42:31: error: dubious one-bit signed bitfield

CC: Jyri Sarha <jsarha at ti.com>
CC: Jie Qiu <jie.qiu at mediatek.com>
CC: Arnaud Pouliquen <arnaud.pouliquen at st.com>
CC: Srinivas Kandagatla <srinivas.kandagatla at linaro.org>
CC: dri-devel at lists.freedesktop.org
CC: stable at vger.kernel.org
Fixes: 09184118a8ab ("ASoC: hdmi-codec: Add hdmi-codec for external HDMI-encoders")
Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
---
 include/sound/hdmi-codec.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/sound/hdmi-codec.h b/include/sound/hdmi-codec.h
index 530c57b..915c435 100644
--- a/include/sound/hdmi-codec.h
+++ b/include/sound/hdmi-codec.h
@@ -36,10 +36,10 @@ struct hdmi_codec_daifmt {
 		HDMI_AC97,
 		HDMI_SPDIF,
 	} fmt;
-	int bit_clk_inv:1;
-	int frame_clk_inv:1;
-	int bit_clk_master:1;
-	int frame_clk_master:1;
+	unsigned int bit_clk_inv:1;
+	unsigned int frame_clk_inv:1;
+	unsigned int bit_clk_master:1;
+	unsigned int frame_clk_master:1;
 };
 
 /*
-- 
2.9.3