[alsa-devel] [PATCH] pcm: softvol: fix conversion of TLVs min_db and max_dB value

[Jörg Krause]

Both, min_dB and max_dB, are floating type whereas the TLV is (always)
unsigned.

The problem with the conversion of a negative floating-point number into an
unsigned integer is, that the behavior is undefined. This may, depending on
the platform, result in a wrong TLV, i.e. for the default values of min_dB
(-51dB) and max_dB (0dB), alsactl generates the following state on an ARM
cpu build with GCC:

	control.1 {
                iface MIXER
                name Master
                value.0 255
                value.1 255
                comment {
                        access 'read write user'
                        type INTEGER
                        count 2
                        range '0 - 255'
                        tlv '00000001000000080000000000000014'
                        dbmin 0
                        dbmax 5100
                        dbvalue.0 5100
                        dbvalue.1 5100
                }
        }

With the fix applied, alsactl stores the correct TLV:

	control.1 {
                iface MIXER
                name Master
                value.0 255
                value.1 255
                comment {
                        access 'read write user'
                        type INTEGER
                        count 2
                        range '0 - 255'
                        tlv '0000000100000008ffffec1400000014'
                        dbmin -5100
                        dbmax 0
                        dbvalue.0 0
                        dbvalue.1 0
                }
        }

Also tested for different combinations of min_dB and max_dB other than the
default values.

Replaces:
http://mailman.alsa-project.org/pipermail/alsa-devel/2016-May/107733.html

Fixes:
http://mailman.alsa-project.org/pipermail/alsa-devel/2016-May/107628.html

Cc: Clemens Ladisch <clemens at ladisch.de>
Signed-off-by: Jörg Krause <joerg.krause at embedded.rocks>
---
 src/pcm/pcm_softvol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/pcm/pcm_softvol.c b/src/pcm/pcm_softvol.c
index 802aa4b..061080d 100644
--- a/src/pcm/pcm_softvol.c
+++ b/src/pcm/pcm_softvol.c
@@ -658,8 +658,8 @@ static int add_tlv_info(snd_pcm_softvol_t *svol, snd_ctl_elem_info_t *cinfo)
 	unsigned int tlv[4];
 	tlv[0] = SND_CTL_TLVT_DB_SCALE;
 	tlv[1] = 2 * sizeof(int);
-	tlv[2] = svol->min_dB * 100;
-	tlv[3] = (svol->max_dB - svol->min_dB) * 100 / svol->max_val;
+	tlv[2] = (int)(svol->min_dB * 100);
+	tlv[3] = (int)((svol->max_dB - svol->min_dB) * 100 / svol->max_val);
 	return snd_ctl_elem_tlv_write(svol->ctl, &cinfo->id, tlv);
 }
 
-- 
2.8.2