[alsa-devel] alsa-lib and floating point support

[Ricard Wanderlof]

We recently came across an issue with the alsa-lib softvol plugin, which 
requires hardware floating-point (i.e. --with-softfloat not enabled during 
build) support in order to use anything but a set of default values (min 
gain -51 dB, max 0 dB, and 256 gain steps). With no hardware floating 
point support, the plugin just prints an error message when loaded.

static int softvol_load_control(snd_pcm_t *pcm, snd_pcm_softvol_t *svol,
 ...
#ifndef HAVE_SOFT_FLOAT
 ...
                        double v = (pow(10.0, db / 20.0) * (double)(1 << 
VOL_SCALE_SHIFT));
 ...
#else
                SNDERR("Cannot handle the given dB range and resolution");
                return -EINVAL;
#endif
        }
 ...

I'm trying to understand why it is critical to have hardware floating 
point support in this case. The pow() function is very resource hungry, 
true, but the function is only called when the plugin is loaded, and not 
for instance on every sample while the stream is running, so on the whole 
I would expect the impact to be minimal. Is there some other rationale 
that I'm missing?

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30