[alsa-devel] alsa-lib and floating point support
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
On Wed, 20 Sep 2017 12:55:30 +0200, Ricard Wanderlof wrote:
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?
The softfloat option is to avoid calculation in float as much as possible, i.e. alsa-lib will be built without any usage of math library.
Takashi
On Wed, 20 Sep 2017, Takashi Iwai wrote:
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?
The softfloat option is to avoid calculation in float as much as possible, i.e. alsa-lib will be built without any usage of math library.
"as much as possible" sounds like the softvol case would be a reasonable exception, on the other hand, if the goal is to completely avoid using the softfloat library I can see the point. Is the reason to avoid dependency issues, or to minimize the total resulting code size, especially on systems with limited memory?
/Ricard
On Wed, 20 Sep 2017 16:45:46 +0200, Ricard Wanderlof wrote:
On Wed, 20 Sep 2017, Takashi Iwai wrote:
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?
The softfloat option is to avoid calculation in float as much as possible, i.e. alsa-lib will be built without any usage of math library.
"as much as possible" sounds like the softvol case would be a reasonable exception, on the other hand, if the goal is to completely avoid using the softfloat library I can see the point. Is the reason to avoid dependency issues, or to minimize the total resulting code size, especially on systems with limited memory?
Well, actually no, because we don't link with -lm when the option is specified, so far. That is, the softfloat option is for controlling whether to allow the usage of math function or not.
Takashi
participants (2)
-
Ricard Wanderlof -
Takashi Iwai