On Tue, 28 Apr 2009, Takashi Iwai wrote:
- leave current fifo_size without change (there will be only small FIFO values to notify applications about small extra delays before samples reaches DAC or leaves ADC)
fifo_size field is supposed to be constant, and shouldn't be changed dynamically after open. This is a problem if the FIFO size varies depending on the parameter.
The alsa-lib's documentation states that one configuration must be chosen (thus hw_params should be called) to obtain fifo_size. It might depend on hardware parameters, but is static then. I would not propose to change anything with it.
- add cache_size value to snd_pcm_hardware struct - here will be stored area of buffer which might be cached in the hardware for large FIFOs including prepared USB's URBs (the upper layer should not work with this buffer area - rewind operation etc.)
I already submitted a similar patch quite ago. Actually, it exposes the delay account in snd_pcm_status() delay calculation. The delay value can be dynamically changed.
The current test patches are found on sound unstable tree topic/pcm-delay branch. git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound-unstable-2.6.git
Looks good. Could you pick these patches to your stable tree so we can stack other patches on top? I need to add 'delay' to hdelta in current elapsed callback to avoid false alarms.
Also, John should initialize this runtime->delay to correct value in frames in the mpc driver.
Jaroslav
----- Jaroslav Kysela perex@perex.cz Linux Kernel Sound Maintainer ALSA Project, Red Hat, Inc.