All drivers in ALSA firewire stack have never reported extra delay for
the runtime of PCM substream. There is some reason, but the main reason
is that the meaning of extra delay differs depending on driver design,
especially for the packet-oriented driver.
Here I define the extra delay for the case of IEC 61883-1/6. It is the
number of PCM frames transferred or should be transferred between the
current isochronous cycle and the isochronous cycle to which the latest
isochronous packet arrived (in IR context) or is scheduled (in IT context).
A commit baa914cd81 ("firewire: add kernel API to access CYCLE_TIME
register") allow unit drivers to read CYCLE_TIME of 1394 OHCI controller.
It allows the drivers to compute the current isochronous cycle.
Additionally, a commit f011712887 ("ALSA: firewire-lib: keep history to
process isochronous packet") enables to save the history processing
packets. It allows the driver to estimate the total number of data blocks
in packets arriving shortly, or calculate the total number of data blocks
in scheduled packets.
Now it is ready. This commit implements the computation of the extra delay.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20230110134933.322794-4-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>
af13842cad