assert() on valid values, and handle acceptable NULL arguments
gracefully.
Note audio_driver_write() and audio_driver_read() used to return
size (with a "XXX: Consider options") when sw was NULL. Imho, it is more
correct to not pretend to have read/written anything by default in the
AudioBackend base abstract class.
Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
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