[PATCH 1/2] pcm: add documentation for -ENODATA error code

[Kai Vehmanen <kai.vehmanen@linux.intel.com>]

This error code can be used e.g. with echo reference PCM devices
(the SND_USE_CASE_MOD_ECHO_REF UCM token).

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
---
 src/pcm/pcm.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/pcm/pcm.c b/src/pcm/pcm.c
index 1064044c9812..488a8dd0493d 100644
--- a/src/pcm/pcm.c
+++ b/src/pcm/pcm.c
@@ -292,6 +292,14 @@ This error can happen when device is physically removed (for example
 some hotplug devices like USB or PCMCIA, CardBus or ExpressCard
 can be removed on the fly).
 
+\par -ENODATA
+
+This error can happen if the device data transfer is dependent on
+an external condition and that condition is not met. For example,
+PCM device for echo reference as described by SND_USE_CASE_MOD_ECHO_REF
+UCM token, may return -ENODATA if the linked playback stream has not been
+started.
+
 \section pcm_params Managing parameters
 
 The ALSA PCM device uses two groups of PCM related parameters. The hardware
-- 
2.26.2