New subject: [Bug 18740] Use of "unsafe" subset of ALSA causes problems with PulseAudio

2 Jun 2009


      http://bugs.winehq.org/show_bug.cgi?id=18740
Summary: Use of "unsafe" subset of ALSA causes problems with
                    PulseAudio
           Product: Wine
           Version: 1.1.22
          Platform: PC
               URL: http://0pointer.de/blog/projects/guide-to-sound-apis.h
                    tml
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: -unknown
        AssignedTo: wine-bugs@winehq.org
        ReportedBy: scott@open-vote.org
            Blocks: 10910
http://0pointer.de/blog/projects/guide-to-sound-apis.html
If we want PulseAudio to work well, we have to limit ourselves to the safe
subset of the ALSA API.
DONTS:
* Do not use "async handlers", e.g. via snd_async_add_pcm_handler() and
friends. Asynchronous handlers are implemented using POSIX signals, which is a
very questionable use of them, especially from libraries and plugins. Even when
you don't want to limit yourself to the safe ALSA subset it is highly
recommended not to use this functionality. Read this for a longer explanation
why signals for audio IO are evil.
    * Do not parse the ALSA configuration file yourself or with any of the ALSA
functions such as snd_config_xxx(). If you need to enumerate audio devices use
snd_device_name_hint() (and related functions). That is the only API that also
supports enumerating non-hardware audio devices and audio devices with drivers
implemented in userspace.
    * Do not parse any of the files from /proc/asound/. Those files only
include information about kernel sound drivers -- user-space plugins are not
listed there. Also, the set of kernel devices might differ from the way they
are presented in user-space. (i.e. sub-devices are mapped in different ways to
actual user-space devices such as surround51 an suchlike.
    * Do not rely on stable device indexes from ALSA. Nowadays they depend on
the initialization order of the drivers during boot-up time and are thus not
stable.
    * Do not use the snd_card_xxx() APIs. For enumerating use
snd_device_name_hint() (and related functions). snd_card_xxx() is obsolete. It
will only list kernel hardware devices. User-space devices such as sound
servers, Bluetooth audio are not included. snd_card_load() is completely
obsolete in these days.
    * Do not hard-code device strings, especially not hw:0 or plughw:0 or even
dmix -- these devices define no channel mapping and are mapped to raw kernel
devices. It is highly recommended to use exclusively default as device string.
If specific channel mappings are required the correct device strings should be
front for stereo, surround40 for Surround 4.0, surround41, surround51, and so
on. Unfortunately at this point ALSA does not define standard device names with
channel mappings for non-kernel devices. This means default may only be used
safely for mono and stereo streams. You should probably prefix your device
string with plug: to make sure ALSA transparently reformats/remaps/resamples
your PCM stream for you if the hardware/backend does not support your sampling
parameters natively.
    * Do not assume that any particular sample type is supported except the
following ones: U8, S16_LE, S16_BE, S32_LE, S32_BE, FLOAT_LE, FLOAT_BE, MU_LAW,
A_LAW.
    * Do not use snd_pcm_avail_update() for synchronization purposes. It should
be used exclusively to query the amount of bytes that may be written/read right
now. Do not use snd_pcm_delay() to query the fill level of your playback
buffer. It should be used exclusively for synchronisation purposes. Make sure
you fully understand the difference, and note that the two functions return
values that are not necessarily directly connected!
    * Do not assume that the mixer controls always know dB information.
    * Do not assume that all devices support MMAP style buffer access.
    * Do not assume that the hardware pointer inside the (possibly mmaped)
playback buffer is the actual position of the sample in the DAC. There might be
an extra latency involved.
    * Do not try to recover with your own code from ALSA error conditions such
as buffer under-runs. Use snd_pcm_recover() instead.
    * Do not touch buffering/period metrics unless you have specific latency
needs. Develop defensively, handling correctly the case when the backend cannot
fulfill your buffering metrics requests. Be aware that the buffering metrics of
the playback buffer only indirectly influence the overall latency in many
cases. i.e. setting the buffer size to a fixed value might actually result in
practical latencies that are much higher.
    * Do not assume that snd_pcm_rewind() is available and works and to which
degree.
    * Do not assume that the time when a PCM stream can receive new data is
strictly dependant on the sampling and buffering parameters and the resulting
average throughput. Always make sure to supply new audio data to the device
when it asks for it by signalling "writability" on the fd. (And similarly for
capturing)
    * Do not use the "simple" interface snd_spcm_xxx().
    * Do not use any of the functions marked as "obsolete".
    * Do not use the timer, midi, rawmidi, hwdep subsystems.
DOS:
* Use snd_device_name_hint() for enumerating audio devices.
    * Use snd_smixer_xx() instead of raw snd_ctl_xxx()
    * For synchronization purposes use snd_pcm_delay().
    * For checking buffer playback/capture fill level use
snd_pcm_update_avail().
    * Use snd_pcm_recover() to recover from errors returned by any of the ALSA
functions.
    * If possible use the largest buffer sizes the device supports to maximize
power saving and drop-out safety. Use snd_pcm_rewind() if you need to react to
user input quickly.
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[Bug 18740] New: Use of "unsafe" subset of ALSA causes problems with PulseAudio