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linux-next/sound/core
Takashi Iwai 42f945970a ALSA: pcm: Add the explicit appl_ptr sync support
Currently x86 platforms use the PCM status/control mmaps for
transferring the PCM status and appl_ptr between kernel and
user-spaces.  The mmap is a most efficient way of communication, but
it has a drawback per its nature, namely, it can't notify the change
explicitly to kernel.

The lack of appl_ptr update notification is a problem on a few
existing drivers, but it's mostly a small issue and negligible.
However, a new type of driver that uses DSP for a deep buffer
management requires the exact position of appl_ptr for calculating the
buffer prefetch size, and the asynchronous appl_ptr update between
kernel and user-spaces becomes a significant problem for it.

How can we enforce user-space to report the appl_ptr update?  The way
is relatively simple.  Just by disabling the PCM control mmap, the
user-space is supposed to fall back to the mode using SYNC_PTR ioctl,
and the kernel gets control over that.  This fallback mode is used in
all non-x86 platforms as default, and also in the 32bit compatible
model on all platforms including x86.  It's been implemented already
over a decade, so we can say it's fairly safe and stably working.

With the help of the knowledge above, this patch introduces a new PCM
info flag SNDRV_PCM_INFO_SYNC_APPLPTR for achieving the appl_ptr sync
from user-space.  When a driver sets this flag at open, the PCM status
/ control mmap is disabled, which effectively switches to SYNC_PTR
mode in user-space side.

In this version, both PCM status and control mmaps are disabled
although only the latter, control mmap, is the target.  It's because
the current alsa-lib implementation supposes that both status and
control mmaps are always coupled, thus it handles a fatal error when
only one of them fails.

Of course, the disablement of the status/control mmaps may bring a
slight performance overhead.  Thus, as of now, this should be used
only for the dedicated devices that deserves.

Note that the disablement of mmap is a sort of workaround.  In the
later patch, we'll introduce the way to identify the protocol version
alsa-lib supports, and keep mmap working while the sync_ptr is
performed together.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
2017-06-23 15:39:47 +02:00
..
oss ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
seq ALSA: seq: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:03 +02:00
compress_offload.c ALSA: compress: fix some missing and misplaced \n in messages 2016-09-16 19:24:13 +02:00
control_compat.c ALSA: ctl: change return value in compatibility layer so that it's the same value in core implementation 2016-03-17 14:11:36 +01:00
control.c ALSA: control: remove entry limitation for list operation 2017-05-24 10:18:28 +02:00
ctljack.c ALSA: declare snd_kcontrol_new structures as const 2017-05-30 10:29:25 +02:00
device.c Merge branch 'topic/hda-unbind' into for-next 2015-03-16 14:48:20 +01:00
hrtimer.c ktime: Get rid of the union 2016-12-25 17:21:22 +01:00
hwdep_compat.c [PATCH] hwdep_compat missed __user annotations 2006-10-10 15:37:21 -07:00
hwdep.c sched/headers: Prepare to move signal wakeup & sigpending methods from <linux/sched.h> into <linux/sched/signal.h> 2017-03-02 08:42:32 +01:00
info_oss.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
info.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
init.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
isadma.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
jack.c ALSA: jack: Allow building the jack layer without input device 2016-02-23 09:03:07 +01:00
Kconfig ALSA: seq: Allow the modular sequencer registration 2017-06-12 08:43:33 +02:00
Makefile ALSA: seq: Allow the modular sequencer registration 2017-06-12 08:43:33 +02:00
memalloc.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
memory.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
misc.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
pcm_compat.c ALSA: pcm: Remove set_fs() in PCM core code 2017-05-23 07:04:05 +02:00
pcm_dmaengine.c ASoC: dmaengine_pcm: Add support for packed transfers 2016-04-27 17:34:11 +01:00
pcm_drm_eld.c ALSA: pcm: use helper function to refer parameter as read-only 2017-05-17 07:24:39 +02:00
pcm_iec958.c ALSA: pcm: Allow 32 bit sample format in IEC958 channel status helper 2016-04-06 14:33:38 -07:00
pcm_lib.c ALSA: pcm: Fix possible inconsistent appl_ptr update via mmap 2017-06-20 07:55:59 +02:00
pcm_local.h ALSA: pcm: unify codes to operate application-side position on PCM buffer 2017-06-12 08:49:22 +02:00
pcm_memory.c ALSA: pcm: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:18:58 +02:00
pcm_misc.c ALSA: pcm: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:18:58 +02:00
pcm_native.c ALSA: pcm: Add the explicit appl_ptr sync support 2017-06-23 15:39:47 +02:00
pcm_param_trace.h ALSA: pcm: use friendly name for id of PCM substream in trace print 2017-06-09 16:27:23 +02:00
pcm_timer.c ALSA: pcm: include pcm_local.h and remove some extraneous tabs 2017-05-30 18:04:47 +02:00
pcm_trace.h ALSA: pcm: use %s instead of %c for format of PCM buffer tracepoints 2017-06-12 08:49:24 +02:00
pcm.c ALSA: pcm: add local header file for snd-pcm module 2017-05-26 08:38:14 +02:00
rawmidi_compat.c ALSA: rawmidi: Fix ioctls X32 ABI 2016-02-28 17:44:51 +01:00
rawmidi.c ALSA: seq: Allow the modular sequencer registration 2017-06-12 08:43:33 +02:00
seq_device.c ALSA: seq: Allow the modular sequencer registration 2017-06-12 08:43:33 +02:00
sgbuf.c ALSA: core: Deletion of unnecessary checks before two function calls 2014-11-21 20:06:57 +01:00
sound_oss.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
sound.c ALSA: core: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:16 +02:00
timer_compat.c ALSA: timer: fix gparams ioctl compatibility for different architectures 2016-03-23 08:06:16 +01:00
timer.c ALSA: timer: Follow standard EXPORT_SYMBOL() declarations 2017-06-16 16:19:10 +02:00
vmaster.c ALSA: core: Use standard printk helpers 2014-02-14 08:14:15 +01:00