mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-09-12 13:55:32 +00:00
42f945970a
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> |
||
|---|---|---|
| .. | ||
| oss | ||
| seq | ||
| compress_offload.c | ||
| control.c | ||
| control_compat.c | ||
| ctljack.c | ||
| device.c | ||
| hrtimer.c | ||
| hwdep.c | ||
| hwdep_compat.c | ||
| info.c | ||
| info_oss.c | ||
| init.c | ||
| isadma.c | ||
| jack.c | ||
| Kconfig | ||
| Makefile | ||
| memalloc.c | ||
| memory.c | ||
| misc.c | ||
| pcm.c | ||
| pcm_compat.c | ||
| pcm_dmaengine.c | ||
| pcm_drm_eld.c | ||
| pcm_iec958.c | ||
| pcm_lib.c | ||
| pcm_local.h | ||
| pcm_memory.c | ||
| pcm_misc.c | ||
| pcm_native.c | ||
| pcm_param_trace.h | ||
| pcm_timer.c | ||
| pcm_trace.h | ||
| rawmidi.c | ||
| rawmidi_compat.c | ||
| seq_device.c | ||
| sgbuf.c | ||
| sound.c | ||
| sound_oss.c | ||
| timer.c | ||
| timer_compat.c | ||
| vmaster.c | ||