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ALSA: pcm: Add the explicit appl_ptr sync support

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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>
This commit is contained in:
Takashi Iwai 2017-06-19 22:39:18 +02:00
parent c2d6af53a4
commit 42f945970a
2 changed files with 22 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/uapi/sound/asound.h
View File

@ -268,6 +268,7 @@ typedef int __bitwise snd_pcm_subformat_t;
#define SNDRV_PCM_INFO_MMAP_VALID 0x00000002 /* period data are valid during transfer */
#define SNDRV_PCM_INFO_DOUBLE 0x00000004 /* Double buffering needed for PCM start/stop */
#define SNDRV_PCM_INFO_BATCH 0x00000010 /* double buffering */
#define SNDRV_PCM_INFO_SYNC_APPLPTR 0x00000020 /* need the explicit sync of appl_ptr update */
#define SNDRV_PCM_INFO_INTERLEAVED 0x00000100 /* channels are interleaved */
#define SNDRV_PCM_INFO_NONINTERLEAVED 0x00000200 /* channels are not interleaved */
#define SNDRV_PCM_INFO_COMPLEX 0x00000400 /* complex frame organization (mmap only) */

23
sound/core/pcm_native.c
View File

@ -3376,10 +3376,29 @@ static int snd_pcm_mmap_control(struct snd_pcm_substream *substream, struct file
area->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
return 0;
}
static bool pcm_status_mmap_allowed(struct snd_pcm_file *pcm_file)
{
if (pcm_file->no_compat_mmap)
return false;
/* Disallow the status/control mmap when SYNC_APPLPTR flag is set;
* it enforces the user-space to fall back to snd_pcm_sync_ptr(),
* thus it effectively assures the manual update of appl_ptr.
* In theory, it should be enough to disallow only PCM control mmap,
* but since the current alsa-lib implementation requires both status
* and control mmaps always paired, we have to disable both of them.
*/
if (pcm_file->substream->runtime->hw.info & SNDRV_PCM_INFO_SYNC_APPLPTR)
return false;
return true;
}
#else /* ! coherent mmap */
/*
* don't support mmap for status and control records.
*/
#define pcm_status_mmap_allowed(pcm_file) false
static int snd_pcm_mmap_status(struct snd_pcm_substream *substream, struct file *file,
struct vm_area_struct *area)
{
@ -3563,11 +3582,11 @@ static int snd_pcm_mmap(struct file *file, struct vm_area_struct *area)
offset = area->vm_pgoff << PAGE_SHIFT;
switch (offset) {
case SNDRV_PCM_MMAP_OFFSET_STATUS:
if (pcm_file->no_compat_mmap)
if (!pcm_status_mmap_allowed(pcm_file))
return -ENXIO;
return snd_pcm_mmap_status(substream, file, area);
case SNDRV_PCM_MMAP_OFFSET_CONTROL:
if (pcm_file->no_compat_mmap)
if (!pcm_status_mmap_allowed(pcm_file))
return -ENXIO;
return snd_pcm_mmap_control(substream, file, area);
default: