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authorTakashi Iwai <tiwai@suse.de>2017-06-19 22:39:18 +0200
committerTakashi Iwai <tiwai@suse.de>2017-06-23 15:39:47 +0200
commit42f945970af9df6216e3d771b4df371d02d8742c (patch)
treee0737461cdf10f91602f80eea364b7f6fd542c27 /sound/core
parentc2d6af53a43fd8bb528eac8f31ffb666e9c74cf7 (diff)
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>
Diffstat (limited to 'sound/core')
-rw-r--r--sound/core/pcm_native.c23
1 files changed, 21 insertions, 2 deletions
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index d35c6614fdab..9ade0c8b54a3 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -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: