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.
Signed-off-by: Takashi Iwai tiwai@suse.de --- include/uapi/sound/asound.h | 1 + sound/core/pcm_native.c | 23 +++++++++++++++++++++-- 2 files changed, 22 insertions(+), 2 deletions(-)
diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h index fd41697cb4d3..7eee52eb7462 100644 --- a/include/uapi/sound/asound.h +++ b/include/uapi/sound/asound.h @@ -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) */ 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: