ALSA: pcm: Add the explicit appl_ptr sync support (42f94597) · Commits · 戴 / test

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>

include/uapi/sound/asound.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -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) */

sound/core/pcm_native.c

+21 −2

Original line number	Diff line number	Diff line
		@@ -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:

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