[alsa-devel] [PATCH 1/2] ALSA: firewire: process packets in 'struct snd_pcm_ops.ack' callback

[Takashi Iwai]

On Wed, 07 Jun 2017 07:59:20 +0200,
Takashi Iwai wrote:
> 
> On Wed, 07 Jun 2017 02:38:05 +0200,
> Takashi Sakamoto wrote:
> > 
> > In recent commit for ALSA PCM core, some arrangement is done for
> > 'struct snd_pcm_ops.ack' callback. This is called when appl_ptr is
> > explicitly moved in intermediate buffer for PCM frames, except for
> > some cases described later.
> > 
> > For drivers in ALSA firewire stack, usage of this callback has a merit to
> > reduce latency between time of PCM frame queueing and handling actual
> > packets in recent isochronous cycle, because no need to wait for software
> > IRQ context from isochronous context of OHCI 1394.
> > 
> > If this works well in a case that mapped page frame is used for the
> > intermediate buffer, user process should execute some commands for ioctl(2)
> > to tell the number of handled PCM frames in the intermediate buffer just
> > after handling them.
> 
> This is one thing that was raised in the discussion with Intel people,
> and my suggestion was to add a new flag to suppress the status/control
> mmap like pcm_file->no_compat_mmap.  Then alsa-lib falls back to the
> sync_ptr ioctl, and the driver can catch each appl_ptr update.

Now I considered this again, and concluded that a simple patch like
below should suffice.

Adding Intel people to Cc, who raised the issue originally.


thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai at suse.de>
Subject: [PATCH] ALSA: pcm: Suppress status/control mmap when ack ops is
 present

The drivers using PCM ack ops require the notification whenever
appl_ptr is updated in general.  But when the PCM status/control page
is mmapped, this notification doesn't happen, per design, thus it's
not guaranteed to receive the fine-grained updates.

For improving the situation, this patch simply suppresses the PCM
status/control mmap when ack ops is defined.  At least, for all
existing drivers with ack, this should give more benefit.

Once when we really need the full optimization with status/control
mmap even using ack ops, we may reconsider the check, e.g. introducing
a new flag.  But, so far, this should be good enough.

Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/pcm_native.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 2bde07a4a87f..b993b0420411 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -3189,6 +3189,10 @@ static int snd_pcm_mmap_status(struct snd_pcm_substream *substream, struct file
 			       struct vm_area_struct *area)
 {
 	long size;
+
+	/* suppress status/control mmap when driver requires ack */
+	if (substream->ops->ack)
+		return -ENXIO;
 	if (!(area->vm_flags & VM_READ))
 		return -EINVAL;
 	size = area->vm_end - area->vm_start;
@@ -3225,6 +3229,10 @@ static int snd_pcm_mmap_control(struct snd_pcm_substream *substream, struct file
 				struct vm_area_struct *area)
 {
 	long size;
+
+	/* suppress status/control mmap when driver requires ack */
+	if (substream->ops->ack)
+		return -ENXIO;
 	if (!(area->vm_flags & VM_READ))
 		return -EINVAL;
 	size = area->vm_end - area->vm_start;
-- 
2.13.0