On Wed, Jun 07, 2017 at 11:20:03PM +0200, Takashi Iwai wrote:
> 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@xxxxxxx>
> 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.
Yes this makes sense and we tested it for us, looks good
Reveiwed-by: Vinod Koul <vinod.koul@xxxxxxxxx>
Tested-by: Subhransu S. Prusty <subhransu.s.prusty@xxxxxxxxx>
>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> ---
> 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
>
--
~Vinod
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