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

[Vinod Koul <vinod.koul@intel.com>]

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@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.

Yes this makes sense and we tested it for us, looks good

Reveiwed-by: Vinod Koul <vinod.koul@intel.com>
Tested-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>

> 
> Signed-off-by: Takashi Iwai <tiwai@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
> 

-- 
~Vinod