Re: [PATCH 3/3] ALSA: pcm: conditionally avoid mmap of control data

[Takashi Iwai <tiwai@suse.de>]

On Tue, 16 May 2017 08:24:39 +0200,
Takashi Sakamoto wrote:
> 
> On May 16 2017 15:18, Takashi Iwai wrote:
> > On Tue, 16 May 2017 07:57:30 +0200,
> > Takashi Sakamoto wrote:
> >>
> >> On May 16 2017 14:46, Takashi Iwai wrote:
> >>> In this case, the problem is that the mmap control allows the appl_ptr
> >>> being changed silently without interaction with the driver.  If the
> >>> driver requires some explicit action for changing the appl_ptr, it
> >>> won't work.
> >>
> >> I think 'struct snd_pcm_ops.pointer' is available for this purpose, too.
> >>
> >> static snd_pcm_sframes_t snd_pcm_playback_rewind(...)
> >> {
> >>     ...
> >>     hw_avail = snd_pcm_playback_hw_avail(runtime);
> >>     (->struct snd_pcm_ops.pointer())
> >>     ...
> >>     (rewind PCM frames)
> >>     ...
> >> +   substream->ops->pointer(...);
> >>     (->struct snd_pcm_ops.pointer())
> >>     ...
> >> }
> >>
> >> If drivers need to handle event to update the appl_ptr, it records
> >> value of the appl_ptr, then compare it to current value to get the
> >> updates.
> >
> > IIRC, the problem isn't about the forward / rewind but about the
> > normal data transfer.  In mmap mode, we transfer data on the mmap
> > buffer, and update appl_ptr via mmap control.  Both are done without
> > notification to the driver (which is intentional for avoiding the
> > context switching).
> >
> > So we want to disable this optimization and always notify to the
> > driver.  Disabling mmap status/control is the straight hack as it
> > falls back to ioctl and then the driver can know the change.
> 
> There's SNDRV_PCM_IOCTL_HW_SYNC command. In kernel land
> implementation, this command is handled with a call of 'struct
> snd_pcm_ops.pointer'.
> 
> In alsa-lib, this command is often executed in most cases to handle
> PCM frames.

The HWSYNC or SYNC_PTR ioctls are used only as the fallback when mmap
failed.  It's the exact goal of this patch :)


Takashi