[alsa-devel] [PATCH 3/3] ALSA: pcm: conditionally avoid mmap of control data

[Takashi Sakamoto]

On May 16 2017 15:34, Takashi Iwai wrote:
>>> 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 :)

Although SYNC_PTR is in the fallback mechanism of alsa-lib, HWSYNC is 
not in.

(alsa-lib)
ioctl(SNDRV_PCM_IOCTL_HWSYNC)
<-snd_pcm_hw_hw_sync()
= struct snd_pcm_fast_ops.hwsync
   <-__snd_pcm_hw_hwsync()
     <-snd_pcm_hwsync()
     <-snd_pcm_avail()
       <-snd_pcm_read_areas()
         <-snd_pcm_mmap_readi()
         <-snd_pcm_mmap_readn()
     <-snd_pcm_avail_delay()
     <-snd_pcm_write_areas()
       <-snd_pcm_mmap_writei()
       <-snd_pcm_mmap_writen()

For a case that applications execute ioctl(2) with 
SNDRV_PCM_IOCTL_READN/READI/WRITEN/WRITEI, in kernel land 
snd_pcm_lib_read1()/snd_pcm_lib_write1() should have calls of 'struct 
snd_pcm_ops.pointer', I think.


Regards

Takashi Sakamoto