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