Re: Question about hdac_ext_link ref_count management

[Ranjani Sridharan <ranjani.sridharan@linux.intel.com>]

On Fri, 2021-01-22 at 15:12 +0100, Takashi Iwai wrote:
> On Fri, 22 Jan 2021 00:23:53 +0100,
> Ranjani Sridharan wrote:
> > Hi Takashi,
> > 
> > While exploring some power optimizations on Intel platforms, I
> > noticed
> > that the hdac_ext_link ref_count is incremented during codec probe
> > in hdac_hda_codec_probe() and the ref_count is held until the codec
> > device is removed. 
> > 
> > I was wondering if it would be possible to call the get/put for the
> > hdac_ext_link in the codec runtime suspend/resume callbacks so that
> > the
> > link is powered up only when the link is in use. Are there any
> > downsides to doing this? 
> 
> Wouldn't the  snd_hdac_ext_bus_link_power_up() / down() calls do the
> runtime PM stuff?  Maybe we need to revisit those link power
> management.  The ext stuff isn't well m
> and, I'm afraid.
Thanks, Takashi.
It looks like snd_hdac_ext_bus_link_power_up/down() are only called
during snd_hdac_ext_bus_link_get/put(). Actually, in my observation
disabling the CORB/RIRB buffer DMAs is what saves us power and this is
done only if snd_hdac_ext_bus_link_put() is called on all links.

> 
> The get() and put() are obviously for fully enabling and disabling
> the
> device, hence it's not suitable for the runtime PM suspend/resume.
> The power_up() / down() should be adjusted to fit with the runtime PM
> call, if any.

The only additional thing that snd_hdac_ext_bus_link_get/put() does on
top of snd_hdac_ext_bus_link_power_up/down() is to stop the CORB/RIRB
DMA when all the link ref_counts are 0. Do you think it is not
advisable to stop the CORB/RIRB DMA during runtime PM?

Thanks,
Ranjani