[alsa-devel] [PATCH 2/7] ALSA: hda: Refactor display power management

[Takashi Iwai]

On Mon, 10 Dec 2018 21:52:05 +0100,
Pierre-Louis Bossart wrote:
> 
> 
> On 12/9/18 3:33 AM, Takashi Iwai wrote:
> > The current HD-audio code manages the DRM audio power via too complex
> > redirections, and this seems even still unbalanced in a corner case as
> > Intel DRM CI has been intermittently reporting.  This patch is a big
> > surgery for addressing the complexity and the possible unbalance.
> >
> > Basically the patch changes the display PM in the following ways:
> >
> > - Both HD-audio controller and codec drivers call a single helper,
> >    snd_hdac_display_power().  (Formerly, the display power control from
> >    a codec was done indirectly via link_power bus ops.)
> >
> > - snd_hdac_display_power() receives the codec address index.  For
> >    turning on/off from the controller, pass HDA_CODEC_IDX_CONTROLLER.
> 
> The need for this virtual index==16 isn't fully clear to me,
> especially if you use the bitfields instead of reference counts.
> 
> Isn't there a risk of the controller setting the bit16 to zero, but
> you still have bit4 on (assuming the idx is 4). If you use this
> virtual index, it should override the actual physical bits when
> set/cleared.

This is the index for a controller, i.e. we'd need bits for the max
number of codecs + 1.

Actually we do support only up to 8 codecs (HDA_MAX_CODECS), so it
should be 8, instead of 16, too.  I'll update to be HDA_MAX_CODECS.

> Or is this meant to actually implement a preemption mechanism, where
> the display power remains on for as long as the controller wishes,
> regardless of what the patch_hdmi and hdac_hdmi code requests?

Right.  That's the mechanism at the initial phase, we need the display
power on while probing the codec, i.e. before identifying the codec
ID.

> Also don't we already have the HDMI codec address already after the
> probe, so couldn't we provide the address directly?

The resume seemed requiring the controller to take the display power
at first, so the same mechanism is used.


thanks,

Takashi