[alsa-devel] [Intel-gfx] [PATCH v5 2/4] drm/i915: implement sync_audio_rate callback

[Yang, Libin]

Hi Ville,

> -----Original Message-----
> From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com]
> Sent: Wednesday, August 26, 2015 8:27 PM
> To: Yang, Libin
> Cc: alsa-devel at alsa-project.org; tiwai at suse.de; intel-
> gfx at lists.freedesktop.org; daniel.vetter at ffwll.ch;
> jani.nikula at linux.intel.com
> Subject: Re: [Intel-gfx] [PATCH v5 2/4] drm/i915: implement
> sync_audio_rate callback
> 
> On Tue, Aug 25, 2015 at 01:48:13AM +0000, Yang, Libin wrote:
> >
> > > -----Original Message-----
> > > From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com]
> > > Sent: Tuesday, August 25, 2015 12:27 AM
> > > To: Yang, Libin
> > > Cc: alsa-devel at alsa-project.org; tiwai at suse.de; intel-
> > > gfx at lists.freedesktop.org; daniel.vetter at ffwll.ch;
> > > jani.nikula at linux.intel.com
> > > Subject: Re: [Intel-gfx] [PATCH v5 2/4] drm/i915: implement
> > > sync_audio_rate callback
> > >
> > > On Mon, Aug 24, 2015 at 03:35:33PM +0000, Yang, Libin wrote:
> > > > > -----Original Message-----
> > > > > From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com]
> > > > > Sent: Monday, August 24, 2015 8:53 PM
> > > > > To: Yang, Libin
> > > > > Cc: alsa-devel at alsa-project.org; tiwai at suse.de; intel-
> > > > > gfx at lists.freedesktop.org; daniel.vetter at ffwll.ch;
> > > > > jani.nikula at linux.intel.com
> > > > > Subject: Re: [Intel-gfx] [PATCH v5 2/4] drm/i915: implement
> > > > > sync_audio_rate callback
> > > > >
> > > > > On Mon, Aug 24, 2015 at 02:38:14AM +0000, Yang, Libin wrote:
> > > > > > > -----Original Message-----
> > > > > > > From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com]
> > > > > > > Sent: Friday, August 21, 2015 11:14 PM
> > > > > > > To: Yang, Libin
> > > > > > > Cc: alsa-devel at alsa-project.org; tiwai at suse.de; intel-
> > > > > > > gfx at lists.freedesktop.org; daniel.vetter at ffwll.ch;
> > > > > > > jani.nikula at linux.intel.com
> > > > > > > Subject: Re: [Intel-gfx] [PATCH v5 2/4] drm/i915: implement
> > > > > > > sync_audio_rate callback
> > > > > > >
> > > > > > > On Tue, Aug 18, 2015 at 02:51:52PM +0800,
> > > libin.yang at intel.com
> > > > > > > wrote:
> > > > > > > > +static int i915_audio_component_sync_audio_rate(struct
> > > device
> > > > > > > *dev,
> > > > > > > > +						int port, int
> > > rate)
> > > > > > > > +{
> > > > > > > > +	struct drm_i915_private *dev_priv = dev_to_i915(dev);
> > > > > > > > +	struct drm_device *drm_dev = dev_priv->dev;
> > > > > > > > +	struct intel_encoder *intel_encoder;
> > > > > > > > +	struct intel_digital_port *intel_dig_port;
> > > > > > > > +	struct intel_crtc *crtc;
> > > > > > > > +	struct drm_display_mode *mode;
> > > > > > > > +	enum pipe pipe = -1;
> > > > > > > > +	u32 tmp;
> > > > > > > > +	int n_low, n_up, n;
> > > > > > > > +
> > > > > > > > +	/* 1. get the pipe */
> > > > > > > > +	for_each_intel_encoder(drm_dev, intel_encoder) {
> > > > > > > > +		if (intel_encoder->type !=
> > > INTEL_OUTPUT_HDMI)
> > > > > > > > +			continue;
> > > > > > > > +		intel_dig_port =
> > > enc_to_dig_port(&intel_encoder-
> > > > > > > >base);
> > > > > > > > +		if (port == intel_dig_port->port) {
> > > > > > > > +			crtc = to_intel_crtc(intel_encoder-
> > > >base.crtc);
> > > > > > >
> > > > > > > This sort of thing would need some locking to safely start
> > > digging
> > > > > at
> > > > > > > the modeset state.
> > > > > > >
> > > > > > > I would probably not do that, and instead add some new
> lock(s)
> > > for
> > > > > this.
> > > > > > > The modeset code would then fill out some relevant
> > > information
> > > > > > > protected
> > > > > > > by that same lock, and this function could then go look at it
> > > > > without
> > > > > > > having to worry about the rest of modeset locking/state.
> > > > > >
> > > > > > >From audio side, there is no competition when calling the
> > > function,
> > > > > > I think.
> > > > > > For protecting the competition between audio and gfx driver,
> > > > > > it seems we need use the lock from gfx side. Do you have
> > > suggested
> > > > > >  lock I can use?
> > > > >
> > > > > To do it like this we'd need pretty much all the modeset locks,
> > > which
> > > > > to me feels a bit troublesome if the audio driver can call this at
> > > > > any point. So I suggested that we may want to add some kind
> of
> > > new
> > > > > audio lock to i915.
> > > >
> > > > If we are using a new audio lock, I believe we still need use the
> > > > lock when gfx is doing the modeset. Only adding the new lock
> > > > in the function i915_audio_component_sync_audio_rate()
> > > > is not enough because gfx driver will still modify it without
> > > > locking.
> > > >
> > > > I'm not sure where to add the lock in the gfx driver.
> > >
> > > The audio enable/disable during modeset would also grab the lock,
> > > and
> > > consult whatever information the audio driver provided. I would
> also
> > > suggest renaming the .sync_audio_rate() to something more clear
> like
> > > .set_sample_rate()
> >
> > The sample rate in audio means the how often the audio data
> > is sampled. Actually this function is none of setting sample rate.
> 
> Well it's more of sample_rate_change_notify() or something like that.
> But I don't mind too much what it's called. We can go with your
> original
> name if you feel it's more clear.

OK, I will keep the original name as it will be more clear for 
audio driver developer.

> 
> > The sample rate setting is done in audio driver. What the function
> > does is only setting the n/cts based on the audio sample rate.
> >
> > >
> > > Anyway, I was thinking of something like this:
> > >
> > > intel_audio_codec_enable()
> > > {
> > > 	lock();
> > > 	.. configure n/cts
> > > 	encoder->audio.enabled = true;
> > > 	unlock();
> > > }
> > >
> > > intel_audio_codec_disable()
> > > {
> > > 	lock();
> > > 	encoder->audio.enabled = false;
> > > 	unlock();
> > > }
> > >
> > > set_sample_rate()
> > > {
> > > 	lock();
> > > 	encoder->audio.sample_rate = rate;
> > > 	if (encoder->audio.enabled) {
> > > 		... reconfigure n/cts
> > > 	}
> > > 	unlock();
> > > }
> >
> > I'm still not sure whether it is safe when gfx driver is doing the
> modeset.
> > Suppose gfx driver will handle the protection, right?
> 
> intel_audio_codec_enable/disable are called during the modeset. With
> my
> idea as long as you're holding the lock and audio.enabled==true you
> could be sure there's nothing bad happening at the same time in i915.
> 
> >
> > If so, I will add spin_lock_irqsave as you suggested.
> 
> Could be a mutex I think.

OK, mutex is OK for audio driver, too.

> 
> >
> > >
> > > Something like that should protect sample rate changes from
> racing
> > > with an ongoing modeset.
> >
> > Btw, it is not changing the sample rate. Audio sample rate will never
> > be changed in these functions.
> 
> The audio driver can change the sample rate at any point, at which
> point
> it'll call into i915 to reconfigure n/cts, no? So we just need to make
> sure things can't blow up if it does that while there's a modeset
> happening at the same time.

The function is called before playing audio.


Regards,
Libin