[alsa-devel] HD-Audio: How to reduce driver initializaton time if multiple codecs present on the bus?

[Lin, Mengdong]

> -----Original Message-----
> From: Takashi Iwai [mailto:tiwai at suse.de]
> Sent: Monday, December 02, 2013 4:24 PM


> > > > > > Btw, using a deferred azx_probe_continue (like we do if
> > > > > > there's a firmware file), would one get more parallelization with
> other drivers?
> > > > > > If so we could consider making that the default.
> > > > >
> > > > > This sounds like a good idea as we have already that code.  The
> > > > > patch would be just a few-liners.
> > > >
> > > > If azx_probe_continue() is delayed, could user space get
> > > > incomplete
> > > audio information after boot?
> > > > I mean is it possible that azx_probe_continue() does not complete
> > > before user space check ALSA info?
> > >
> > > Not impossible.  But then user-space should be triggered in event
> > > driven way by udev or such.
> >
> > Hi Takashi/David,
> >
> > So is it okay to defer azx_probe_continue() by default?
> 
> Yes.
> 
> > If yes, is it also necessary to defer snd_card_create() at the end of
> azx_probe_continue()?
> 
> No, that's a significant difference.  The snd_card_create() invocation isn't
> related with the hardware access itself, thus it's no point to delay.  If you
> delay the card instance creation, this may result in races between the card
> order, which we'd like to avoid.
> 
> > So user space can only see the sound card when azx_probe_continue()
> completes.
> 
> The completion of the card creation is notified only after
> snd_card_register(), so it doesn't matter when to call snd_card_create().
> 
> The below is an untested patch.  I should have added in the previous
> reply...

Hi Takashi,

Many thanks for the patch and information!

Is my understanding below is right?
snd_card_register() will create the device files under /sys/devices/pci0000:00/0000:00:1b.0/sound/cardx 
(or /sys/devices/pci0000:00/0000:00:03.0/sound/cardx for Haswell display audio), and trigger uevent to notify user space that an audio device is ready.

And for the patch, 

>  #ifdef CONFIG_SND_HDA_PATCH_LOADER
>  	if (patch[dev] && *patch[dev]) {
> @@ -3855,25 +3855,17 @@ static int azx_probe(struct pci_dev *pci,
>  					      azx_firmware_cb);
>  		if (err < 0)
>  			goto out_free;
> -		probe_now = false; /* continued in azx_firmware_cb() */
> +		schedule_probe = false; /* continued in azx_firmware_cb() */
>  	}

Should it be 'schedule_probe = true'?
I guess the driver intends to defer firmware loading.

Thanks
Mengdong