[alsa-devel] ASoC and a codec that can't be controlled

[Liam Girdwood]

On Wed, 2007-05-30 at 13:10 -0500, Timur Tabi wrote:
> Liam Girdwood wrote:
> 
> >> I guess I just don't understand why the codec driver is acting like the "master" driver of 
> >> ASOC.  IMHO, the codec driver should be doing two things:
> > 
> > I guess this is due to an ordering issue we had during early
> > development. We had to make sure that the I2C probe of the codec
> > succeeded before registering the card, pcms, etc. Fwiw, I do intend to
> > address this in future versions (I should probably add a road map to the
> > wiki now).
> 

Wiki roadmap now added.

https://bugtrack.alsa-project.org/wiki/wikka.php?wakka=ASoCRoadMap

> In that case, I think we can solve this problem as well as the PowerPC device tree problem 
> in one shot, because they're basically the same problem.
> 
> Anyway, let me see if I get this right:
> 
> 1. The first function called is eti_b1_init().
> 2. eti_b1_init() calls platform_device_add() to add an soc-audio class device and register 
> the eti_b1_snd_devdata structure.
> 3. The registration causes a number of things to happen, one of which is a call to 
> soc_probe().
> 4. soc_probe() sees that the snd_soc_machine_eti_b1.probe is NULL, so that part is skipped.
> 3. soc_probe() sees that snd_soc_machine_eti_b1.dai_link->cpu_dai->probe is also NULL, so 
> that part is skipped. (eti_b1_snd_devdata.machine == snd_soc_machine_eti_b1)
> 4. soc_probe() sees that soc_codec_dev_wm8731.probe is not NULL, so it calls wm8731_probe().
> 5. wm8731_probe() registers an I2C driver.
> 6. The I2C class driver calls wm8731_i2c_driver.attach_adapter, which is wm8731_i2c_attach().
> 7. wm8731_i2c_attach() calls i2c_probe(), which in turn calls wm8731_codec_probe()
> 8. wm8731_codec_probe() allocates a snd_soc_codec structure.
> 8. wm8731_codec_probe() calls wm8731_init().
> 
> This means that if there is no I2C support, wm8731_init() will never be called.
> 
> 9. wm8731_init() initializes the snd_soc_codec structure.
> 
> Question: why doesn't wm8731_probe() initialize the non-I2C parts of the snd_soc_codec 
> structure?  For example, snd_soc_codec.dai, snd_soc_codec.name, and snd_soc_codec.owner? 
> That way, the snd_soc_codec structure will be properly initialized even when there's no I2C.
> 

This codec only supports I2C, so it shouldn't matter too much. But it
would be better for codecs using other IO mechanisms.

> 10. wm8731_init() calls snd_soc_new_pcms.
> 11. wm8731_init() exits, wm8731_codec_probe() exits, wm8731_i2c_attach() exits, and then 
> wm8731_probe() exits.  Control returns to soc_probe().
> 12. soc_probe() notices that eti_b1_snd_devdata.platform.probe is NULL, so it skips that.
> 
> Question: why don't you define a function for at91_soc_platform.probe, and in that 
> function you can call snd_soc_new_pcms()?  That way, you guarantee the codec driver's I2C 
> interface is initialized before snd_soc_new_pcms() is called, and you avoid have PCM calls 
> in the codec driver.
> 

I think the best place to call snd_soc_new_pcms is in the machine
driver. This means we don't have to add any pcms that are not used. 

> >> ASOC and the machine driver should then work in tandem to decide which driver will do what 
> >> and which capabilities are *actually* supported.  *Something* needs to look at the entire 
> >> system and say to each device, "Well, yes, I know about this little feature of yours, but 
> >> we're just not going to support that today."
> >>
> > 
> > The machine driver pretty much already does this. It can override valid
> > hardware configurations and return -EINVAl if required. 
> 
> Do you have an example of that?  Would that be eti_b1_hw_params()?

Yes, the hw_params could return error on anything the machine didn't
like. e.g. this could be used to workaround quirks. 

Liam