Re: [alsa-devel] [PATCH 5/5] ALSA/i915: Check power well API existense before calling

[Takashi Iwai <tiwai@suse.de>]

At Mon, 13 May 2013 10:55:46 +0200,
Jaroslav Kysela wrote:
> 
> Date 13.5.2013 10:28, David Henningsson wrote:
> > On 05/13/2013 09:37 AM, Wang Xingchao wrote:
> >> I915 module maybe loaded after snd_hda_intel, the power-well
> >> API doesnot exist in such case. This patch intended to avoid
> >> loading dependency between snd-hda-intel and i915 module.
> > 
> > Hi Xingchao and thanks for working on this.
> > 
> > This patch seems to re-do some of the work done in other patches (a lot 
> > of lines removed), so it's a little hard to follow. But I'll try to 
> > write some overall comments on how I have envisioned things...
> > 
> > 1. I don't think there's any need to create an additional kernel module, 
> > we can just let hda_i915.c be in the snd-hda-intel.ko module, and only 
> > compile it if DRM_I915 is defined.
> > 
> > 2. We don't need an IS_HSW macro on the audio side. Instead declare a 
> > new AZX_DCAPS_NEED_I915_POWER (or similar) driver quirk.
> > 
> > 3. Somewhere in the beginning of the probing in hda_intel.c, we'll need 
> > something like:
> > 
> > if (driver_caps & AZX_DCAPS_NEED_I915_POWER)
> >    hda_i915_init(chip);
> > 
> > 4. The hda_i915_init does not need to be exported (they're now both in 
> > the same module). hda_i915.h could have something like:
> > 
> > #ifdef DRM_I915
> >    void hda_i915_init(chip);
> > #else
> >    #define hda_i915_init(chip) do {} while(0)
> > #endif
> > 
> > 5. You're saying this patch is about avoid loading dependency between 
> > snd-hda-intel and i915 module. That does not make sense to me, since the 
> > powerwell API is in the i915 module, snd-hda-intel must load it when it 
> > wants to enable power on haswell, right? Thus there is a loading 
> > dependency. If the i915 module is not loaded at that point, we must wait 
> > for it to load, so we can have proper power, instead of continuing 
> > probing without the power well?
> 
> Looking to the code, if the drm code requires a callback to the audio
> code, I would just register it in the audio driver init phase and
> unregister it when snd-hda-intel is unloaded. This cross module loading
> dependency looks really bad. Or it is not allowed to load the audio
> driver later than DRM one?

It's not allowed.  The drm/i915 must be initialized before the audio.
And yet, we don't want this dependency in a hard way in the hda
driver, because the driver is not only for i915 but for other vendor's
controllers, too.

In the previous meeting, I suggested to split snd-hda-intel for
Haswell as an alternative solution.  But this has obvious
disadvantages, and since the dynamic symbol lookup is already used in
a few other kernel codes, we decided to try this first.


Takashi