Re: [PATCH] ALSA: hda - PCH HDA controller not controlled by i915 power

[Takashi Iwai <tiwai@suse.de>]

At Tue, 09 Jun 2015 10:51:19 +0200,
David Henningsson wrote:
> 
> 
> 
> On 2015-06-09 10:30, Takashi Iwai wrote:
> > At Tue, 09 Jun 2015 08:46:35 +0200,
> > David Henningsson wrote:
> >>
> >>
> >>
> >> On 2015-06-09 08:31, Takashi Iwai wrote:
> >>> At Tue,  9 Jun 2015 14:05:18 +0800,
> >>> libin.yang@intel.com wrote:
> >>>>
> >>>> From: Libin Yang <libin.yang@intel.com>
> >>>>
> >>>> On some Intel platforms, the HDMI codec is connected to PCH HDA
> >>>> controller. In this case, AZX_DCAPS_I915_POWERWELL is set and
> >>>> the i915 power well failure should not block the hda controller
> >>>> initialization.
> >>>>
> >>>> Signed-off-by: Libin Yang <libin.yang@intel.com>
> >>>
> >>> Is this fix needed for 4.1, or it's only for 4.2?
> >>
> >> It's a bug fix, and as such should go to all kernels, even with cc to
> >> stable. That said, it mainly concerns Skylake, so kernels that don't
> >> support Skylake would not need a backport.
> >
> > The patch can't be applied even to 4.1 as is because of the code
> > structure change.  So, Cc to stable doesn't work, in anyway.
> >
> >>> I vaguely remember of a bug report.  If there is any relevant bug
> >>> report, please give the link, too.
> >>
> >> I think this was raised to Intel by us. The use case is when the
> >> integrated GPU is disabled and a discrete GPU is used. In this case the
> >> i915 module fails to load. If then the HDMI and analog codec are both on
> >> the same controller, the entire controller fails instead of just the
> >> HDMI codec.
> >>
> >> I'll see if I can get the patch tested ASAP.
> >
> > OK, the bug seems needed for 4.1 and earlier.  But Libin's patch is
> > only for 4.2.  And even worse, backporting this isn't
> > straightforward due to the lack of need_i915_power field.  Hmm.
> >
> > I think we can make it easier by just allowing to continue the probe.
> > A totally untested patch for 4.1 is below.
> 
> As part of the wider discussion, I think we could continue instead of 
> failing also on Haswell and Broadwell. This is to some degree 
> hypothetical, but if the i915 module does not load for some reason, then 
> the i915 module would not turn the power well off either, so it remains 
> on the entire time (and then audio could potentially work).

Without graphics HDMI/DP never works, so practically it's useless.
The problem can be reproduced easily by passing nomodeset boot
option.

OTOH, it'd simplify the change, and this was indeed my initial patch
before posting.  Then I thought this might lead to a regression; a
non-working HDMI/DP sound card appears while it wasn't before.  So I
added the check of HSW/BDW.

Meanwhile, nomodeset or failing i915 is a more or less special
situation, so we might not consider about this behavior change so
much, and take rather for the code simplicity.

Comments?


Takashi