Re: [PATCH v2 1/3] drm/i915/hda: Add audio component stub

[Takashi Iwai <tiwai@suse.de>]

On Tue, 17 May 2016 14:56:16 +0200,
Daniel Vetter wrote:
> 
> On Tue, May 17, 2016 at 02:47:44PM +0200, Takashi Iwai wrote:
> > On Tue, 17 May 2016 14:34:21 +0200,
> > Daniel Vetter wrote:
> > > 
> > > On Tue, May 17, 2016 at 03:11:02PM +0300, Imre Deak wrote:
> > > > On ti, 2016-05-17 at 13:10 +0200, Daniel Vetter wrote:
> > > > > On Tue, May 17, 2016 at 01:22:52PM +0300, Imre Deak wrote:
> > > > > > On ti, 2016-05-17 at 11:59 +0200, Takashi Iwai wrote:
> > > > > > > On Tue, 17 May 2016 11:42:17 +0200,
> > > > > > > Daniel Vetter wrote:
> > > > > > > > 
> > > > > > > > On Tue, May 17, 2016 at 09:37:12AM +0200, Takashi Iwai wrote:
> > > > > > > > > On Tue, 17 May 2016 09:20:48 +0200,
> > > > > > > > > Daniel Vetter wrote:
> > > > > > > > > > 
> > > > > > > > > > On Thu, May 12, 2016 at 09:06:53PM +0300, Imre Deak wrote:
> > > > > > > > > > > User may pass nomodeset or i915.modeset=0 option to
> > > > > > > > > > > disable i915 KMS
> > > > > > > > > > > explicitly.  Although this itself works fine, it breaks
> > > > > > > > > > > the weak
> > > > > > > > > > > dependency the HD-audio driver requires, and it's the
> > > > > > > > > > > reason the
> > > > > > > > > > > delayed component binding isn't implemented in HD-
> > > > > > > > > > > audio.  Since i915
> > > > > > > > > > > doesn't notify its disablement, HD-audio would be blocked
> > > > > > > > > > > unnecessarily endlessly, waiting for the bind with i915.
> > > > > > > > > > > 
> > > > > > > > > > > This patch introduces a stub audio component binding when
> > > > > > > > > > > i915 driver
> > > > > > > > > > > is loaded with KMS off like the boot options above.  Then
> > > > > > > > > > > i915 driver
> > > > > > > > > > > still registers the slave component but with the new
> > > > > > > > > > > "disabled" ops
> > > > > > > > > > > flag, so that the master component (HD-audio) can know
> > > > > > > > > > > clearly the
> > > > > > > > > > > slave state.
> > > > > > > > > > > 
> > > > > > > > > > > v2:
> > > > > > > > > > > - Fail the probe in case component registration fails,
> > > > > > > > > > > instead of
> > > > > > > > > > >   suppressing the error. (Ville)
> > > > > > > > > > > - Register the component only for the real PCI device
> > > > > > > > > > > function.
> > > > > > > > > > > 
> > > > > > > > > > > CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > > > > > > > > > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > > > > > > > > > > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > > > > > > > > > 
> > > > > > > > > > We don't support not running with modesetting. Why do we
> > > > > > > > > > suddenly care?
> > > > > > > > > 
> > > > > > > > > This is needed for the patch 2 and 3.  Right now we have no
> > > > > > > > > blocking
> > > > > > > > > or deferred component binding, so far, in HD-audio
> > > > > > > > > side.  This caused
> > > > > > > > > problems when async module probe was done.
> > > > > > > > > 
> > > > > > > > > So, the patch 3 implements the blocking behavior of HD-audio
> > > > > > > > > side.  It
> > > > > > > > > would lead to another regression when i915 doesn't notify its
> > > > > > > > > disabled
> > > > > > > > > state by this patch.  Otherwise the HD-audio driver will be
> > > > > > > > > blocked
> > > > > > > > > endlessly of unnecessarily long.
> > > > > > > > > 
> > > > > > > > > > Same for users creating a .config that fails to boot or
> > > > > > > > > > work ...
> > > > > > > > > 
> > > > > > > > > The config isn't cared much, but the problem is about the
> > > > > > > > > runtime boot
> > > > > > > > > option.
> > > > > > > > > 
> > > > > > > > > > If HDA needs to coporate with gfx to get things done, then
> > > > > > > > > > imo we should
> > > > > > > > > > just require that i915.ko is there.
> > > > > > > > > 
> > > > > > > > > Other way round: we do already require i915 in HD-audio
> > > > > > > > > side.  But in
> > > > > > > > > this case, we do *not* want to require i915 when it's
> > > > > > > > > disabled in
> > > > > > > > > runtime. 
> > > > > > > > 
> > > > > > > > That's what I mean: If you boot with i915.nomodeset you're
> > > > > > > > explicitly fine
> > > > > > > > with a somewhat non-useable system - that option is for
> > > > > > > > debugging only
> > > > > > > > really. If that means audio also doesn't work, then I think
> > > > > > > > that's ok.
> > > > > > > 
> > > > > > > It's not only "it doesn't work".  The module load gets stuck.  So
> > > > > > > we
> > > > > > > need some notification for the blocked component binding.
> > > > > > > 
> > > > > > > > Adding complexity for this case (which means more error paths
> > > > > > > > we don't
> > > > > > > > ever test and hence _will_ break) seems over the top.
> > > > > > > > 
> > > > > > > > I'm quite opposed to adding error handling for every condition
> > > > > > > > in general
> > > > > > > > because the combinatorial testing madness just can't be
> > > > > > > > handled. The one
> > > > > > > > exception in the i915.ko driver is that when the render side
> > > > > > > > died
> > > > > > > > (terminal gpu hang) we'll try our best to keep the display
> > > > > > > > alive. But
> > > > > > > > that's it, and the justification for that is "we want users to
> > > > > > > > be able to
> > > > > > > > get the bug report out". I don't see a justification of that
> > > > > > > > magnitude for
> > > > > > > > this feature here at all.
> > > > > > > 
> > > > > > > Well, actually the patchset was proposed just because Intel CI
> > > > > > > tests
> > > > > > > failed due to async module probes.  If Intel is happy with
> > > > > > > continued
> > > > > > > CI test failures, I'm also happy with the current situation ;)
> > > > > > 
> > > > > > It's not only due to those particular failures. That was caused by
> > > > > > a
> > > > > > kmod bug and as such would be good to not depend on that mechanism.
> > > > > > But
> > > > > > things will fail atm even in the normal case when audio is built-in 
> > > > > > and
> > > > > > i915 is a module. This patchset would solve that too.
> > > > > 
> > > > > I'm not against patch 3 from this series, which seems to be the
> > > > > bugfix we
> > > > > want. I'm against the kludge in patch 1 here only (and maybe patch 2,
> > > > > not
> > > > > sure about that). Patch 1 here looks like a workaround purely for the
> > > > > i915.modeset=0 case. And I don't want special code for debug knobs if
> > > > > we
> > > > > can avoid it.
> > > > 
> > > > We have discussed this in more detail, see:
> > > > https://bugs.freedesktop.org/show_bug.cgi?id=94566#c26
> > > 
> > > I disagree with Takashi here. modeset=0 is a debug option, and we should
> > > probably mark it as _unsafe.
> > 
> > Well, even if it's a debug option, making the system unusable isn't a
> > nice move.  With the blocking component binding, the audio side stalls
> > unexpectedly (and silently).  How user would know it?
> > 
> > I'm no big fan of the current implementation, but I really would like
> > to avoid users hitting the unexpected results.
> 
> Ok, I looked at patch 3, and that indeed would lead to trouble without
> patch 1. But the real trouble is the unconditional wait_completion in
> there - blocking for another driver to complete loading from a driver load
> function is a no-go. The correct way to do this is to bail out with
> EPROBE_DEFER if not all the parts are available there. Also, throw out
> that request_module.
> 
> By bailing out with EPROBE_DEFER you avoid deadlocks, and the driver core
> also knows what's going on. Which is incidentally what's used to
> implicitly order suspend/resume. Driver core will restart your probe as
> soon as some new devices/drivers have registers (assuming that hopefully
> then you're unblocking), but if you're unlucky your driver can go through
> that loop a few times.
> 
> But that was just a very quick look, we definitely shouldn't need any
> wait_completion in driver load to handle cross-module depencies.

Yeah, I admit that wait_completion() is hackish.  OTOH, EPROBE_DEFER
doesn't work in the case of HD-audio because we want to give up
binding and continue without i915 but only with onboard audio, instead
of endlessly reprobing for the never-appearing component.  The i915
binding is no hard dependency; i.e. it isn't (always) mandatory, and
EPROBE_DEFER can't handle such a fallback, AFAIK.

If there is a good way to deal with it, please let me know.  I'd love
to rewrite to a cleaner way.


thanks,

Takashi
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