Re: [RFC 04/15] drm/i915: Add headers for non-HDAudio HDMI interface

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Mar 15, 2016 at 09:35:26AM +0100, Daniel Vetter wrote:
> On Mon, Mar 14, 2016 at 04:20:19PM +0200, Ville Syrjälä wrote:
> > On Mon, Mar 14, 2016 at 10:04:00AM +0100, Daniel Vetter wrote:
> > > On Fri, Mar 11, 2016 at 09:09:12PM +0200, Ville Syrjälä wrote:
> > > > On Fri, Mar 11, 2016 at 11:27:13AM -0600, Pierre-Louis Bossart wrote:
> > > > > Thanks for the review Ville
> > > > > 
> > > > > [snip]
> > > > > 
> > > > > > Kinda hard to see where everything gets used due to the way the patches
> > > > > > are split up.
> > > > > 
> > > > > Yes, it's far from great...
> > > > > 
> > > > > > At least the hotplug/mode change events are not needed. We only have the
> > > > > > two points where i915 should inform the audio driver about this stuff,
> > > > > > and those are the intel_audio_code_enable/disable(). For that we
> > > > > > already have the .pin_eld_notify() hook.
> > > > > >
> > > > > > The interrupt stuff should mostly vanish from i915 with the subdevice
> > > > > > approach. As in i915 would just call the interrupt handler of the audio
> > > > > > driver based on the LPE bits in IIR, and the audio driver can then do
> > > > > > whatever it wants based on its own status register.
> > > > > 
> > > > > Are you saying that the subdevice would provide a read/write interface 
> > > > > for the audio driver to look at display registers, and the i915 driver 
> > > > > would only provide a notification interface (EDID and interrupts) to the 
> > > > > audio driver?
> > > > 
> > > > The audio driver would simply ioremap the appropriate range of
> > > > registers itself.
> > > > 
> > > > > If yes, would there be two component framework links, one between 
> > > > > i915/audio driver and one between subdevice/audio driver.
> > > > 
> > > > Yeah sort of. i915 registers the platform device for the audio, the
> > > > audio driver can then bind to the device via the platform driver .probe
> > > > callback. It can then register with the audio component stuff at some
> > > > point to get the relevant notifications on the display state. When
> > > > i915 gets unloaded we remove the platform device, at which point the
> > > > audio driver's platform driver .remove() callback gets invoked and
> > > > it should unregister/cleanup everything.
> > > > 
> > > > I just tried to frob around with the VED code a bit, and got it to load
> > > > at least. It's not quite happy about reloading i915 while the ipvr
> > > > driver was loaded though. Not sure what's going on there, but I do
> > > > think this approach should work. So the VED patch could serve as a
> > > > decent enough model to follow.
> > > 
> > > platform devices registerd by modules are apparently inherently racy and
> > > in an unfixable way. At least I remember something like that from VED
> > > discussion.
> > > 
> > > In short you _must_ unload VED manually before unloading i915, or it all
> > > goes boom. If this is the only thing that went boom it's acceptable.
> > 
> > VED goes boom due drm_global_mutex deadlock at least if you load
> > i915 while ipvr is already loaded. I didn't check to hard if there
> > were any booms on pure unload so far.
> 
> Oh right another boom that happened, but can be avoided by dropping the
> ->load callback and directly calling drm_dev_alloc/register. Shouldn't be
> a problem with a non-drm driver though.
> 
> > Anyways, I was a bit worried about the races so I did a pair of 
> > small test modules to try out this model, and to me it looked to
> > work so far. I just unregistered the platform device from the parent
> > pci driver .remove() hook, and it got blocked until the platform
> > driver's .remove() hook was done.
> > 
> > In any case if this is broken, then I assume mfd must be broken. And
> > that thing is at least used quite extensively.
> 
> Hm, I don't remember the exact details, but iirc the problem was that the
> struct device is refcounted, but you can't add a module ref for the vtable
> is has (specifically the final release method) since that would result in
> a ref-loop. Usually it works, but if someone keeps the device alive
> (through sysfs or whatever) and you manage to unload everything before
> that last ref gets dropped it goes boom.
> 
> So "works", but not in a safe way.

I was worried that it's something like that. I guess I'll need to try
grab a ref on something in my test module and see how it fares.

-- 
Ville Syrjälä
Intel OTC
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