[alsa-devel] Need your advice: Add a new communication inteface between HD-Audio and Gfx drivers for hotplug notification/ELD update

[Takashi Iwai]

At Thu, 23 Jan 2014 06:35:12 +0000,
Lin, Mengdong wrote:
> 
> > -----Original Message-----
> > From: Takashi Iwai [mailto:tiwai at suse.de]
> > Sent: Thursday, January 23, 2014 1:19 AM
> > To: Daniel Vetter
> > Cc: Lin, Mengdong; Barnes, Jesse; Zanoni, Paulo R;
> > alsa-devel at alsa-project.org; intel-gfx at lists.freedesktop.org; dri-devel
> > Subject: Re: Need your advice: Add a new communication inteface
> > between HD-Audio and Gfx drivers for hotplug notification/ELD update
> > 
> > At Wed, 22 Jan 2014 15:18:21 +0100,
> > Daniel Vetter wrote:
> > >
> > > On Wed, Jan 22, 2014 at 12:48:04PM +0000, Lin, Mengdong wrote:
> > > > > -----Original Message-----
> > > > > From: daniel.vetter at ffwll.ch [mailto:daniel.vetter at ffwll.ch] On
> > > > > Behalf Of Daniel Vetter
> > > > > Sent: Tuesday, January 21, 2014 9:11 PM
> > > > > To: Lin, Mengdong
> > > > > Cc: Takashi Iwai (tiwai at suse.de); Barnes, Jesse; Zanoni, Paulo R;
> > > > > alsa-devel at alsa-project.org; intel-gfx at lists.freedesktop.org
> > > > > Subject: Re: Need your advice: Add a new communication inteface
> > > > > between HD-Audio and Gfx drivers for hotplug notification/ELD
> > > > > update
> > > > >
> > > > > On Tue, Jan 21, 2014 at 1:35 PM, Lin, Mengdong
> > > > > <mengdong.lin at intel.com>
> > > > > wrote:
> > > > > > Dear audio and gfx stakeholders,
> > > > > >
> > > > > >
> > > > > >
> > > > > > We hope to add a new interface between audio and gfx driver, for
> > > > > > gfx driver to notify audio about HDMI/DP hot-plug and ELD update.
> > > > > >
> > > > > > Would you please share some comments on the proposal below?
> > > > > >
> > > > > >
> > > > > >
> > > > > > Background of this issue: On Intel Haswell/Broadwell platforms,
> > > > > > there is a HW restriction that after the display HD-Audio
> > > > > > controller is in D3,
> > > > > >
> > > > > > it cannot be waken up by HDMI/DP hot-plug. Consequently,
> > > > > > although the gfx driver can still detect the HDMI/DP hot-plug,
> > > > > >
> > > > > > audio driver has no idea about this and cannot notify user space
> > > > > > whether the external HDMI/DP monitor is available for audio
> > > > > > playback,
> > > > > >
> > > > > > because the audio controller cannot wake up to D0 and receive HW
> > > > > > unsolicited event about hot-plug from the audio codec.
> > > > > >
> > > > > > This limitation will affect user space to decide whether we can
> > > > > > output audio over HDMI/DP.
> > > > > >
> > > > > >
> > > > > >
> > > > > > To solve the above limitation, Takashi suggested to add a new
> > > > > > communication interface between audio and gfx driver: create a
> > > > > common
> > > > > > object
> > > > > >
> > > > > > containing the ops registered by both graphics and audio
> > > > > > drivers, then communicate through it, something like
> > vga_switcheroo.
> > > > > >
> > > > > >
> > > > > >
> > > > > > Is it okay to create this kernel object in i915 driver?
> > > > > >
> > > > > >
> > > > > >
> > > > > > I915 can export an API like "display_register_audio_client" for
> > > > > > audio driver to register a client and hot-plug notification ops.
> > > > > >
> > > > > >
> > > > > >
> > > > > > I915 can also call some API like "display_register_gfx_client"
> > > > > > itself and register ops for audio driver to query monitor
> > > > > > presence and ELD info on a specific port.
> > > > > >
> > > > > > It would be faster for audio driver than quering ELD by
> > > > > > command/response over the HD-A bus, thus avoid delay in i915
> > > > > > mode
> > > > > set.
> > > > > >
> > > > > > This will also avoid waking up the audio devices unnecessarily
> > > > > > if the user space does not really want to use HDMI/DP for audio
> > playback.
> > > > > >
> > > > > >
> > > > > >
> > > > > > Whenever i195 enables/disables audio on a port in modeset, it
> > > > > > can call some API like "display_set_audio_state()" on this
> > > > > > kernel object and trigger notifications to the audio driver.
> > > > > >
> > > > > >
> > > > > >
> > > > > > When the audio driver is probed (in the delayed probe stage), it
> > > > > > can request
> > > > > > i915 API symbol to register the audio client for this
> > > > > > communication kernel object.
> > > > > >
> > > > > > Since the 1st i915 mode set may happen before audio driver
> > > > > > registers the ops, we'll let audio driver check ELD once after
> > > > > > registering the audio client ops.
> > > > > >
> > > > > > And for the platforms which uses this communication interface,
> > > > > > we can disable unsolicited event for HDMI/DP hot-plug in the audio
> > driver.
> > > > > >
> > > > > >
> > > > > >
> > > > > > We hope to hear your feedback and start to work out more details.
> > > >
> > > > Thanks for your advice, Daniel!
> > > >
> > > > > Yeah, I've discussed this at KS with Takashi and we've agreed that
> > > > > some common object to facilitate driver interactions. A few things
> > > > > though:
> > > > > - This should be common infrastructure useable by all alsa and drm
> > > > > drivers, not just i915 and snd-hda. Especially on embedded
> > > > > platforms this issue is fairly rampant ...
> > > >
> > > > Agree. Where to put this common object?
> > > > Is it okay to put it under /driver/gpu/drm, similar to vga_switchroo?
> > > > Shall we divide clients into audio and gfx categories, and define
> > > > different ops for them? Since different info/request flow in
> > > > different direction between audio and gfx.
> > >
> > > I guess we could place them into drivers/gpu, yeah. For a name I'd
> > > suggest avsink or something like that, to make it clear that it's the
> > > combination of audio+video. For the actual interfaces I guess we just
> > > need one object in the device model, but the interface should be split
> > > into things called from the audio side only, functions for the video
> > > driver side only and stuff which can be called from both sides. This
> > > matters mostly just so we don't end up with deadlocks since we need a
> > > lock to protect the avsink state itself (e.g. the EDL or the
> > audio_output_connected state).
> > >
> > > > > - While at it it should also encompass power management handling
> > > > > of the shared hw imo so that we can get rid of the hsw specific
> > > > > hacks for the power well code. Or at least we need to rework the
> > > > > power well code to reuse this new infrastructure, I don't really
> > > > > want to maintain a few copies of the lazy symbol_get logic this kind
> > of stuff requires.
> > > >
> > > > Sounds good.
> > > >
> > > > > - I think the biggest problem is figuring out who should register
> > > > > these device nodes. I think it makes the most sense if we do this
> > > > > in the gfx driver, but that requires some trickery on the alsa
> > > > > side (probably with using -EPROBE_DEFER or something like that.
> > > >
> > > > Can the new infrastructure allow audio driver to query whether gfx
> > driver is ready?
> > > > Maybe audio can wait until gfx is ready. For HD-Audio driver, the
> > > > most time consuming part is delayed after the probe stage, and
> > > > actually we can wait in the delayed phase.
> > >
> > > Tbh I haven't really thought about this really. EPROBE_DEFER looks
> > > like the technique used by embedded platforms, but there's also the
> > > new aggregate device driver infrastructure that Russell King is
> > > working on for the imx driver. Or maybe we need to hand-roll our own
> > notification scheme.
> > >
> > > On a hunch it's probably best if the gfx side registers this device
> > > (since it also owns the output state in general) and that the audio
> > > side waits until the gfx side has registered everything if it's not
> > > there yet. I also haven't though about how the audio side could probe
> > > for the right avsink node really ...
> > 
> > Yes, I think doing it first in the gfx side makes sense, too.
> > 
> > > > > - I agree that passing ELD and all the other information through
> > > > > this new structure makes lot more sense than the current mess we
> > > > > have with passing the ELD through some hardware buffer.
> > > >
> > > > > - Finally I think we should assign some identifier to this link
> > > > > which will get exposed both on the drm side and in alsa, so that
> > > > > userspace can figure out which display connects to which output.
> > > > > With that media player could do the Right Thing and automatically
> > > > > place the audio stream on the right pin in alsa.
> > > >
> > > > Is there something that blocks media player from doing the right thing
> > now?
> > > > For HD-Audio, the eld entries under /proc/asound/cardx expose the
> > > > ELD info and can help user space to check if a monitor is usable on a
> > pin.
> > > > Current limitation is these eld entries cannot update if the audio
> > > > controller is in D3, so we need the new infrastructure to notify
> > > > audio driver to update them. But I'm not sure about embedded audio,
> > > > maybe Takashi would like to share more info.
> > >
> > > ELD doesn't contain the serial number from the EDID, so if you have
> > > two monitors of the same model userspace can't figure out which audio
> > > output is connected to which screen.
> > 
> > Right.  And, comparing two different data just for knowing whether A
> > and B are relevant is merely a pain.
> 
> Thanks for clarification!
> Maybe we can add output info (eg. display port number) to the eld entries under /proc/asound/cardx. Is it okay?

It's possible, but the proc file is just a help.  It can't be the
API.  For accessing the information, we'll need some new API, or let
inform via sysfs of the new device.

> And I have a question: how to assure the audio/gfx client find its right peer?
> On a x86 platform, there can be an integrated GPU and an discrete GPU. So there can be two audio controllers and two GPUs.
> We need to assure audio controller find the proper GPU, and vice versa. Maybe we need the peer audio/gfx to register with a same identifier (something like vendor ID) for peering.

Yes, it's an open issue.  So far, the binding with vga_switcheroo is
done by a rough guess with PCI ID.


Takashi