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

Wed Jan 22 22:35:12 PST 2014

> -----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?

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.

Regards
Mengdong