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

Wed Jan 22 16:04:14 CET 2014

On Wed, Jan 22, 2014 at 9:18 AM, Daniel Vetter <daniel at ffwll.ch> 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).

sorry to jump into this a bit late, so maybe this was covered already earlier..

For drm/msm the hdmi code needs something along these lines from audio:

int hdmi_audio_info_setup(struct platform_device *pdev, bool enabled,
        uint32_t num_of_channels, uint32_t channel_allocation,
        uint32_t level_shift, bool down_mix);
void hdmi_audio_set_sample_rate(struct platform_device *pdev, int rate);

(former is mainly to setup avi audio infoframe, latter for clks)

in addition to hotplug and ELD stuff

BR,
-R

>> > - 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 ...
>
>> > - 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.
>
> Cheers, Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
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