[alsa-devel] [PATCH 13/13] drm: bridge/dw_hdmi-ahb-audio: parse ELD from HDMI driver

[Daniel Vetter]

On Wed, May 27, 2015 at 7:31 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Wed, May 27, 2015 at 12:43:08PM +0200, Daniel Vetter wrote:
>> On Sat, May 09, 2015 at 11:26:57AM +0100, Russell King wrote:
>> > Parse the ELD (EDID like data) stored from the HDMI driver to restrict
>> > the sample rates and channels which are available to ALSA.  This causes
>> > the ALSA device to reflect the capabilities of the overall audio path,
>> > not just what is supported at the HDMI source interface level.
>> >
>> > Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
>>
>> I completely missed the new eld helpers in this series, so fairly late
>> with a few questions here.
>>
>> In x86/desktop gpus we pass around the eld in hw, and there's an interrupt
>> on the snd side every time that materially changes. Same goes for other
>> state changes like the display pipe getting lit up/disabled again.
>>
>> How is hotplug handling done here? You update the eld in get_modes, but I
>> don't see any notification from hdmi->audio (it only seems to get updated
>> at open time, no idea whether that's enough). And it looks like there's a
>> potential use-after-free when the drm side updates the eld and frees the
>> old one, while the snd side tries to access this.
>
> It doesn't handle hotplug at all, per se.
>
> The problem with hotplug is that (afaik) ALSA has no way to notify
> userspace that something has changed: if I were to add such a
> notification from the DRM side of the driver to the audio side, I
> wouldn't know what to do with it when the audio side received it.
>
> In any case, I tend to think of unplugging the display as being much
> like unplugging the 3.5mm line out jack to your hifi system - just
> because you've disconnected the speakers does not stop the audio
> playing - the audio continues despite you not being able to hear it.

Yeah, afaik that's the same that happens on intel.

> Sure, there's the problem that if you plug in a display with different
> capabilities, the existing format may not be supported, but I don't
> see a reasonable method which could provoke what effectively would be
> a restart on the ALSA side.
>
> In any case, hotplug is kind-of outside of the remit of these helpers -
> these helpers are more to do with decoding an existing ELD and limiting
> the capabilities of the audio side to that specified in the ELD.  It's
> up to the driver how it gets the ELD, and it's left to the driver to
> determine how to handle the change-of-ELD notification.
>
> Given your description of i915 hardware above, I think this makes sense
> anyway: if you have a hardware notification system, it makes sense to use
> that rather than some software implementation, and requiring a software
> implementation for everything to me sounds too limiting.
>
> That's not to say that we couldn't have a software implementation in the
> future: right now, I merely concentrated on the parsing issue.
>
> The dw-hdmi audio driver isn't ready for mainlining yet - there's two
> variants of this at the moment (one for iMX6 and a different one for
> Rockchip - same IP but configured with different interfaces for getting
> the audio data into the HDMI block.)  There's still some work to be done
> to properly unify the two approaches into something sane.

I agree that it makes sense to start with the parsing issues. And some
of the reconfiguration work we need to do in i915 like retuning
derived clocks when the master display clock changes is definitely
solved better already in the arm world I think ;-) I also don't think
that hotplug handling is all that important, at least a quick read
through the intel snd stuff suggest it doesn't handle any of the
reconfiguration all that well at all. The only issue that might be
there with your sw approach is that a concurrent probe/hotplug in drm
might free the edid and hence the eld, while the snd side is trying to
copy that. I'm not sure how that's prevented. Not really a problem
with the helpers though.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch