Re: HDCP blob still present

[Denis 'GNUtoo' Carikli]

On Thu, 14 Nov 2024 10:23:24 +0000
Leah Rowe <info@minifree.org> wrote:

> afaik linux simply doesn't implement hdcp stuff at all
I found the time to look into it and apparently an userspace
application can decide to enable it or not[1] with the i915 driver.

According to collabora, in 2017: "Sinks (such as TVs) cannot demand an
upstream link provide HDCP, either."[2], so as I understand that is
probably the standard for video at least in theory.

But then farnz commented on the lwn.net article of 2017 that in
practice with audio things can be different:
> On the other hand, HDCP support has uses outside DRM, thanks to buggy
> sinks that require HDCP before they handle the data at all (this is
> not just a hypothetical - I've encountered an A/V amp that would only
> play HDMI Basic Audio unless the source had HDCP, which was fun to
> debug when both ends said they had valid audio data - and all my
> "consumer" kit does HDCP if the sink supports it, so a normal user
> won't know that their sink has this bug until they switch to a free
> OS).

And from nybble41:
> [...]
> One area where this interface to enable HDCP might come in handy,
> outside of a locked-down system, is supplying Dolby TrueHD audio over
> HDMI. The relevant standards mandate that TrueHD audio can only be
> carried over HDMI interfaces which have HDCP enabled. It makes no
> difference that the _source_ of the audio was not encumbered by
> DRM—without HDCP, a compliant receiver won't accept TrueHD input.

In any case if it's just for audio, it's probably not a big issue and
it might be a good idea to just remove this blob instead of
implementing some fake HDCP just to make the other side happy.

This information might be relevant in the discussion with
distributions. 

Do you intend to contact distributions that support RK3399 about this
issue?

References:
-----------
[1]https://lwn.net/Articles/740916/
[2]https://www.collabora.com/news-and-blog/blog/2017/12/11/why-linux-hdcp-isnt-the-end-of-the-world/

Denis.

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