HDMI Audio Patches Posted For Raspberry Pi's VC4 Driver
Written by Michael Larabel in Hardware on 7 February 2017 at 04:31 PM EST. 5 Comments
HARDWARE --
If these patches land soon, the Raspberry Pi could beat newer AMD graphics cards to having mainline HDMI audio support via their respective Linux kernel DRM drivers (with the AMDGPU audio support still being held up by DAL/DC mainlining efforts). Eric Anholt managed to finally put out the VC4 HDMI audio code for review.

For many weeks now we've heard about Broadcom's Eric Anholt hacking away at Raspberry Pi HDMI audio support for the newer open-source VC4 driver stack. Today he sent out the patches for review on the DRI-devel list for adding the HDMI audio support to the VC4 DRM kernel driver.


In particular, with this patch, for wiring it all up.
The HDMI encoder IP embeds all needed blocks to output audio, with a custom DAI called MAI moving audio between the two parts of the HDMI core. This driver now exposes a sound card to let users stream audio to their display.

Using the hdmi-codec driver has been considered here, but MAI meant having to significantly rework hdmi-codec, and it would have left little shared code with the I2S mode anyway.

The encoder requires that the audio be SPDIF-formatted frames only, which alsalib will format-convert for us.

This patch is the combined work of Eric Anholt (initial register setup with a separate dmaengine driver and using simple-audio-card) and Boris Brezillon (moving it all into HDMI, massive debug to get it actually working), and which Eric has the permission to release.

The work though is probably a bit late for seeing it in the Linux 4.11 kernel, so may not make it until Linux 4.12, around which point we'll hopefully be seeing AMDGPU with mainline DC/DAL support for having HDMI audio too.
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