Question regarding HDMI Audio support on i.MX6 with vanilla kernel

[Russell King - ARM Linux]

On Tue, Nov 07, 2017 at 07:23:37PM +0000, Luís Mendes wrote:
> You don't need to waste your time with this issue... I will develop my own
> hardware later, my own carrier board, it is not a big issue for me.
> I was just speaking about this in terms of having a workaround in the
> device driver for situations like this, but since you say that all HDMI
> monitors must have EDID then this doesn't make much sense to change,
> forcing the EDID is enough.

It's useful to get to the bottom of it as it's likely that other people
will run into the same issue - that means we can be prepared for similar
issues in the future.

Looking at the schematics, it raises several concerns:

1. the 5V supplied to HDMI is taken direct from the board power
   connector.  HDMI wants the voltage at the HDMI connector to be
   within 4.8V to 5.3V - if the power adapter supplying the board is
   outside that spec, it's going to be hit and miss whether HDMI works.

   Obviously, HDMI conformance with the 5V supply to the HDMI sink is
   entirely dependent on the accuracy of whatever supply is powering
   the Wandboard, and losses in the cable, and the current consumption
   of the Wandboard.

2. there is an (optional?) TXS0102 level converter on the DDC signals
   which, although marketed for I2C buses, doesn't actually conform to
   the I2C bus spec.  The I2C bus spec calls for 30% and 70% of the
   bus voltage for input thresholds, which would be 1.5V and 3.5V.
   The TXS0102 datasheet gives more stringent levels, which are 0.15V
   and 4.6V assuming a 5V "B" supply.

   (Note: HDMI calls for the DDC bus to conform to the I2C bus spec.)

So, I think this Wandboard design has some question marks over the
hardware design, and so I wouldn't expect HDMI to be reliable.

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