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

[Luís Mendes]

Hi,

On Tue, Nov 7, 2017 at 8:07 PM, Russell King - ARM Linux <
linux at armlinux.org.uk> wrote:

> 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.
>

The manufacturer specifies a tolerance of 5% for the power supply, if I
remember... The AC adpater I am using has 5% tolerance, so it represents
0.25V so it may drop down to 4.75V, so it shouldn't be that bad.


>
> 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.
>

This chip must be the culprit, it really does a bad job at translating I2C
signals. The logic levels are completing wrong then.
If the chip is not populated because iMX6 may have some 5V tolerant pins,
then it is barely able to drive the logic 1 level for the monitor, although
it can receive properly
...


>
> --
> RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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> up
> According to speedtest.net: 8.21Mbps down 510kbps up
>

Luís
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