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My I2C writes seem to work, but when I read the value back it's not what I wrote.  What happened?

Many of the HDMI parts have multiple device addresses for different register maps.  Some of them have programmable device addresses and some are fixed.  When writes appear to work, but reads don't read back the correct value but ACK correctly, it's very possibly an issue of bus contention with another device on the I2C bus.

It's also worthwhile to note that not all register maps in all devices are available when whe part is in power down so that is a possiblity as well.