[Bug 92685] [i915 Skylake] HDMI output does not work with some adapters

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92685

Stefan Brüns <stefan.bruens at rwth-aachen.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      i915 platform|                            |I965GM

--- Comment #30 from Stefan Brüns <stefan.bruens at rwth-aachen.de> ---
I can see the same issue (HDMI-to-VGA not working) on a Laptop with 965GM (Dell
Inspiron 11z).

The same adapter works when used behind a DP-to-HDMI adapter, the obvious
difference is the AUX-to-I2C bridge in the DP/HDMI adapter.

I hooked up a logic analyzer to the HDMI connection (that is, the DDC lines).
There is no obvious protocol violation when connected to the 965GM, but there
are a few observations:

-- using i2c-dev/i2cdump 4 0x50, on 965GM --
- when reading each byte individually (i.e. write offset, read byte), the whole
EEPROM is read correctly
- when reading in blocks (write offset, read 32 bytes), the first byte is
always correct, the second byte occasionally

-- comparing 965GM as master and DP adapter as master --
965GM: releases SDA (ACK) at the exact same time as SCL is pulled down
DP: SDA is kept stable for 500ns after SCL is pulled down

My current assuption is the HDMI-to-VGA adapter samples the SDA line with the
falling edge of SCL, i.e. at a time where the SDA line is already pulled up
again.

LA traces can be made available.

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