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Reverse Design Target Display Mode on iMac

I have a computer lab with several old iMacs (circa 2011). Most users just want to use them as displays for their laptops, and in MacOS target display mode this (mostly) works great.

However, I would also like to support this under Linux, which is installed on all iMacs in parallel with MacOS. I am pretty experienced with the Linux kernel (I already wrote kernel drivers), but I have no idea about MacOS, so I'm looking for pointers on how to determine how TDM actually starts (that is, what happens after Cmd-F2).

I guess this is mainly due to the writing of some Intel GPU registers, but I don't know which bit of MacOS is actually responsible for this. Any clues?

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So I decided it on my own. Looking at libsmc, SMCKit, smc_util, and the applesmc driver, I found that writing 0x02 to the "MVMR" SMC key would cause the display to switch to DisplayPort input.

See my short blog post for more details: http://floe.butterbrot.org/matrix/hacking/tdm/

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Source: https://habr.com/ru/post/1675145/

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