[Nouveau] HDMI pixel clock limits

[Ilia Mirkin]

Hello,

Various HDMI versions enable higher and higher pixel clocks. However
individual GPUs are not required to support the maximum pixel clock
supported by the spec in order to be compliant. It appears that some
GPUs max out at 225MHz while others at 297MHz (while others still, I
assume, are limited to 165MHz, esp among the older ones).

We've been unable to find this in the VBIOS (I had a thought that it
was in the table pointed to by the 'T' table, but we have a
counterexample to that). Could you suggest a way to find this
information either from the VBIOS or based on the GPU? Are there
differences between regular HDMI and DP -> HDMI (passive)?

This is becoming more and more relevant as 2560x1440/3840x2160
displays are become more common, while (dual-link) DVI-D is on its way
out.

Thanks,

  -ilia