A few more items for the glossary, and understanding GPU switching

[Yann Dirson]

Hello,

Back to reading some amdgpu code, I stumbled on a few things that probably ought
to be added to the glossary.  Well, to be fair, I actually just had to read the kernel
logs to see most of them, so those likely make sense to explain.

* ATPX (could not find anything about that on the net). "PX" alone seems to be used 
  as a synonym.  Referred to as "ATPX power control", and closely linked to vga_switcheroo.
  This seems to be a preferred alternative to
  BOCO and BACO, but those are only tersely mentioned in the glossary, and could maybe
  use more details.  There also seem to be the notion of "ATPX hybrid" (which apparently
  applies to the RENOIR in my laptop)
* MACO and BAMACO looks similarly related (I may have missed some ;)
* CRAT seems to be known as "Component Resource Attribute Table" ACPI table according
  to various Internet sources (it is listed as "Component Resource Association Table"
  in kfd_crat.h, do we want to adjust this?), but I could not locate any details on this
  table (ACPI 6.5 seems to list it as deprecated, but even 6.4 just gives erroneous
  information, "Links to ACPI-related Documents" in　https://uefi.org/acpi has no info)
  Is it useful to add to the glossary, with some info about its role in the amdgpu context?


Now about the init on my MSI Bravo17 (Renoir APU, Navi10 dGPU)...

What I understand about ATPX is that this is a "PX system", as shown by amdgpu_atpx_detect()
logging:
 amdgpu: vga_switcheroo: detected switching method \_SB_.PCI0.GP17.VGA_.ATPX handle

So amdgpu seems to imply vga_switcheroo is supposed to get initialized at some point, right?
Similarly with another similar-looking log:

 snd_hda_intel 0000:03:00.1: Handle vga_switcheroo audio client

But no other log see vga_switcheroo actually getting enabled (and the debugfs dir does not
appear either, confirming this).

That is, no call to vga_switcheroo_enable() gets done (both would trigger a
"vga_switcheroo: enabled" log).  The 2 call chains from amdgpu would be:

amdgpu_init()
-> amdgpu_register_atpx_handler()
   -> vga_switcheroo_register_handler()

amdgpu_device_init()
-> vga_switcheroo_register_client()
   -> register_client()

Booting with "tp_printk ftrace=function ftrace_filter=vga_switcheroo_register_handler,vga_switcheroo_register_client,vga_switcheroo_ready" it gets obvious vga_switcheroo_register_client() never not gets called.

The answer seems to lie in amdgpu_device_supports_px(), but I'm not sure to get everything straight here:
amdgpu_device_supports_px() is documented as "Returns true if the device is a dGPU with ATPX power control",
but amdgpu_is_atpx_hybrid() it calls seems to take info from a single static struct, so does the quoted
docstring really make sense?  And if an "APTX hybrid" system does not support vga_switcheroo in the end,
couldn't we just avoid any mention of vga_switcheroo in the logs (since it is indeed amdgpu itself sending
the user on that track), or at least just print once that it is not supported and a short reason?

Best regards,
-- 
Yann