(In reply to OptionalRealName from comment #210)
> Is anyone from AMD aware of this at all?

I opened a ticket with them on 25-Feb-2018.  

I received a response on 28-Feb-2018, to which I replied as follows:

[start_______________________________________________________________________
On 28/02/18 10:51, <email address hidden> wrote:
> Dear Chris,
>
> Your service request : SR #{ticketno:[8200794428]} has been reviewed and 
> updated.
>
> Response and Service Request History:
>
> Thank you for the response.
>
> I understand you are experiencing an issue on your PC with Ryzen processor
> when C6 state is enabled on the BIOS.

Yes, my PC freezes at random when it is idle.  Typically it will freeze when left overnight, roughly every 2 or 3 days.

Having disabled C6 -- using the 'zenstates.py' script, from https://github.com/r4m0n/ZenStates-Linux -- my machine has not frozen for 7 days.

> This issue has been fixed with the latest BIOS updates, but the option to fix
> it may not be available in all BIOS.

What is the root cause of the issue ?

In what way has it been fixed ?

> I request you to update to the latest BIOS and see if you have the Power 
> Supply Control option in the MB BIOS. Try toggling this option between 
> the different settings to see if it fixes it. If the specific option is 
> not available I would suggest you keep C6 off for now.

I have the latest available "PRIME X370-PRO BIOS 3803" from ASUS.  That apparently includes:

  2.Update to AGESA 1000a for new upcoming processors

I understand that means AGESA 1.0.0.0a (?) -- I have no idea what that means, since AMD seem to keep the release notes for AGESA as a deep, dark secret.  A previous BIOSes had (according to ASUS) "AGESA 1071", and before that were "AGESA 1.0.0.6B" and "AGESA 1.0.0.6a"... so I admit to being baffled.

What does this new BIOS "Power Supply Option" do ?

Are you telling me that this is problem with my power supply ?

If so, does this mean I need a better power supply ?

Disabling C6 is not really a long term solution... since that disables both (a) the maximum single core performance, and (b) the minimum power consumption state.  While these are arguably marginal, I have wasted a lot of time and energy trying to get my machine to work reliably.

I am seriously disappointed that the only information available is buried in kernel bug report(s) and in various support forums.

Having (eventually) found Linux Kernel Bug 196683, I have been hoping that AMD would leap into action to: (a) inject some clarity into the discussion, and (b) provide a proper solution.

<sigh>

For completeness, let me repeat my questions:

  1) What is the root cause of the issue ?

  2) In what way has it been fixed ?

  3) What does this new BIOS "Power Supply Option" do ?

  4) Are you telling me that this is problem with my power supply ?

  5) If so, does this mean I need a better power supply ?
__________________________________________________________________________end]

I prompted them on 12-Mar-2018, and today (14-Mar-2018) have received:

[start_______________________________________________________________________

Your service request : SR #{ticketno:[8200794428]} has been reviewed and updated.

Response and Service Request History:

Thanks for getting back to me, i really appreciate your patience. 

This issue is related to the power supply.  Most PC power supplies (PSUs) are designed to handle a wide range of power consumption from your PC components, but not all PSUs are created equal. 

Because of this, there are some rare conditions where the power draw of an efficient PC does not meet the minimum power consumption requirements for one or more circuits inside some PSUs. 

This scenario (called “minimal loading supply”) can cause such PSUs to output poor quality power, or shut off entirely. 

To prevent this issue from happening, it is important to ensure that the power supply supports 0A minimum load on the +12V circuit. These PSUs became commonplace starting in 2013 for the Intel “Haswell” platform.

This specification can be found printed on the sticker affixed to most PSUs, or it may be available on the manufacturer’s website. If you cannot locate this information related to your PSU, you will need to contact the manufacturer directly. 

If you have not already, please download and update to the latest Motherboard bios and check to see if you have this new bios option and let me know the outcome of toggling this option on and off, and between the various preset settings. 
__________________________________________________________________________end]

I admit that the PSU I was using was (a) old and (b) cheap.  I have now replaced that by a brand-new, more efficient PSU which supports 0A loads across all voltages.  I am running that with C6 enabled, and await developments.