[Bug 111482] Sapphire Pulse RX 5700 XT power consumption

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111482

--- Comment #26 from Andrew Sheldon <asheldon55 at gmail.com> ---
(In reply to Eduardo from comment #25) 
> I have a PowerColor RedDevil 5700XT and for me, Kernel 5.4-rc1 just works.  
> Memory clocks always at 100Mhz when idle,
even using KDE (Plasma 5.16).

> I'm using Display Port, with FreeSync ON, on a 75HZ monitor, with 2560x1080
> resolution.

It works for you because your resolution is below a certain threshold in
resolution and refresh rate, and not a multi-monitor setup.

Anyway, I think I have a clearer picture of things now.

Firstly, Navi does still have a few additional power consumption issues (as
compared to Vega). One such issue is that if during the boot sequence the
monitor switches to 144hz (the default on one of my monitors), than that's it,
it's impossible to clock down from the maximum mclk at any point. One
workaround is to boot with the monitor unplugged, then plug it in after. It
seems to happen later in the boot sequence (but before X) so it's possible this
can be worked around by changing framebuffer settings.  Furthermore,
2560x1440 at 144hz in general forces the card to the maximum clock (even with the
workaround, although you can switch down to a different mode at least), whereas
Vega stays at a lower mclk.

Secondly, multi-monitor configurations will force the card to the maximum
clock, by design (on all GPUs). You can workaround this by setting both cards
to the same resolution/refresh rate, provided you have a newer kernel. However,
this doesn't work with Navi. Another hacky workaround is to write data to the
powerplay tables while in dual-monitor setup with mismatched modes, but this is
just a hack, and I can't promise stability (although it worked for me, again,
only with Vega).

Thirdly, Navi uses a lot more power at idle compared to Vega, even when both
are
in the maximum mclk.  E.g. Vega uses around 15W, in a multi-monitor
configuration (2560x1440 at 90 + 2560x1440 at 144).
Whereas Navi will use 36W for pretty much any configuration that hits the
maximum mclk. It could be that HBM is more efficient, lower voltages, or even a
reporting error (I haven't tested at the wall, yet).

So in short:
- Navi + 144hz at boot completely breaks mclk switching 
- Navi + 144hz uses unnecessarily high mclk (compared to Vega)
- Multi-monitor high mclk is by design (all GPUs)
- Navi uses a lot more power at idle than Vega, when at the same mclk

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