Re: [RFC] drm/amd/display: add SI support to AMD DC

[sylvain.bertrand-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org]

On Sun, Oct 14, 2018 at 11:47:18PM +0200, Mauro Rossi wrote:
> DOUBT: I think that it would make sense to set "power level 0" i.e.
> the "lower state" as safe default,
> considering that powerplay smu6/hwmgr are not implemented for SI and
> smu7 CIK functions do not work,
> the AS-IS dpm is the only available option. (and it seams to be
> working, looking at the framerates 250-280 in the V1 Vulkan benchmark)

dpm not implemented for SI? Are you sure? Because I recall passing "messages"
to the smu6 to switch between major power modes. And currently in amdgpu I
switch back and forth between "high" and "auto". And if I stay on "high" my GPU
fans are going crazy all the time, which forces me to switch back to "auto" in
order to quiet the fans.
Any major state change for any block would have to go through the dpm block
(smu), because it has to be accounted by this block for proper operation. One
of the hard thing to code, coze no proper documentation, was to set the blocks
in an initial state from a poweron/hard reset/etc which the dpm will know
how to start/catch-up its operations from. In a perfect world, the dpm block
would have the knowledge on how to program any block in an initial working
state from any unknown state (and in case of hard block hang, would know how to
hard reset it and set it in a initial state).

> If freesync is about reducing the framerate rate for power saving,
> provided that I've seen it be mentioned the first time for GCN 2nd generation,
> I'm not expecting freesync as a mandatory capability for the series.

Well, there are the low FPS games, and the movies. I am a regular gamer, and I
know that sustained 60 fps is the very lower bound for many games on a desktop
computer display.
It does not apply to mobile size display, and "couch playing" on a TV.  The
readibilty of the game depends directly on the amplitude of movements in
physical distance on the display, their speed, the viewing distance, and the
FPS. From my personal comfort point of view and types of games I play, on a
desktop display (dpi, typical viewing distance, typical size), I would target a
minimum of 80-100 fps (120/144 fps seems to be "perfect" targets).
Played rise of the tomb raider (vulkan) on my tahiti part, and even with a blur
effect to "smooth" the perceived fps like in movies, sub-60 fps is really
uncomfortable. I was about to stop playing to this game even though it is a
rather "slow" video game.

> In amd-staging-drm-next dce_clock_source is generic, SI specifics are
> not necessary anymore.

If I recall properly, "bandwidths" and "watermarks" calcs had asic generations
specific code paths. bandwidth is important for displayport programming or
maybe you can presume the maximum all the time (and sacrifice some unkown
amount of power) and watermarks, I know in my old driver I was giving the
highest priority the the dce block anyway (something related to a "memory
aribtrer" and "line buffer").

> Any other testing tools worth a run?

AAA games (vulkan: rize of the tomb raider, GL:bioshock infinite) and AAAA games
(dota2(gl and vulkan), cs:go...)

-- 
Sylvain
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