[PATCH] drm/amdgpu: set bypass mode when uvd is idle.

[Deucher, Alexander]

> -----Original Message-----
> From: amd-gfx [mailto:amd-gfx-bounces at lists.freedesktop.org] On Behalf
> Of Andy Furniss
> Sent: Sunday, November 06, 2016 3:31 PM
> To: Zhu, Rex; Deucher, Alexander; amd-gfx at lists.freedesktop.org
> Subject: Re: [PATCH] drm/amdgpu: set bypass mode when uvd is idle.
> 
> Zhu, Rex wrote:
> >>>> Is there any harm in just always putting it into bypass mode or
> >>>> does it interact badly with PG?  Presumably it does (otherwise
> >>>> we wouldn't need this patch), it would be good to note why.
> >
> > Rex: when UVD PG enabled, DCLK/VCLK will be turn off when uvd is
> > idle(DCLK=OFF). If we set bypass mode=1, dclk/vclk will be bypassed
> > to an external ‘Bypass’ clock(DCLK = 100MHz)
> >
> > So it is unnecessary to set bypass mode when PG enabled.
> >
> > +uvd_v5_0_set_bypass_mode(adev, !enable); This change is because
> > tom's commit 72cb64c1f6a3a8129af341e90418a687c4971a40 Fix the
> > sequence of UVD powergate function in smu7_clockgating.c.
> 
> I was about to file a bug till I tried this which fixes UVD perf
> on my R9285 + agd5f drm-next-4.10-wip.
> 
> Additional unrelated question = I notice that UVD does not seem
> to set other clocks quite high enough when used.
> 
> For playback the vo may bump things up a bit, but even then it can be a bit
> borderline for playing high bitrate UHD with powerplay on auto.
> 
> Pure decode benchmarks like
> 
> ffmpeg -hwaccel vdpau -i high-bitrate-2160p60-vid -pix_fmt nv12 -f null -
> 
> go from 63 -> 81 fps, powerplay auto -> high.

The UVD and gfx clocks are separate.  The gfx load for video decode operations is not generally great enough (CSC and maybe scaling) to generate enough gfx load to boost the gfx clocks to their highest level.  We plan to add an API to allow userspace applications to request a minimum floor for specific contexts, but it hasn't been implemented yet.  This is useful if you are trying to hit maximum decode rates, but may not always be the best choice for power usage.  You really only want to set the clocks high enough to hit the target frame rate.

Alex