[PATCH] drm/i915/display: Prevent DC6 while vblank is enabled for Panel Replay

[Ville Syrjälä]

On Wed, Sep 11, 2024 at 01:14:33PM +0000, Hogander, Jouni wrote:
> On Wed, 2024-09-11 at 16:00 +0300, Ville Syrjälä wrote:
> > On Wed, Sep 11, 2024 at 03:40:15PM +0300, Jouni Högander wrote:
> > > We need to block DC6 entry in case of Panel Replay as enabling VBI
> > > doesn't
> > > prevent DC6 in case of Panel Replay.
> > 
> > This doesn't make sense to me. I *think* we are currently
> > supposed to always operate in the "main link on" mode for panel
> > replay.
> 
> This is not true. Check bspec 68920:
> 
> "When performing PR on an eDP port the Source will allow advanced link
> power management (ALPM) to turn the Main Link OFF when not sending an
> SDP or update region."

Right, it seems to be a thing for eDP only.

> 
> And if you check block_dc6_needed in my patch that is checking eDP.
> 
> I was originally planning to handle this by preventing PR entry when
> VBLANK is enabled, but that would be more expensive from power
> managements point of view -> decided to go with blocking DC6.

None of this explains how DC6 vs. DC5 is somehow different.
DC5 should already turn of all the clocks/etc so nothing real
can actually happen anymore. The only thing DC6 adds on top
of DC5 is turning off some extra power wells.

Hmm. So get_allowed_dc_mask() seems to be telling me that new
platforms only have DC6 but no DC5. Is that correct or not?
No idea. But that means we are in fact disabling all DC states
and that at least explains how something might happen due to
this patch.

The one thing that still doesn't quite make sense is that I would
assume that the main link would get turned off regardless of DC6
or not, which I would think causes the timing generator to stop
anyway and should still give us no vblanks...

-- 
Ville Syrjälä
Intel