Re: [RFC PATCH] drm/i915/skl: Add DC6 disabling as a power well

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Sep 21, 2015 at 10:00:45AM +0200, Patrik Jakobsson wrote:
> On Wed, Sep 16, 2015 at 11:10:07PM +0300, Ville Syrjälä wrote:
> > On Fri, Sep 11, 2015 at 01:55:22PM +0200, Patrik Jakobsson wrote:
> > > We need to be able to control if DC6 is allowed or not. Much like
> > > requesting power to a specific piece of the hardware we need to be able
> > > to request that we don't enter DC6 during certain hw access.
> > > 
> > > To solve this without introducing too much infrastructure I'm hooking
> > > into the power well / power domain framework. DC6 prevention is modeled
> > > much like an enabled power well. Thus I'm using the terminology on/off
> > > for DC states instead of enable/disable.
> > > 
> > > The problem that started this work is the need for DC6 to be disabled
> > > when accessing DP_AUX_A during CRTC on/off. That is also fixed in this
> > > patch.
> > > 
> > > This is posted as an RFC since DMC and DC state handling is being
> > > reworked and will possibly affect the outcome of this patch. The patch
> > > has known warnings.
> > > 
> > > Signed-off-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_ddi.c        |  9 +++++
> > >  drivers/gpu/drm/i915/intel_drv.h        |  2 +
> > >  drivers/gpu/drm/i915/intel_runtime_pm.c | 69 +++++++++++++++++++++++++--------
> > >  3 files changed, 64 insertions(+), 16 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_ddi.c b/drivers/gpu/drm/i915/intel_ddi.c
> > > index 4823184..c2c1ad2 100644
> > > --- a/drivers/gpu/drm/i915/intel_ddi.c
> > > +++ b/drivers/gpu/drm/i915/intel_ddi.c
> > > @@ -2288,6 +2288,8 @@ static void intel_ddi_pre_enable(struct intel_encoder *intel_encoder)
> > >  	if (type == INTEL_OUTPUT_DISPLAYPORT || type == INTEL_OUTPUT_EDP) {
> > >  		struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
> > >  
> > > +		intel_display_power_get(dev_priv, POWER_DOMAIN_AUX_A);
> > > +
> > 
> > These I think shouldn't be necessary with my
> > intel_display_port_aux_power_domain() stuff since intel_dp_aux_ch() will
> > itself grab the appropriate power domain.
> > 
> > That's of course assuming that AUX is the only reason why we need to
> > keep DC6 disabled here.
> > 
> 
> The upside with having get/put around bigger aux transfers is that we don't get
> tons of enable/disable lines in the log. My vote is that we keep this but also
> have your fine-grained get/puts.

Imo the correct solution to avoid this is by adding a slight bit of
hystersis to the power well code. Which means that yes, we reinvent
another feature of the core power_domain code in our home-grown solution -
I hate it when my years old predictions come true ;-)

Sprinkling higher-level get/put calls all over the place is imo just
leaking the abstraction, which isn't good.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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