Re: [PATCH v3] drm/i915: Apply Wa Display #1183 on skl, kbl, and cfl

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Nov 13, 2017 at 01:47:26PM -0800, Lucas De Marchi wrote:
> Hi Ville,
> 
> On Thu, Nov 9, 2017 at 8:58 AM, Ville Syrjälä
> <ville.syrjala@linux.intel.com> wrote:
> > On Thu, Nov 09, 2017 at 08:02:40AM -0800, Lucas De Marchi wrote:
> >> On Thu, Nov 9, 2017 at 5:11 AM, Ville Syrjälä
> >> <ville.syrjala@linux.intel.com> wrote:
> >> > On Thu, Nov 09, 2017 at 02:58:04AM -0800, Lucas De Marchi wrote:
> >> >> Wa Display #1183 was recently added to workaround
> >> >> "Failures when enabling DPLL0 with eDP link rate 2.16
> >> >> or 4.32 GHz and CD clock frequency 308.57 or 617.14 MHz
> >> >> (CDCLK_CTL CD Frequency Select 10b or 11b) used in this
> >> >>  enabling or in previous enabling."
> >> >>
> >> >> This Workaround was designed to minimize the impact only
> >> >> to save the bad case with that link rates. But HW engineers
> >> >> indicated that it should be safe to apply broadly. Although
> >> >> they were expecting the DPLL0 link rate to be unchanged on
> >> >> runtime.
> >> >>
> >> >> We need to cover 2 cases: when we are in fact enabling DPLL0
> >> >> and when we are just changing the frequency. The workaround
> >> >> for those cases are similar but different enough to have them
> >> >> done in different places.
> >> >>
> >> >> This is based on previous patch by Rodrigo Vivi with suggestions
> >> >> from Ville Syrjälä.
> >> >
> >> > Still doesn't look like what I suggested.
> >>
> >> I agree with your suggestion of moving stuff to skl_set_cdclk() to
> >> cover the case in which
> >> vco isn't changing. However see the paragraph I added above on why I
> >> need to do it
> >> differently. In short: the sequence on the WA for enabling and
> >> updating cdclck are different,
> >> with some code duplication unfortunately. I don't see you covering
> >> that case in your
> >> suggestion. Have I missed anything?
> >
> > Even if we follow the spec literally I think we can do it all
> > in skl_set_cdclk(). I think the following should dtrt:
> 
> There are some subtle differences wrt to the initialize and update
> sequences according to the WA
> that I'd like to clarify.
> 
> >
> > pcu start
> >
> > if (...)
> >         disable_dpll0()
> >
> > cdclk_sel = real
> 
> We should only do this if we are enabling, but not when updating. In
> the latter case
> cdclk_sel should only be touched after setting divmux to 1.

Seems like a pointless distinction to me. We'll be doing the 0->real
toggle anyway while divmux_override==1. But if we want to be pedantic,
then we could of course just skip this if the DPLL is already enabled.

> 
> >
> > if (need_wa)
> >         divmux=1
> 
> Reading the WA to the letter, in the enabling case this should happen between
> DPLL_CTRL1 and LCPLL1_CTL are written.  Here you are moving it to happen before
> the write to DPLL_CTRL1.

I assume that until the DPLL is enabled the settings in DPLL_CTRL1
don't actually matter.

> 
> >
> > if (...)
> >         enable_dpll0()
> >
> > if (need_wa) {
> >         cdclk_sel = 0
> >         cdclk_sel = real
> 
> When updating we should set both freq_sel and and freq_decimal. When
> enabling, only freq_sel (but I guess
> it doesn't matter since we set this same register above).

I think the implication is just that the "decimal" frequency doesn't
matter until something actually starts to use cdclk. The safe bet
would be to always program it to match the frequency select.

> 
> 
> >         divmux=0
> > }
> 
> With this sequence you would actually not change the frequency for the
> cases in which the WA is not
> required. AFAIU from previous version of this patch it's ok to always
> follow the WA path so we wouldn't
> have a "need_wa". Is that ok?  I can come up with a patch that shares
> more code, but I don't think your
> approach is following the spec literally.

Yeah, the exact conditions for need_wa seem a bit unlcear to me since it
says "... used in this enabling or in previous enabling". I'm not sure
if it's referring to the DPLL or CDCLK frequency or both. Maybe safer to
just always follow the w/a sequence.

-- 
Ville Syrjälä
Intel OTC
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