Re: [PATCH] drm/i915/cdclk: Rename intel_cdclk_needs_modeset to intel_cdclk_params_changed

From: Gustavo Sousa <gustavo.sousa@intel.com>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: <intel-gfx@lists.freedesktop.org>
Subject: Re: [PATCH] drm/i915/cdclk: Rename intel_cdclk_needs_modeset to intel_cdclk_params_changed
Date: Wed, 14 Feb 2024 16:56:50 -0300	[thread overview]
Message-ID: <170794061034.56490.5633508531160366518@gjsousa-mobl2> (raw)
In-Reply-To: <ZcEAIfeJyyztDa2-@intel.com>

Hi, Ville.

Sorry for taking long to get back to this.

Quoting Ville Syrjälä (2024-02-05 12:34:57-03:00)
>On Sat, Feb 03, 2024 at 10:25:18AM -0300, Gustavo Sousa wrote:
>> Quoting Ville Syrjälä (2024-02-02 16:58:37-03:00)
>> >On Fri, Feb 02, 2024 at 10:12:08AM -0300, Gustavo Sousa wrote:
>> >> Looks like the name and description of intel_cdclk_needs_modeset()
>> >> became inacurate as of commit 59f9e9cab3a1 ("drm/i915: Skip modeset for
>> >> cdclk changes if possible"), when it became possible to update the cdclk
>> >> without requiring disabling the pipes when only changing the cd2x
>> >> divider was enough.
>> >> 
>> >> Later on we also added the same type of support with squash and crawling
>> >> with commit 25e0e5ae5610 ("drm/i915/display: Do both crawl and squash
>> >> when changing cdclk"), commit d4a23930490d ("drm/i915: Allow cdclk
>> >> squasher to be reconfigured live") and commit d62686ba3b54
>> >> ("drm/i915/adl_p: CDCLK crawl support for ADL").
>> >> 
>> >> As such, update that function's name and documentation to something more
>> >> appropriate, since the real checks for requiring modeset are done
>> >> elsewhere.
>> >> 
>> >> Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
>> >> ---
>> >> 
>> >> One thing worth noting here is that, with this change, we are left with an
>> >> awkward situation where two function names related to checking changes in cdclk:
>> >> 
>> >>   intel_cdclk_params_changed() and intel_cdclk_changed()
>> >> 
>> >> ,
>> >> 
>> >> and I find it weird that we have intel_cdclk_changed(), which checks for the
>> >> voltage level as well. Shouldn't the voltage level be a function of cdclk and
>> >> ddi clock? Why do we need that?
>> >> 
>> >>  drivers/gpu/drm/i915/display/intel_cdclk.c        | 15 +++++++--------
>> >>  drivers/gpu/drm/i915/display/intel_cdclk.h        |  4 ++--
>> >>  .../drm/i915/display/intel_display_power_well.c   |  4 ++--
>> >>  3 files changed, 11 insertions(+), 12 deletions(-)
>> >> 
>> >> diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
>> >> index 26200ee3e23f..caadd880865f 100644
>> >> --- a/drivers/gpu/drm/i915/display/intel_cdclk.c
>> >> +++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
>> >> @@ -2233,17 +2233,16 @@ static bool intel_cdclk_can_squash(struct drm_i915_private *dev_priv,
>> >>  }
>> >>  
>> >>  /**
>> >> - * intel_cdclk_needs_modeset - Determine if changong between the CDCLK
>> >> - *                             configurations requires a modeset on all pipes
>> >> + * intel_cdclk_params_changed - Check whether CDCLK parameters changed
>> >>   * @a: first CDCLK configuration
>> >>   * @b: second CDCLK configuration
>> >>   *
>> >>   * Returns:
>> >> - * True if changing between the two CDCLK configurations
>> >> - * requires all pipes to be off, false if not.
>> >> + * True if parameters changed in a way that requires programming the CDCLK
>> >> + * and False otherwise.
>> >>   */
>> >> -bool intel_cdclk_needs_modeset(const struct intel_cdclk_config *a,
>> >> -                               const struct intel_cdclk_config *b)
>> >> +bool intel_cdclk_params_changed(const struct intel_cdclk_config *a,
>> >> +                                const struct intel_cdclk_config *b)
>> >
>> >The new name isn't very descriptive either.
>> 
>> Yeah... I would much rather use intel_cdclk_changed(), but that one is
>> already taken.
>> 
>> >
>> >Outside the cd2x/crawl/squash cases we stil have to consider
>> >two cases:
>> >1. cdclk frequency/pll changes (voltage level can change or not)
>> >2. cdclk frequency/pll doesn't change, but voltage level needs to change
>> >
>> >And that difference is what intel_cdclk_needs_modeset() is trying
>> >convey. And intel_cdclk_changed() tells us whether anything at all
>> >is changing.
>> 
>> I might be missing something, but, by going through the specs, it looked
>> to me that voltage level was dependent on cdclk (as well as on ddi
>> clock) and not the other way around. That's why I find it odd that we
>> need an intel_cdclk_changed() that, besides looking for changes in
>> cdclk, also checks for the voltage level.
>> 
>> In intel_set_cdclk(), we check intel_cdclk_changed() before continuing.
>> If, for example, there is a change in ddi clock that requires a change
>> in voltage level but no changes in cdclk, intel_cdclk_changed() would
>> return true, right? Wouldn't that make us unnecessarily go through
>> intel_set_cdclk()?
>
>intel_set_cdclk() is the thing that does the voltage change.

Yep and perhaps I provided an incomplete response above. Sorry.

I was wondering if handling voltage level should really be
intel_set_cdclk()'s responsibility.

I might be missing the big picture here, but, at least for the recent
platforms, I get the understanding that voltage level handling should be
a separate step in the hardware commit process.

Would it be possible to have a commit containing (i) update(s) to ddi
clk and (ii) no update to cdclk such that (i) require an update to
voltage level, right?

--
Gustavo Sousa