Re: [Intel-gfx] [PATCH 11/15] drm/i915: Nuke intel_bw_calc_min_cdclk()

["Lisovskiy, Stanislav" <stanislav.lisovskiy@intel.com>]

On Tue, Feb 01, 2022 at 12:05:13PM +0200, Ville Syrjälä wrote:
> On Tue, Feb 01, 2022 at 10:52:39AM +0200, Lisovskiy, Stanislav wrote:
> > On Tue, Jan 18, 2022 at 11:23:50AM +0200, Ville Syrjala wrote:
> > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > 
> > > intel_bw_calc_min_cdclk() is entirely pointless. All it manages to do is
> > > somehow conflate the per-pipe min cdclk with dbuf min cdclk. There is no
> > > (at least documented) dbuf min cdclk limit on pre-skl so let's just get
> > > rid of all this confusion.
> > > 
> > > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > I think we constantly have a bit contradictional attitude towards such situation.
> > >From one perspective you can say, that those kind of "leagcy" callbacks are
> > pointless, from the other hand one might say. that we need to have a unified
> > approach for all platforms and I think we got, some legacy handlers for old
> > platforms for similar purpose as well.
> > I'm fine with both approaches, however for example when I was submitting
> > that patch, I was asked by reviewer to add this kind of legacy callback, so that we have
> > a "uniform" approach.
> > We just then need to have some standard agreement on those, which doesn't
> > depend on today's cosmic radiation levels :)
> 
> Yes in general I prefer a unified approach across all platforms.
> But in this case there is nothing to do for the old platforms as they
> don't have any kind of dbuf cdclk limit, or if there is one we don't
> know what it is since it's not documented.
> 
> So the only thing the code was really doing was conflating the
> per-pipe cdclk limit (which is handled elsewhere for all platforms
> in a  unified fashion) with something that doesn't even exist.
> 
> Also I don't think it was even correct anyway since it was
> using the per-pipe cdclk_state->min_cdclk[] already during
> intel_cdclk_atomic_check(), but cdclk_state->min_cdclk[]
> isn't even computed until intel_modeset_calc_cdclk() which 
> is called later. So I guess it was basically just computing 
> the max of the min_cdclk[] for all the pipes for the _old_
> state, not the new state.

No, I think actually the idea was that it was first calculating
new_bw_state->min_cdclk, based on plane and dbuf bandwidth requirements
in intel_atomic_check_cdclk, however then the final decision which
cdclk to choose was is done in intel_cdclk.c, which calculated new_cdclk_state->min_cdclk
and then we just choose maximum of those.
And intel_compute_min_cdclk is the final arbiter:

static int intel_compute_min_cdclk(struct intel_cdclk_state *cdclk_state)
{
        struct intel_atomic_state *state = cdclk_state->base.state;
        struct drm_i915_private *dev_priv = to_i915(state->base.dev);
        struct intel_bw_state *bw_state = NULL;
        struct intel_crtc *crtc;
        struct intel_crtc_state *crtc_state;
        int min_cdclk, i;
        enum pipe pipe;

        for_each_new_intel_crtc_in_state(state, crtc, crtc_state, i) {
                int ret;

                min_cdclk = intel_crtc_compute_min_cdclk(crtc_state);
                if (min_cdclk < 0)
                        return min_cdclk;

                bw_state = intel_atomic_get_bw_state(state);
                if (IS_ERR(bw_state))
                        return PTR_ERR(bw_state);

                if (cdclk_state->min_cdclk[crtc->pipe] == min_cdclk)
                        continue;

                cdclk_state->min_cdclk[crtc->pipe] = min_cdclk;

                ret = intel_atomic_lock_global_state(&cdclk_state->base);
                if (ret)
                        return ret;
        }

        min_cdclk = cdclk_state->force_min_cdclk;
        for_each_pipe(dev_priv, pipe) {
                min_cdclk = max(cdclk_state->min_cdclk[pipe], min_cdclk);

                if (!bw_state)
                        continue;

                min_cdclk = max(bw_state->min_cdclk, min_cdclk);
        }

        return min_cdclk;
}

Stan

> 
> -- 
> Ville Syrjälä
> Intel