[Intel-gfx] [PATCH] drm/i915: Force state->modeset=true when distrust_bios_wm==true

[Lisovskiy, Stanislav]

On Wed, 2020-02-12 at 17:01 +0200, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Currently when we load the driver we set distrust_bios_wm=true, which
> will cause active_pipe_changes to get flagged even when we're not
> toggling any pipes on/off. The reason being that we want to fully
> redistribute the dbuf among the active pipes and ignore whatever
> state the firmware left behind.
> 
> Unfortunately when the code flags active_pipe_changes it doesn't
> set state->modeset to true, which means the hardware dbuf state
> won't actually get updated. Hence the hardware and software
> states go out of sync, which can result in planes trying to use a
> disabled dbuf slice. Suprisingly that only seems to corrupt the
> display rather than making the whole display engine keel over.
> 
> Let's fix this for now by flagging state->modeset whenever
> distrust_bios_wm is set.
> 
> Eventually we'll likely want to rip out all of this mess and
> introduce proper statye tracking for dbuf. But that requires
> more work. Toss in a FIXME to that effect.
> 
> Cc: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
> Fixes: ff2cd8635e41 ("drm/i915: Correctly map DBUF slices to pipes")
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_display.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c
> b/drivers/gpu/drm/i915/display/intel_display.c
> index 61ba1f2256a0..9e4f99ae81fb 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -15027,6 +15027,20 @@ static int intel_atomic_check(struct
> drm_device *dev,
>  	if (new_cdclk_state && new_cdclk_state-
> >force_min_cdclk_changed)
>  		any_ms = true;
>  
> +	/*
> +	 * distrust_bios_wm will force a full dbuf recomputation
> +	 * but the hardware state will only get updated accordingly
> +	 * if state->modeset==true. Hence distrust_bios_wm==true &&
> +	 * state->modeset==false is an invalid combination which
> +	 * would cause the hardware and software dbuf state to get
> +	 * out of sync. We must prevent that.
> +	 *
> +	 * FIXME clean up this mess and introduce better
> +	 * state tracking for dbuf.

I disagree here in that sense, that this is actually not a DBUF
issue :) 
But merely state->modeset and active_pipe_changes going out of sync.
However refactoring here definitely will help to unite those
to somekind of unified new state. At least having this
active_pipe_changes seems to be redundant.

> +	 */
> +	if (dev_priv->wm.distrust_bios_wm)
> +		any_ms = true;
> +

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>

>  	if (any_ms) {
>  		ret = intel_modeset_checks(state);
>  		if (ret)