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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 12 Feb 2020, Ville Syrjala <ville.syrjala@linux.intel.com> wrote:
> From: Ville Syrjälä <ville.syrjala@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.

I have a hard time following all the implications of this change. Would
this under some circumstances lead to a case where we use the state read
at probe, and do a full modeset on that state?

DSC, lacking full state readout, fails badly with this change [1]. We'll
do DSC enable using a mostly zeroed out DSC config in state. Leading to
division by zero.

BR,
Jani.


[1] https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_16544/fi-tgl-dsi/boot0.txt

>
> Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> Fixes: ff2cd8635e41 ("drm/i915: Correctly map DBUF slices to pipes")
> Signed-off-by: Ville Syrjälä <ville.syrjala@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.
> +	 */
> +	if (dev_priv->wm.distrust_bios_wm)
> +		any_ms = true;
> +
>  	if (any_ms) {
>  		ret = intel_modeset_checks(state);
>  		if (ret)

-- 
Jani Nikula, Intel Open Source Graphics Center
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