Re: [PATCH] drm/i915: Fix legacy cursor vs. watermarks for ILK-BDW

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 17-02-17 om 16:01 schreef ville.syrjala@linux.intel.com:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> In order to make cursor updates actually safe wrt. watermark programming
> we have to clear the legacy_cursor_update flag in the atomic state. That
> will cause the regular atomic update path to do the necessary vblank
> wait after the plane update if needed, otherwise the vblank wait would
> be skipped and we'd feed the optimal watermarks to the hardware before
> the plane update has actually happened.
>
> To make the slow vs. fast path determination in
> intel_legacy_cursor_update() a little simpler we can ignore the actual
> visibility of the plane (which can only get computed once we've already
> chosen out path) and instead we simply check whether the fb is being
> set or cleared by the user. This means a fully clipped but logically
> visible cursor will be considered visible as far as watermark
> programming is concerned. We can do that for the cursor since it's a
> fixed size plane and the clipped size doesn't play a role in the
> watermark computation.
>
> This should fix underruns that can occur when the cursor gets
> enable/disabled or the size gets changed. Hopefully it's good enough
> that only pure cursor movement and flips go through unthrottled.
>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Uwe Kleine-König <uwe@kleine-koenig.org>
> Reported-by: Uwe Kleine-König <uwe@kleine-koenig.org>
> Fixes: f79f26921ee1 ("drm/i915: Add a cursor hack to allow converting legacy page flip to atomic, v3.")
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 29 ++++++++++++++++++-----------
>  drivers/gpu/drm/i915/intel_pm.c      | 20 ++++++++++++--------
>  2 files changed, 30 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index b05d9c85384b..356ac04093e8 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -13031,6 +13031,17 @@ static int intel_atomic_commit(struct drm_device *dev,
>  	struct drm_i915_private *dev_priv = to_i915(dev);
>  	int ret = 0;
>  
> +	/*
> +	 * The intel_legacy_cursor_update() fast path takes care
> +	 * of avoiding the vblank waits for simple cursor
> +	 * movement and flips. For cursor on/off and size changes,
> +	 * we want to perform the vblank waits so that watermark
> +	 * updates happen during the correct frames. Gen9+ have
> +	 * double buffered watermarks and so shouldn't need this.
> +	 */
> +	if (INTEL_GEN(dev_priv) < 9)
> +		state->legacy_cursor_update = false;
Could we perhaps add a check in ilk_compute_pipe_wm which unsets the legacy_cursor_update flag so we keep things unsynced as much as possible?
>  	ret = drm_atomic_helper_setup_commit(state, nonblock);
>  	if (ret)
>  		return ret;
> @@ -13455,8 +13466,7 @@ intel_legacy_cursor_update(struct drm_plane *plane,
>  	    old_plane_state->src_h != src_h ||
>  	    old_plane_state->crtc_w != crtc_w ||
>  	    old_plane_state->crtc_h != crtc_h ||
> -	    !old_plane_state->visible ||
> -	    old_plane_state->fb->modifier != fb->modifier)
> +	    !old_plane_state->fb != !fb)
>  		goto slow;
>  
>  	new_plane_state = intel_plane_duplicate_state(plane);
> @@ -13479,10 +13489,6 @@ intel_legacy_cursor_update(struct drm_plane *plane,
>  	if (ret)
>  		goto out_free;
>  
> -	/* Visibility changed, must take slowpath. */
> -	if (!new_plane_state->visible)
> -		goto slow_free;
> -
>  	ret = mutex_lock_interruptible(&dev_priv->drm.struct_mutex);
>  	if (ret)
>  		goto out_free;
Those 2 hunks are needed. If you move the cursor out of the visible area or back you need to update.
Easiest way to trigger a bug is load a 256 width cursor out of the visible area, move it back in and you get
a fifo underrun.

Why is switching fb's synced? Identical sized fb should be unsynced if possible.
> @@ -13522,9 +13528,12 @@ intel_legacy_cursor_update(struct drm_plane *plane,
>  	new_plane_state->fb = old_fb;
>  	to_intel_plane_state(new_plane_state)->vma = old_vma;
>  
> -	intel_plane->update_plane(plane,
> -				  to_intel_crtc_state(crtc->state),
> -				  to_intel_plane_state(plane->state));
> +	if (plane->state->visible)
> +		intel_plane->update_plane(plane,
> +					  to_intel_crtc_state(crtc->state),
> +					  to_intel_plane_state(plane->state));
> +	else
> +		intel_plane->disable_plane(plane, crtc);
>  
>  	intel_cleanup_plane_fb(plane, new_plane_state);
>  
> @@ -13534,8 +13543,6 @@ intel_legacy_cursor_update(struct drm_plane *plane,
>  	intel_plane_destroy_state(plane, new_plane_state);
>  	return ret;
>  
> -slow_free:
> -	intel_plane_destroy_state(plane, new_plane_state);
>  slow:
>  	return drm_atomic_helper_update_plane(plane, crtc, fb,
>  					      crtc_x, crtc_y, crtc_w, crtc_h,
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index fe243c65de1a..4de8c40acc7e 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -1831,20 +1831,24 @@ static uint32_t ilk_compute_cur_wm(const struct intel_crtc_state *cstate,
>  				   const struct intel_plane_state *pstate,
>  				   uint32_t mem_value)
>  {
> +	int cpp;
> +
>  	/*
> -	 * We treat the cursor plane as always-on for the purposes of watermark
> -	 * calculation.  Until we have two-stage watermark programming merged,
> -	 * this is necessary to avoid flickering.
> +	 * Treat cursor with fb as always visible since cursor updates
> +	 * can happen faster than the vrefresh rate, and the current
> +	 * watermark code doesn't handle that correctly. Cursor updates
> +	 * which set/clear the fb or change the cursor size are going
> +	 * to get throttled by intel_legacy_cursor_update() to work
> +	 * around this problem with the watermark code.
>  	 */
> -	int cpp = 4;
> -	int width = pstate->base.visible ? pstate->base.crtc_w : 64;
> -
> -	if (!cstate->base.active)
> +	if (!cstate->base.active || !pstate->base.fb)
>  		return 0;
>  
> +	cpp = pstate->base.fb->format->cpp[0];
> +
>  	return ilk_wm_method2(cstate->pixel_rate,
>  			      cstate->base.adjusted_mode.crtc_htotal,
> -			      width, cpp, mem_value);
> +			      pstate->base.crtc_w, cpp, mem_value);
>  }
>  
>  /* Only for WM_LP. */


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