Re: [PATCH 2/3] drm/i915/skl: Flush the WM configuration

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Oct 15, 2014 at 06:34:43PM +0100, Damien Lespiau wrote:
> When we write new values for the DDB allocation and WM parameters, we now
> need to trigger the double buffer update for the pipe to take the new
> configuration into account.
> 
> As the DDB is a global resource shared between planes, enabling or
> disabling one plane will result in changes for all planes that are
> currently in use, thus the need write PLANE_SURF/CUR_BASE for more than
> the plane we're touching.
> 
> v2: Don't wait for pipes that are off
> 
> v3: Split the staging results structure to not exceed the 1Kb stack
>     allocation in skl_update_wm()
> 
> v4: Rework and document the algorithm after Ville found that it was all
>     wrong.
> 
> Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_pm.c | 135 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 135 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 118eb95..99f7c40 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -3507,6 +3507,140 @@ static void skl_write_wm_values(struct drm_i915_private *dev_priv,
>  	}
>  }
>  
> +/*
> + * When setting up a new DDB allocation arrangement, we need to correctly
> + * sequence the times at which the new allocations for the pipes are taken into
> + * account or we'll have pipes fetching from space previously allocated to
> + * another pipe.
> + *
> + * Roughly the sequence looks like:
> + *  1. re-allocate the pipe(s) with the allocation being reduced and not
> + *     overlapping with a previous light-up pipe (another way to put it is:
> + *     pipes with their new allocation strickly included into their old ones).
> + *  2. re-allocate the other pipes that get their allocation reduced
> + *  3. allocate the pipes having their allocation increased
> + *
> + * Steps 1. and 2. are here to take care of the following case:
> + * - Initially DDB looks like this:
> + *     |   B    |   C    |
> + * - enable pipe A.
> + * - pipe B has a reduced DDB allocation that overlaps with the old pipe C
> + *   allocation
> + *     |  A  |  B  |  C  |
> + *
> + * We need to sequence the re-allocation: C, B, A (and not B, C, A).
> + */
> +
> +static void skl_wm_flush_pipe(struct drm_i915_private *dev_priv, enum pipe pipe)
> +{
> +	struct drm_device *dev = dev_priv->dev;
> +	int plane;
> +
> +	for_each_plane(pipe, plane) {
> +		I915_WRITE(PLANE_SURF(pipe, plane),
> +			   I915_READ(PLANE_SURF(pipe, plane)));
> +	}
> +	I915_WRITE(CURBASE(pipe), I915_READ(CURBASE(pipe)));
> +}
> +
> +static bool
> +skl_ddb_allocation_included(const struct skl_ddb_allocation *old,
> +			    const struct skl_ddb_allocation *new,
> +			    enum pipe pipe)
> +{
> +	uint16_t old_size, new_size;
> +
> +	old_size = skl_ddb_entry_size(&old->pipe[pipe]);
> +	new_size = skl_ddb_entry_size(&new->pipe[pipe]);
> +
> +	return old_size != new_size &&

First I was thinking this size check is redundant, but it's actually
needed to avoid the vblank wait when things didn't change.

The algorithm itself looks solid to me. Well, assuming that the
allocated chunks are always sorted the same way. Which they since the
allocation uses a for_each_pipe() loop. Anything else would be mad
anyway because then we might not be able to reallocate without
temporarily disabling some planes.

The only issue with this is that it actually shouldn't be hiding in the
wm code AFAICS, and instead we should drive the whole atomic commit
from this same algorithm so that the DDB allocation, watermarks and plane
config all agree with each other, all the time. So basically
skl_wm_flush_pipe() should eventually become something like:

{
 for_each_plane_on_crtc()
 	plane.commit();
 crtc.commit(); // for global pipe properties
}

But it seems good enough for now given that we're not yet living in the
nuclear age.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> +	       new->pipe[pipe].start >= old->pipe[pipe].start &&
> +	       new->pipe[pipe].end <= old->pipe[pipe].end;
> +}
> +
> +static void skl_flush_wm_values(struct drm_i915_private *dev_priv,
> +				struct skl_wm_values *new_values)
> +{
> +	struct drm_device *dev = dev_priv->dev;
> +	struct skl_ddb_allocation *cur_ddb, *new_ddb;
> +	bool reallocated[I915_MAX_PIPES] = {false, false, false};
> +	struct intel_crtc *crtc;
> +	enum pipe pipe;
> +
> +	new_ddb = &new_values->ddb;
> +	cur_ddb = &dev_priv->wm.skl_hw.ddb;
> +
> +	/*
> +	 * First pass: flush the pipes with the new allocation contained into
> +	 * the old space.
> +	 *
> +	 * We'll wait for the vblank on those pipes to ensure we can safely
> +	 * re-allocate the freed space without this pipe fetching from it.
> +	 */
> +	for_each_intel_crtc(dev, crtc) {
> +		if (!crtc->active)
> +			continue;
> +
> +		pipe = crtc->pipe;
> +
> +		if (!skl_ddb_allocation_included(cur_ddb, new_ddb, pipe))
> +			continue;
> +
> +		skl_wm_flush_pipe(dev_priv, pipe);
> +		intel_wait_for_vblank(dev, pipe);
> +
> +		reallocated[pipe] = true;
> +	}
> +
> +
> +	/*
> +	 * Second pass: flush the pipes that are having their allocation
> +	 * reduced, but overlapping with a previous allocation.
> +	 *
> +	 * Here as well we need to wait for the vblank to make sure the freed
> +	 * space is not used anymore.
> +	 */
> +	for_each_intel_crtc(dev, crtc) {
> +		if (!crtc->active)
> +			continue;
> +
> +		pipe = crtc->pipe;
> +
> +		if (reallocated[pipe])
> +			continue;
> +
> +		if (skl_ddb_entry_size(&new_ddb->pipe[pipe]) <
> +		    skl_ddb_entry_size(&cur_ddb->pipe[pipe])) {
> +			skl_wm_flush_pipe(dev_priv, pipe);
> +			intel_wait_for_vblank(dev, pipe);
> +		}
> +
> +		reallocated[pipe] = true;
> +	}
> +
> +	/*
> +	 * Third pass: flush the pipes that got more space allocated.
> +	 *
> +	 * We don't need to actively wait for the update here, next vblank
> +	 * will just get more DDB space with the correct WM values.
> +	 */
> +	for_each_intel_crtc(dev, crtc) {
> +		if (!crtc->active)
> +			continue;
> +
> +		pipe = crtc->pipe;
> +
> +		/*
> +		 * At this point, only the pipes more space than before are
> +		 * left to re-allocate.
> +		 */
> +		if (reallocated[pipe])
> +			continue;
> +
> +		skl_wm_flush_pipe(dev_priv, pipe);
> +	}
> +}
> +
>  static bool skl_update_pipe_wm(struct drm_crtc *crtc,
>  			       struct skl_pipe_wm_parameters *params,
>  			       struct intel_wm_config *config,
> @@ -3598,6 +3732,7 @@ static void skl_update_wm(struct drm_crtc *crtc)
>  
>  	skl_update_other_pipe_wm(dev, crtc, &config, results);
>  	skl_write_wm_values(dev_priv, results);
> +	skl_flush_wm_values(dev_priv, results);
>  
>  	/* store the new configuration */
>  	dev_priv->wm.skl_hw = *results;
> -- 
> 1.8.3.1
> 
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-- 
Ville Syrjälä
Intel OTC
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