Re: [PATCH 15/18] drm/i915: alloc/free the FBC CFB during enable/disable

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Oct 20, 2015 at 11:50:01AM -0200, Paulo Zanoni wrote:
> One of the problems with the current code is that it frees the CFB and
> releases its drm_mm node as soon as we flip FBC's enable bit. This is
> bad because after we disbale FBC the hardware may still use the CFB
> for the rest of the frame, so in theory we should only release the
> drm_mm node one frame after we disable FBC. Otherwise, a stolen memory
> allocation done right after an FBC disable may result in either
> corrupted memory for the new owner of that memory region or corrupted
> screen/underruns in case the new owner changes it while the hardware
> is still reading it. This case is not exactly easy to reproduce since
> we currently don't do a lot of stolen memory allocations, but I see
> patches on the mailing list trying to expose stolen memory to user
> space, so races will be possible.
> 
> I thought about three different approaches to solve this, and they all
> have downsides.
> 
> The first approach would be to simply use multiple drm_mm nodes and
> freeing the unused ones only after a frame has passed. The problem
> with this approach is that since stolen memory is rather small,
> there's a risk we just won't be able to allocate a new CFB from stolen
> if the previous one was not freed yet. This could happen in case we
> quickly disable FBC from pipe A and decide to enable it on pipe B, or
> just if we change pipe A's fb stride while FBC is enabled.
> 
> The second approach would be similar to the first one, but maintaining
> a single drm_mm node and keeping track of when it can be reused. This
> would remove the disadvantage of not having enough space for two
> nodes, but would create the new problem where we may not be able to
> enable FBC at the point intel_fbc_update() is called, so we would have
> to add more code to retry updating FBC after the time has passed. And
> that can quickly get too complex since we can get invalidate, flush,
> flip_prepare, disable and other calls in the middle of the wait.
> 
> Both solutions above - and also the current code - have the problem
> that we unnecessarily free+realloc FBC during invalidate+flush
> operations even if the CFB size doesn't change.
> 
> The third option would be to move the allocation/deallocation to
> enable/disable. This makes sure that the pipe is always disabled when
> we allocate/deallocate the CFB, so there's no risk that the FBC
> hardware may read or write to the memory right after it is freed from
> drm_mm. The downside is that it is possible for user space to change
> the buffer stride without triggering a disable/enable - only
> deactivate/activate -, so we'll have to handle this case somehow, even
> though it is uncommon - see igt's kms_frontbuffer_tracking test,
> fbc-stridechange subtest. It could be possible to implement a way to
> free+alloc the CFB during said stride change, but it would involve a
> lot of book-keeping - exactly as mentioned above - just for a rare
> case, so for now I'll keep it simple and just deactivate FBC. Besides,
> we may not even need to disable FBC since we do CFB over-allocation.
> 
> Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_fbc.c | 132 ++++++++++++++++++++-------------------
>  1 file changed, 68 insertions(+), 64 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_fbc.c b/drivers/gpu/drm/i915/intel_fbc.c
> index bf855b2..48d8cfc 100644
> --- a/drivers/gpu/drm/i915/intel_fbc.c
> +++ b/drivers/gpu/drm/i915/intel_fbc.c
> @@ -59,6 +59,45 @@ static unsigned int get_crtc_fence_y_offset(struct intel_crtc *crtc)
>  	return crtc->base.y - crtc->adjusted_y;
>  }
>  
> +/*
> + * For SKL+, the plane source size used by the hardware is based on the value we
> + * write to the PLANE_SIZE register. For BDW-, the hardware looks at the value
> + * we wrote to PIPESRC.
> + */
> +static void intel_fbc_get_plane_source_size(struct intel_crtc *crtc,
> +					    int *width, int *height)
> +{
> +	struct intel_plane_state *plane_state =
> +			to_intel_plane_state(crtc->base.primary->state);
> +	int w, h;
> +
> +	if (intel_rotation_90_or_270(plane_state->base.rotation)) {
> +		w = drm_rect_height(&plane_state->src) >> 16;
> +		h = drm_rect_width(&plane_state->src) >> 16;
> +	} else {
> +		w = drm_rect_width(&plane_state->src) >> 16;
> +		h = drm_rect_height(&plane_state->src) >> 16;
> +	}
> +
> +	if (width)
> +		*width = w;
> +	if (height)
> +		*height = h;
> +}
> +
> +static int intel_fbc_calculate_cfb_size(struct intel_crtc *crtc,
> +					struct drm_framebuffer *fb)
> +{
> +	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
> +	int lines;
> +
> +	intel_fbc_get_plane_source_size(crtc, NULL, &lines);
> +	if (INTEL_INFO(dev_priv)->gen >= 7)
> +		lines = min(lines, 2048);
> +
> +	return lines * fb->pitches[0];

Why are we even looking at fb->pitches here? I thought that for fbc we
only compress what we actually scan out, i.e. the source rectangle of the
plane in pixels. Maybe some rounding involved perhaps.

Or did I miss something?
-Daniel

> +}
> +
>  static void i8xx_fbc_deactivate(struct drm_i915_private *dev_priv)
>  {
>  	u32 fbc_ctl;
> @@ -604,11 +643,17 @@ again:
>  	}
>  }
>  
> -static int intel_fbc_alloc_cfb(struct drm_i915_private *dev_priv, int size,
> -			       int fb_cpp)
> +static int intel_fbc_alloc_cfb(struct intel_crtc *crtc)
>  {
> +	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
> +	struct drm_framebuffer *fb = crtc->base.primary->state->fb;
>  	struct drm_mm_node *uninitialized_var(compressed_llb);
> -	int ret;
> +	int size, fb_cpp, ret;
> +
> +	WARN_ON(dev_priv->fbc.compressed_fb.allocated);
> +
> +	size = intel_fbc_calculate_cfb_size(crtc, fb);
> +	fb_cpp = drm_format_plane_cpp(fb->pixel_format, 0);
>  
>  	ret = find_compression_threshold(dev_priv, &dev_priv->fbc.compressed_fb,
>  					 size, fb_cpp);
> @@ -685,64 +730,6 @@ void intel_fbc_cleanup_cfb(struct drm_i915_private *dev_priv)
>  	mutex_unlock(&dev_priv->fbc.lock);
>  }
>  
> -/*
> - * For SKL+, the plane source size used by the hardware is based on the value we
> - * write to the PLANE_SIZE register. For BDW-, the hardware looks at the value
> - * we wrote to PIPESRC.
> - */
> -static void intel_fbc_get_plane_source_size(struct intel_crtc *crtc,
> -					    int *width, int *height)
> -{
> -	struct intel_plane_state *plane_state =
> -			to_intel_plane_state(crtc->base.primary->state);
> -	int w, h;
> -
> -	if (intel_rotation_90_or_270(plane_state->base.rotation)) {
> -		w = drm_rect_height(&plane_state->src) >> 16;
> -		h = drm_rect_width(&plane_state->src) >> 16;
> -	} else {
> -		w = drm_rect_width(&plane_state->src) >> 16;
> -		h = drm_rect_height(&plane_state->src) >> 16;
> -	}
> -
> -	if (width)
> -		*width = w;
> -	if (height)
> -		*height = h;
> -}
> -
> -static int intel_fbc_calculate_cfb_size(struct intel_crtc *crtc)
> -{
> -	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
> -	struct drm_framebuffer *fb = crtc->base.primary->fb;
> -	int lines;
> -
> -	intel_fbc_get_plane_source_size(crtc, NULL, &lines);
> -	if (INTEL_INFO(dev_priv)->gen >= 7)
> -		lines = min(lines, 2048);
> -
> -	return lines * fb->pitches[0];
> -}
> -
> -static int intel_fbc_setup_cfb(struct intel_crtc *crtc)
> -{
> -	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
> -	struct drm_framebuffer *fb = crtc->base.primary->fb;
> -	int size, cpp;
> -
> -	size = intel_fbc_calculate_cfb_size(crtc);
> -	cpp = drm_format_plane_cpp(fb->pixel_format, 0);
> -
> -	if (dev_priv->fbc.compressed_fb.allocated &&
> -	    size <= dev_priv->fbc.compressed_fb.size * dev_priv->fbc.threshold)
> -		return 0;
> -
> -	/* Release any current block */
> -	__intel_fbc_cleanup_cfb(dev_priv);
> -
> -	return intel_fbc_alloc_cfb(dev_priv, size, cpp);
> -}
> -
>  static bool stride_is_valid(struct drm_i915_private *dev_priv,
>  			    unsigned int stride)
>  {
> @@ -904,8 +891,19 @@ static void __intel_fbc_update(struct intel_crtc *crtc)
>  		goto out_disable;
>  	}
>  
> -	if (intel_fbc_setup_cfb(crtc)) {
> -		set_no_fbc_reason(dev_priv, "not enough stolen memory");
> +	/* It is possible for the required CFB size change without a
> +	 * crtc->disable + crtc->enable since it is possible to change the
> +	 * stride without triggering a full modeset. Since we try to
> +	 * over-allocate the CFB, there's a chance we may keep FBC enabled even
> +	 * if this happens, but if we exceed the current CFB size we'll have to
> +	 * disable FBC. Notice that it would be possible to disable FBC, wait
> +	 * for a frame, free the stolen node, then try to reenable FBC in case
> +	 * we didn't get any invalidate/deactivate calls, but this would require
> +	 * a lot of tracking just for a case we expect to be uncommon, so we
> +	 * just don't have this code for now. */
> +	if (intel_fbc_calculate_cfb_size(crtc, fb) >
> +	    dev_priv->fbc.compressed_fb.size * dev_priv->fbc.threshold) {
> +		set_no_fbc_reason(dev_priv, "CFB requirements changed");
>  		goto out_disable;
>  	}
>  
> @@ -958,7 +956,6 @@ out_disable:
>  		DRM_DEBUG_KMS("unsupported config, deactivating FBC\n");
>  		__intel_fbc_deactivate(dev_priv);
>  	}
> -	__intel_fbc_cleanup_cfb(dev_priv);
>  }
>  
>  /*
> @@ -1102,6 +1099,11 @@ void intel_fbc_enable(struct intel_crtc *crtc)
>  		goto out;
>  	}
>  
> +	if (intel_fbc_alloc_cfb(crtc)) {
> +		set_no_fbc_reason(dev_priv, "not enough stolen memory");
> +		goto out;
> +	}
> +
>  	DRM_DEBUG_KMS("Enabling FBC on pipe %c\n", pipe_name(crtc->pipe));
>  	dev_priv->fbc.no_fbc_reason = "FBC enabled but not active yet\n";
>  
> @@ -1129,6 +1131,8 @@ static void __intel_fbc_disable(struct drm_i915_private *dev_priv)
>  
>  	DRM_DEBUG_KMS("Disabling FBC on pipe %c\n", pipe_name(crtc->pipe));
>  
> +	__intel_fbc_cleanup_cfb(dev_priv);
> +
>  	dev_priv->fbc.enabled = false;
>  	dev_priv->fbc.crtc = NULL;
>  }
> -- 
> 2.6.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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