[Intel-gfx] [PATCH] drm/i915: Avoid clearing the base drm_crtc_state

[Daniel Vetter]

On Fri, Mar 03, 2017 at 03:46:44PM +0000, Chris Wilson wrote:
> To prevent having to preserve the drm_crtc_state as we clear the
> intel_crtc_state, only memset our extended state.
> 
> Fixes:
> drivers/gpu/drm/i915/intel_display.c: In function ‘clear_intel_crtc_state’:
> drivers/gpu/drm/i915/intel_display.c:11301:1: error: the frame size of 1056 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index e6881a69a88f..cfab4d135af3 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -11269,7 +11269,6 @@ clear_intel_crtc_state(struct intel_crtc_state *crtc_state)
>  {
>  	struct drm_i915_private *dev_priv =
>  		to_i915(crtc_state->base.crtc->dev);
> -	struct drm_crtc_state tmp_state;
>  	struct intel_crtc_scaler_state scaler_state;
>  	struct intel_dpll_hw_state dpll_hw_state;
>  	struct intel_shared_dpll *shared_dpll;
> @@ -11281,7 +11280,6 @@ clear_intel_crtc_state(struct intel_crtc_state *crtc_state)
>  	 * fixed, so that the crtc_state can be safely duplicated. For now,
>  	 * only fields that are know to not cause problems are preserved. */
>  
> -	tmp_state = crtc_state->base;
>  	scaler_state = crtc_state->scaler_state;
>  	shared_dpll = crtc_state->shared_dpll;
>  	dpll_hw_state = crtc_state->dpll_hw_state;
> @@ -11289,9 +11287,9 @@ clear_intel_crtc_state(struct intel_crtc_state *crtc_state)
>  	if (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv))
>  		wm_state = crtc_state->wm;
>  
> -	memset(crtc_state, 0, sizeof *crtc_state);
> +	memset(&crtc_state->base + 1, 0,
> +	       sizeof(*crtc_state) - sizeof(crtc_state->base));

Maybe add a comment like /* Only clear our part of the overall struct. */
or similar, since this is not entirely obvious what's going on. Also

	COMPILE_BUG_ON(offsetof(struct intel_crtc_state, base) != 0);

maybe? Anyway, I'll let you decide on both, either way:

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>  
> -	crtc_state->base = tmp_state;
>  	crtc_state->scaler_state = scaler_state;
>  	crtc_state->shared_dpll = shared_dpll;
>  	crtc_state->dpll_hw_state = dpll_hw_state;
> -- 
> 2.11.0
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch