[PATCH 4/4] drm/atomic-helpers: Make encoder picking more robust

[Thierry Reding]

On Mon, Aug 03, 2015 at 05:24:11PM +0200, Daniel Vetter wrote:
> We've had a few issues with atomic where subtle bugs in the encoder
> picking logic lead to accidental self-stealing of the encoder,
> resulting in a NULL connector_state->crtc in update_connector_routing
> and subsequent.
> 
> Linus applied some duct-tape for an mst regression in
> 
> commit 27667f4744fc5a0f3e50910e78740bac5670d18b
> Author: Linus Torvalds <torvalds at linux-foundation.org>
> Date:   Wed Jul 29 22:18:16 2015 -0700
> 
>     i915: temporary fix for DP MST docking station NULL pointer dereference
> 
> But that was incomplete (the code will still oops when debuggin is
> enabled) and mangled the state even further. So instead WARN and bail
> out as the more future-proof option.
> 
> Cc: Theodore Ts'o <tytso at mit.edu>
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 8694ca9beee3..9dcc7280e572 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -234,13 +234,14 @@ update_connector_routing(struct drm_atomic_state *state, int conn_idx)
>  		}
>  	}
>  
> +	if (WARN_ON(!connector_state->crtc))
> +		return -EINVAL;
> +
>  	connector_state->best_encoder = new_encoder;
> -	if (connector_state->crtc) {
> -		idx = drm_crtc_index(connector_state->crtc);
> +	idx = drm_crtc_index(connector_state->crtc);
>  
> -		crtc_state = state->crtc_states[idx];
> -		crtc_state->mode_changed = true;
> -	}
> +	crtc_state = state->crtc_states[idx];
> +	crtc_state->mode_changed = true;
>  
>  	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] using [ENCODER:%d:%s] on [CRTC:%d]\n",
>  			 connector->base.id,

Reviewed-by: Thierry Reding <treding at nvidia.com>
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