[REGRESSION] Re: i915 driver crashes on T540p if docking station attached

[Daniel Vetter]

On Wed, Jul 29, 2015 at 10:18:16PM -0700, Linus Torvalds wrote:
>  drivers/gpu/drm/drm_atomic_helper.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 5b59d5ad7d1c..aac212297b49 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -230,10 +230,12 @@ update_connector_routing(struct drm_atomic_state *state, int conn_idx)
>  	}
>  
>  	connector_state->best_encoder = new_encoder;
> -	idx = drm_crtc_index(connector_state->crtc);
> +	if (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;
> +	}

This shouldn't happen since if it does we ended up stealing the encoder
from the connector itself (we do check for connector_state->crtc earlier)
and that would be a bug. I haven't figured out a precise theory but my
guess is on the best_encoder selection, and indeed dp mst encoder
selection seems to have gone belly up in 4.2 with the bisected commit.

I have 4 patches in git://people.freedesktop.org/~danvet/drm fixes-stuff
but I couldn't test them yet since no dp mst here and I didn't find
anything that would ship faster than 1-2 weeks yet. I'll try to get some
other people here to test it meanwhile too.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch