Re: [Intel-gfx] [PATCH] drm/i915: WARN if not all pipes are in bigjoiner mask, when copying plane state

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

On Tue, May 09, 2023 at 02:14:41PM +0300, Stanislav Lisovskiy wrote:
> There is a suspicion that we might not have all bigjoiner pipes
> set in correspodent mask, which leads to that not all crtc are added to the state,
> however because we are copying for instance crtc reference from master crtc
> to slave crtc, we might be trying to get it via intel_atomic_get_new_crtc_state,
> which might the return NULL.
> This is surely not a fix, but at least the WARN should give us some clue and
> "red light" when this happens.
> In future we might need to evaluate the logic of adding crtc to the state,
> to make sure that we always have all affected crtcs in the state,
> even though such functions already exist, there seem to be still some
> glitches in this logic.
> 
> Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_atomic_plane.c | 13 +++++++++++++
>  drivers/gpu/drm/i915/display/intel_display.c      |  2 +-
>  drivers/gpu/drm/i915/display/intel_display.h      |  1 +
>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_atomic_plane.c b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> index 4125ee07a271..03cbd755261b 100644
> --- a/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> +++ b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> @@ -695,6 +695,19 @@ int intel_plane_atomic_check(struct intel_atomic_state *state,
>  
>  		new_master_plane_state =
>  			intel_atomic_get_new_plane_state(state, master_plane);
> +
> +		/*
> +		 * We would be copying plane state from master crtc
> +		 * however if crtc_state->bigjoiner_pipes doesn't contain both
> +		 * master and slave, that means that quite likely we didn't call
> +		 * intel_atomic_get_crtc_state for both, which can cause issues,
> +		 * like intel_atomic_get_new_crtc_state returning NULL suddently,
> +		 * when we for example try to use hw.crtc from that plane state.
> +		 * This WARN might sched some light on out existing issues, also
> +		 * prevent others from happening in future.
> +		 */
> +		drm_WARN_ON(state->base.dev, intel_bigjoiner_num_pipes(new_crtc_state) < 2);

What you are doing here is basically just
 if (bigjoiner_pipes)
  	assert(bigjoiner_pipes != 1);
which is not going to catch anything.

We can trivially see that it will never happen given
how bigjoiner_pipes is initialized.

-- 
Ville Syrjälä
Intel