Re: [PATCH v2 16/24] drm/msm: dpu: Add modeset lock checks where applicable

Daniel Vetter
2019-10-09T22:21:09Z

On Fri, Nov 16, 2018 at 7:44 PM Sean Paul <sean@poorly.run> wrote:
>
> From: Sean Paul <seanpaul@chromium.org>
>
> Add modeset lock checks to functions that could be called outside the
> core atomic stack.
>
> Changes in v2:
> - None
>
> Signed-off-by: Sean Paul <seanpaul@chromium.org>
> ---
> drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c | 2 ++
> drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c | 1 +
> 2 files changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> index a008a87a8113..cd0a0bea4335 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> @@ -284,6 +284,8 @@ enum dpu_intf_mode dpu_crtc_get_intf_mode(struct drm_crtc *crtc)
> return INTF_MODE_NONE;
> }
>
> + WARN_ON(!drm_modeset_is_locked(&crtc->mutex));
> +
> /* TODO: Returns the first INTF_MODE, could there be multiple values? */
> drm_for_each_encoder_mask(encoder, crtc->dev, crtc->state->encoder_mask)
> return dpu_encoder_get_intf_mode(encoder);
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c
> index 64134d619748..5104fc01147e 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_kms.c
> @@ -358,6 +358,7 @@ void dpu_kms_encoder_enable(struct drm_encoder *encoder)
> if (funcs && funcs->commit)
> funcs->commit(encoder);
>
> + WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex));
> drm_for_each_crtc(crtc, dev) {
> if (!(crtc->state->encoder_mask & drm_encoder_mask(encoder)))
> continue;

I'm fairly sure this is called in the atomic_commit path, and in there
you might not actually hold these locks (if you do a nonblocking
modeset).

The locking rules for ->state are pretty fun: Either hold the lock, or
be in atomic commit. In the later case atomic helpers' commit ordering
guarantees that you can safely access ->state (but read-only only)
without hodling any locks. You might want to revert.

Don't ask why I stumbled over this.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch