drm: Add DRM_MODESET_LOCK_BEGIN/END helpers

This patch adds a couple of helpers to remove the boilerplate involved
in grabbing all of the modeset locks.

I've also converted the obvious cases in drm core to use the helpers.

The only remaining instance of drm_modeset_lock_all_ctx() is in
drm_framebuffer. It's complicated by the state clear that occurs on
deadlock. ATM, there's no way to inject code in the deadlock path with
the helpers, so it's unfit for conversion.

Changes in v2:
- Relocate ret argument to the end of the list (Daniel)
- Incorporate Daniel's doc suggestions (Daniel)

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129150423.239081-4-sean@poorly.run

1 files changed, 59 insertions, 0 deletions

diff --git a/include/drm/drm_modeset_lock.h b/include/drm/drm_modeset_lock.h
index a685d1bb21f2..a308f2d6496f 100644
--- a/include/drm/drm_modeset_lock.h
+++ b/include/drm/drm_modeset_lock.h
@@ -130,4 +130,63 @@ void drm_warn_on_modeset_not_all_locked(struct drm_device *dev);
 int drm_modeset_lock_all_ctx(struct drm_device *dev,
 			     struct drm_modeset_acquire_ctx *ctx);
 
+/**
+ * DRM_MODESET_LOCK_ALL_BEGIN - Helper to acquire modeset locks
+ * @dev: drm device
+ * @ctx: local modeset acquire context, will be dereferenced
+ * @flags: DRM_MODESET_ACQUIRE_* flags to pass to drm_modeset_acquire_init()
+ * @ret: local ret/err/etc variable to track error status
+ *
+ * Use these macros to simplify grabbing all modeset locks using a local
+ * context. This has the advantage of reducing boilerplate, but also properly
+ * checking return values where appropriate.
+ *
+ * Any code run between BEGIN and END will be holding the modeset locks.
+ *
+ * This must be paired with DRM_MODESET_LOCK_ALL_END(). We will jump back and
+ * forth between the labels on deadlock and error conditions.
+ *
+ * Drivers can acquire additional modeset locks. If any lock acquisition
+ * fails, the control flow needs to jump to DRM_MODESET_LOCK_ALL_END() with
+ * the @ret parameter containing the return value of drm_modeset_lock().
+ *
+ * Returns:
+ * The only possible value of ret immediately after DRM_MODESET_LOCK_ALL_BEGIN()
+ * is 0, so no error checking is necessary
+ */
+#define DRM_MODESET_LOCK_ALL_BEGIN(dev, ctx, flags, ret)		\
+	drm_modeset_acquire_init(&ctx, flags);				\
+modeset_lock_retry:							\
+	ret = drm_modeset_lock_all_ctx(dev, &ctx);			\
+	if (ret)							\
+		goto modeset_lock_fail;
+
+/**
+ * DRM_MODESET_LOCK_ALL_END - Helper to release and cleanup modeset locks
+ * @ctx: local modeset acquire context, will be dereferenced
+ * @ret: local ret/err/etc variable to track error status
+ *
+ * The other side of DRM_MODESET_LOCK_ALL_BEGIN(). It will bounce back to BEGIN
+ * if ret is -EDEADLK.
+ *
+ * It's important that you use the same ret variable for begin and end so
+ * deadlock conditions are properly handled.
+ *
+ * Returns:
+ * ret will be untouched unless it is -EDEADLK on entry. That means that if you
+ * successfully acquire the locks, ret will be whatever your code sets it to. If
+ * there is a deadlock or other failure with acquire or backoff, ret will be set
+ * to that failure. In both of these cases the code between BEGIN/END will not
+ * be run, so the failure will reflect the inability to grab the locks.
+ */
+#define DRM_MODESET_LOCK_ALL_END(ctx, ret)				\
+modeset_lock_fail:							\
+	if (ret == -EDEADLK) {						\
+		ret = drm_modeset_backoff(&ctx);			\
+		if (!ret)						\
+			goto modeset_lock_retry;			\
+	}								\
+	drm_modeset_drop_locks(&ctx);					\
+	drm_modeset_acquire_fini(&ctx);
+
 #endif /* DRM_MODESET_LOCK_H_ */