drm/i915/dsb: fix extra warning on error path handling

When we call intel_dsb_get(), the dsb initialization may fail for
various reasons. We already log the error message in that path, making
it unnecessary to trigger a warning that refcount == 0 when calling
intel_dsb_put().

So here we simplify the logic and do lazy shutdown: leaving the extra
refcount alive so when we call intel_dsb_put() we end up calling
i915_vma_unpin_and_release().
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111205024.22853-3-lucas.demarchi@intel.com

drivers/gpu/drm/i915/display/intel_dsb.c

@@ -102,6 +102,7 @@ intel_dsb_get(struct intel_crtc *crtc)
 	struct intel_dsb *dsb = &crtc->dsb;
 	struct drm_i915_gem_object *obj;
 	struct i915_vma *vma;
+	u32 *buf;
 	intel_wakeref_t wakeref;
 
 	if (!HAS_DSB(i915))
@@ -110,7 +111,6 @@ intel_dsb_get(struct intel_crtc *crtc)
 	if (dsb->refcount++ != 0)
 		return dsb;
 
-	dsb->id = DSB1;
 	wakeref = intel_runtime_pm_get(&i915->runtime_pm);
 
 	obj = i915_gem_object_create_internal(i915, DSB_BUF_SIZE);
@@ -123,22 +123,29 @@ intel_dsb_get(struct intel_crtc *crtc)
 	if (IS_ERR(vma)) {
 		DRM_ERROR("Vma creation failed\n");
 		i915_gem_object_put(obj);
-		dsb->refcount--;
 		goto err;
 	}
 
-	dsb->cmd_buf = i915_gem_object_pin_map(vma->obj, I915_MAP_WC);
-	if (IS_ERR(dsb->cmd_buf)) {
+	buf = i915_gem_object_pin_map(vma->obj, I915_MAP_WC);
+	if (IS_ERR(buf)) {
 		DRM_ERROR("Command buffer creation failed\n");
-		i915_vma_unpin_and_release(&vma, 0);
-		dsb->cmd_buf = NULL;
-		dsb->refcount--;
 		goto err;
 	}
+
+	dsb->id = DSB1;
 	dsb->vma = vma;
+	dsb->cmd_buf = buf;
 
 err:
+	/*
+	 * Set cmd_buf to NULL so the writes pass-through, but leave the
+	 * dangling refcount to be removed later by the corresponding
+	 * intel_dsb_put(): the important error message will already be
+	 * logged above.
+	 */
+	dsb->cmd_buf = NULL;
 	intel_runtime_pm_put(&i915->runtime_pm, wakeref);
+
 	return dsb;
 }