drm/tegra: gem: Add a clarifying comment

Clarify when a fixed IOV address can be used and when a buffer has to
be mapped before the IOVA can be used.
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/plane.c

@@ -119,6 +119,14 @@ static int tegra_dc_pin(struct tegra_dc *dc, struct tegra_plane_state *state)
 		dma_addr_t phys_addr, *phys;
 		struct sg_table *sgt;
 
+		/*
+		 * If we're not attached to a domain, we already stored the
+		 * physical address when the buffer was allocated. If we're
+		 * part of a group that's shared between all display
+		 * controllers, we've also already mapped the framebuffer
+		 * through the SMMU. In both cases we can short-circuit the
+		 * code below and retrieve the stored IOV address.
+		 */
 		if (!domain || dc->client.group)
 			phys = &phys_addr;
 		else