[PATCH ipsec-next] xfrm: add comments in the xfrm mtu calc function

Document the [p]mtu values used by, and calculated for xfrms.

Signed-off-by: Christian Hopps <redacted>
---
net/xfrm/xfrm_policy.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 5c61ec04b839..2480717654b6 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c

@@ -3858,15 +3858,19 @@ static void xfrm_init_pmtu(struct xfrm_dst **bundle, int nr)

		u32 pmtu, route_mtu_cached;
		struct dst_entry *dst;

+		/* Get the xfrm's dst [p]mtu (i.e., the xfrm's intf/route). */
		dst = &xdst->u.dst;
		pmtu = dst_mtu(xfrm_dst_child(dst));
		xdst->child_mtu_cached = pmtu;

+		/* Subtract the xfrm mode overhead. */
		pmtu = xfrm_state_mtu(dst->xfrm, pmtu);

+		/* Get the inner traffic route's MTU. */
		route_mtu_cached = dst_mtu(xdst->route);
		xdst->route_mtu_cached = route_mtu_cached;

+		/* Update the xfrm's dst MTU to the minimum of these. */
		if (pmtu > route_mtu_cached)
			pmtu = route_mtu_cached;

--
2.41.0