From d238ead08297f1e4a46a72953d2c5377fbe3111c Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Thu, 23 Nov 2023 05:36:43 +0900
Subject: [PATCH 0050/1160] network: the maximum MTU size for CAN interface may
 be changed later

So we should not reduce the requested size to the current maximum
before applying CAN FD mode.

(cherry picked from commit 15be80428204b57ca55272d2b45703047ad6f28d)
---
 src/network/networkd-setlink.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/src/network/networkd-setlink.c b/src/network/networkd-setlink.c
index 954fd7947d..2298f9ea3a 100644
--- a/src/network/networkd-setlink.c
+++ b/src/network/networkd-setlink.c
@@ -848,7 +848,7 @@ int link_request_to_set_master(Link *link) {
 
 int link_request_to_set_mtu(Link *link, uint32_t mtu) {
         const char *origin;
-        uint32_t min_mtu;
+        uint32_t min_mtu, max_mtu;
         Request *req;
         int r;
 
@@ -876,10 +876,19 @@ int link_request_to_set_mtu(Link *link, uint32_t mtu) {
                 mtu = min_mtu;
         }
 
-        if (mtu > link->max_mtu) {
+        max_mtu = link->max_mtu;
+        if (link->iftype == ARPHRD_CAN)
+                /* The maximum MTU may be changed when FD mode is changed.
+                 * See https://docs.kernel.org/networking/can.html#can-fd-flexible-data-rate-driver-support
+                 *   MTU = 16 (CAN_MTU)   => Classical CAN device
+                 *   MTU = 72 (CANFD_MTU) => CAN FD capable device
+                 * So, even if the current maximum is 16, we should not reduce the requested value now. */
+                max_mtu = MAX(max_mtu, 72u);
+
+        if (mtu > max_mtu) {
                 log_link_warning(link, "Reducing the requested MTU %"PRIu32" to the interface's maximum MTU %"PRIu32".",
-                                 mtu, link->max_mtu);
-                mtu = link->max_mtu;
+                                 mtu, max_mtu);
+                mtu = max_mtu;
         }
 
         if (link->mtu == mtu)
-- 
2.33.0