[PATCH 3/5][SRU][J] thunderbolt: Do not allow subtracting more NFC credits than configured

[AceLan Kao]

From: Mika Westerberg <mika.westerberg at linux.intel.com>

BugLink: https://bugs.launchpad.net/bugs/1955016

This might happen if the boot firmware uses different amount of NFC
credits than what the router suggests, or we are dealing with pre-USB4
device.

Signed-off-by: Mika Westerberg <mika.westerberg at linux.intel.com>
(cherry picked from commit 6cb27a04fb779717c4a3d20233b93596885838cf)
Signed-off-by: Chia-Lin Kao (AceLan) <acelan.kao at canonical.com>
---
 drivers/thunderbolt/switch.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/thunderbolt/switch.c b/drivers/thunderbolt/switch.c
index 3014146081c1..463cfdc0b42f 100644
--- a/drivers/thunderbolt/switch.c
+++ b/drivers/thunderbolt/switch.c
@@ -623,6 +623,9 @@ int tb_port_add_nfc_credits(struct tb_port *port, int credits)
 		return 0;
 
 	nfc_credits = port->config.nfc_credits & ADP_CS_4_NFC_BUFFERS_MASK;
+	if (credits < 0)
+		credits = max_t(int, -nfc_credits, credits);
+
 	nfc_credits += credits;
 
 	tb_port_dbg(port, "adding %d NFC credits to %lu", credits,
-- 
2.25.1