linux-kernel - [PATCH v2] serial: core: only stop transmit when HW fifo is empty

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date: Sun,  3 Mar 2024 16:08:07 +0100
From: Jonas Gorski <jonas.gorski@...il.com>
To: linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	stable@...r.kernel.org
Subject: [PATCH v2] serial: core: only stop transmit when HW fifo is empty

If the circular buffer is empty, it just means we fit all characters to
send into the HW fifo, but not that the hardware finished transmitting
them.

So if we immediately call stop_tx() after that, this may abort any
pending characters in the HW fifo, and cause dropped characters on the
console.

Fix this by only stopping tx when the tx HW fifo is actually empty.

Fixes: 8275b48b2780 ("tty: serial: introduce transmit helpers")
Cc: stable@...r.kernel.org
Signed-off-by: Jonas Gorski <jonas.gorski@...il.com>
---
(this is v2 of the bcm63xx-uart fix attempt)

v1 -> v2
* replace workaround with fix for core issue
* add Cc: for stable

I'm somewhat confident this is the core issue causing the broken output
with bcm63xx-uart, and there is no actual need for the UART_TX_NOSTOP.

I wouldn't be surprised if this also fixes mxs-uart for which
UART_TX_NOSTOP was introduced.

If it does, there is no need for the flag anymore.
 include/linux/serial_core.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 55b1f3ba48ac..bb0f2d4ac62f 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -786,7 +786,8 @@ enum UART_TX_FLAGS {
 	if (pending < WAKEUP_CHARS) {					      \
 		uart_write_wakeup(__port);				      \
 									      \
-		if (!((flags) & UART_TX_NOSTOP) && pending == 0)	      \
+		if (!((flags) & UART_TX_NOSTOP) && pending == 0 &&	      \
+		    __port->ops->tx_empty(__port))			      \
 			__port->ops->stop_tx(__port);			      \
 	}								      \
 									      \
-- 
2.34.1