[PATCH 1/1] tty: n_gsm: Fix SW flow control encoding/handling

From: Daniel Starke
Date: Tue Jan 11 2022 - 02:36:19 EST


According to 3GPP 27.010 chapter 5.2.7.3 DC1 and DC3 (SW flow control) are to
be treated according to ISO/IEC 646. That means the MSB shall be ignored.
This patch applies the needed changes to handle this correctly.

Signed-off-by: Daniel Starke <daniel.starke@xxxxxxxxxxx>
---
drivers/tty/n_gsm.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0b96b14bbfe1..9ee0643fc9e2 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -322,6 +322,7 @@ static int addr_cnt;
#define GSM1_ESCAPE_BITS 0x20
#define XON 0x11
#define XOFF 0x13
+#define ASCII_MASK 0x7F

static const struct tty_port_operations gsm_port_ops;

@@ -521,7 +522,7 @@ static void gsm_print_packet(const char *hdr, int addr, int cr,
* @output: output buffer
* @len: length of input
*
- * Expand a buffer by bytestuffing it. The worst case size change
+ * Expand a buffer by byte stuffing it. The worst case size change
* is doubling and the caller is responsible for handing out
* suitable sized buffers.
*/
@@ -531,7 +532,8 @@ static int gsm_stuff_frame(const u8 *input, u8 *output, int len)
int olen = 0;
while (len--) {
if (*input == GSM1_SOF || *input == GSM1_ESCAPE
- || *input == XON || *input == XOFF) {
+ || (*input & ASCII_MASK) == XON
+ || (*input & ASCII_MASK) == XOFF) {
*output++ = GSM1_ESCAPE;
*output++ = *input++ ^ GSM1_ESCAPE_BITS;
olen++;
--
2.25.1