[PATCH 5.17 0781/1126] serial: 8250: fix XOFF/XON sending when DMA is used

Greg Kroah-Hartman
2022-04-05T09:32:02Z

From: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

[ Upstream commit f58c252e30cf74f68b0054293adc03b5923b9f0e ]

When 8250 UART is using DMA, x_char (XON/XOFF) is never sent
to the wire. After this change, x_char is injected correctly.

Create uart_xchar_out() helper for sending the x_char out and
accounting related to it. It seems that almost every driver
does these same steps with x_char. Except for 8250, however,
almost all currently lack .serial_out so they cannot immediately
take advantage of this new helper.

The downside of this patch is that it might reintroduce
the problems some devices faced with mixed DMA/non-DMA transfer
which caused revert f967fc8f165f (Revert "serial: 8250_dma:
don't bother DMA with small transfers"). However, the impact
should be limited to cases with XON/XOFF (that didn't work
with DMA capable devices to begin with so this problem is not
very likely to cause a major issue, if any at all).

Fixes: 9ee4b83e51f74 ("serial: 8250: Add support for dmaengine")
Reported-by: Gilles Buloz <gilles.buloz@kontron.com>
Tested-by: Gilles Buloz <gilles.buloz@kontron.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220314091432.4288-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/tty/serial/8250/8250_dma.c | 11 ++++++++++-
drivers/tty/serial/8250/8250_port.c | 4 +---
drivers/tty/serial/serial_core.c | 14 ++++++++++++++
include/linux/serial_core.h | 2 ++
4 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index 890fa7ddaa7f..b3c3f7e5851a 100644
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -64,10 +64,19 @@ int serial8250_tx_dma(struct uart_8250_port *p)
struct uart_8250_dma *dma = p->dma;
struct circ_buf *xmit = &p->port.state->xmit;
struct dma_async_tx_descriptor *desc;
+ struct uart_port *up = &p->port;
int ret;

- if (dma->tx_running)
+ if (dma->tx_running) {
+ if (up->x_char) {
+ dmaengine_pause(dma->txchan);
+ uart_xchar_out(up, UART_TX);
+ dmaengine_resume(dma->txchan);
+ }
return 0;
+ } else if (up->x_char) {
+ uart_xchar_out(up, UART_TX);
+ }

if (uart_tx_stopped(&p->port) || uart_circ_empty(xmit)) {
/* We have been called from __dma_tx_complete() */
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index fd0339d22491..9f116e75956e 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1819,9 +1819,7 @@ void serial8250_tx_chars(struct uart_8250_port *up)
int count;

if (port->x_char) {
- serial_out(up, UART_TX, port->x_char);
- port->icount.tx++;
- port->x_char = 0;
+ uart_xchar_out(port, UART_TX);
return;
}
if (uart_tx_stopped(port)) {
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 0db90be4c3bc..f67540ae2a88 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -644,6 +644,20 @@ static void uart_flush_buffer(struct tty_struct *tty)
tty_port_tty_wakeup(&state->port);
}

+/*
+ * This function performs low-level write of high-priority XON/XOFF
+ * character and accounting for it.
+ *
+ * Requires uart_port to implement .serial_out().
+ */
+void uart_xchar_out(struct uart_port *uport, int offset)
+{
+ serial_port_out(uport, offset, uport->x_char);
+ uport->icount.tx++;
+ uport->x_char = 0;
+}
+EXPORT_SYMBOL_GPL(uart_xchar_out);
+
/*
* This function is used to send a high-priority XON/XOFF character to
* the device
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index c58cc142d23f..8c32935e1059 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -458,6 +458,8 @@ extern void uart_handle_cts_change(struct uart_port *uport,
extern void uart_insert_char(struct uart_port *port, unsigned int status,
unsigned int overrun, unsigned int ch, unsigned int flag);

+void uart_xchar_out(struct uart_port *uport, int offset);
+
#ifdef CONFIG_MAGIC_SYSRQ_SERIAL
#define SYSRQ_TIMEOUT (HZ * 5)

--
2.34.1