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serial: 8250: fix XOFF/XON sending when DMA is used 

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 f967fc8f16 (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: 9ee4b83e51 ("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>
This commit is contained in:
Ilpo Järvinen 2022-03-14 11:14:32 +02:00 committed by Greg Kroah-Hartman
parent 1db536f95d
commit f58c252e30
4 changed files with 27 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
drivers/tty/serial/8250/8250_dma.c
View File

@ -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() */

4
drivers/tty/serial/8250/8250_port.c
View File

@ -1822,9 +1822,7 @@ 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)) {

14
drivers/tty/serial/serial_core.c
View File

@ -641,6 +641,20 @@ 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

2
include/linux/serial_core.h
View File

@ -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)