Re: [PATCH] tty: serial: atmel: Preserve previous USART mode if RS485 disabled
From: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Date: 2022-08-24 12:02:49
Also in:
linux-serial, lkml
On Wed, 24 Aug 2022, Sergiu Moga wrote:
Whenever the atmel_rs485_config driver method would be called, the USART mode is reset to normal mode before even checking if RS485 flag is set, thus resulting in losing the previous USART mode in the case where the checking fails. Some tools, such as `linux-serial-test`, lead to the driver calling this method when doing the setup of the serial port: after setting the port mode (Hardware Flow Control, Normal Mode, RS485 Mode, etc.), `linux-serial-test` tries to enable/disable RS485 depending on the commandline arguments passed. If we were to, for example, enable Hardware Flow Control through `linux-serial-test`, the tool would make the driver set the corresponding bit to 1 (ATMEL_US_USMODE_HWHS bit in the ATMEL_US_MR register) through the atmel_set_termios method and then proceed to disabling RS485. This, in turn, causes the ATMEL_US_USMODE_HWHS bit of the ATMEL_US_MR mode register to be unset and, if the checking for RS485 fails, leads to having the mode set back to the ATMEL_US_USMODE_NORMAL normal mode. Since in hardware flow control mode the meanings of the ATMEL_US_RTSDIS and ATMEL_US_RTSEN bits are swapped, this leads to our endpoint leaving the RTS line to high when wanting to receive, which is the opposite of what the other endpoint is expecting in order to start transmitting. This fix ensures that this reset is done only if the checking for RS485 succeeds.
Could you please try to split this long paragraph to a slightly shorter bits such that it would be easier to read.
quoted hunk ↗ jump to hunk
Fixes: e8faff7330a35 ("ARM: 6092/1: atmel_serial: support for RS485 communications") Signed-off-by: Sergiu Moga <redacted> --- drivers/tty/serial/atmel_serial.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-)diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c index 0a0b46ee0955..c29b1fb48694 100644 --- a/drivers/tty/serial/atmel_serial.c +++ b/drivers/tty/serial/atmel_serial.c@@ -298,9 +298,6 @@ static int atmel_config_rs485(struct uart_port *port, struct ktermios *termios, mode = atmel_uart_readl(port, ATMEL_US_MR); - /* Resetting serial mode to RS232 (0x0) */ - mode &= ~ATMEL_US_USMODE; - if (rs485conf->flags & SER_RS485_ENABLED) { dev_dbg(port->dev, "Setting UART to RS485\n"); if (rs485conf->flags & SER_RS485_RX_DURING_TX)@@ -310,6 +307,7 @@ static int atmel_config_rs485(struct uart_port *port, struct ktermios *termios, atmel_uart_writel(port, ATMEL_US_TTGR, rs485conf->delay_rts_after_send); + mode &= ~ATMEL_US_USMODE; mode |= ATMEL_US_USMODE_RS485; } else { dev_dbg(port->dev, "Setting UART to RS232\n");
Makes sense. Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com> Unrelated to this patch but I came across it while reviewing yours... Do you BTW have any idea why atmel_serial_probe() sets ATMEL_US_USMODE_NORMAL inside rs485_enabled block? I'd have expected it wanted to do ATMEL_US_USMODE_RS485 there too like is done in atmel_config_rs485(). -- i.