From bca476139d2ded86be146dae09b06e22548b67f3 Mon Sep 17 00:00:00 2001 From: Dick Hollenbeck Date: Wed, 9 Dec 2009 12:31:34 -0800 Subject: [PATCH] serial: 8250: add serial transmitter fully empty test When controlling an industrial radio modem it can be necessary to manipulate the handshake lines in order to control the radio modem's transmitter, from userspace. The transmitter should not be turned off before all characters have been transmitted. serial8250_tx_empty() was reporting that all characters were transmitted before they actually were. === Discovered in parallel with more testing and analysis by Kees Schoenmakers as follows: I ran into an NetMos 9835 serial pci board which behaves a little different than the standard. This type of expansion board is very common. "Standard" 8250 compatible devices clear the 'UART_LST_TEMT" bit together with the "UART_LSR_THRE" bit when writing data to the device. The NetMos device does it slightly different I believe that the TEMT bit is coupled to the shift register. The problem is that after writing data to the device and very quickly after that one does call serial8250_tx_empty, it returns the wrong information. My patch makes the test more robust (and solves the problem) and it does not affect the already correct devices. Alan: We may yet need to quirk this but now we know which chips we have a way to do that should we find this breaks some other 8250 clone with dodgy THRE. Signed-off-by: Dick Hollenbeck Signed-off-by: Alan Cox Cc: Kees Schoenmakers Signed-off-by: Andrew Morton Cc: stable Signed-off-by: Greg Kroah-Hartman --- drivers/serial/8250.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c index c3e37c8e7e26..e9b15c3746fa 100644 --- a/drivers/serial/8250.c +++ b/drivers/serial/8250.c @@ -83,6 +83,9 @@ static unsigned int skip_txen_test; /* force skip of txen test at init time */ #define PASS_LIMIT 256 +#define BOTH_EMPTY (UART_LSR_TEMT | UART_LSR_THRE) + + /* * We default to IRQ0 for the "no irq" hack. Some * machine types want others as well - they're free @@ -1792,7 +1795,7 @@ static unsigned int serial8250_tx_empty(struct uart_port *port) up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS; spin_unlock_irqrestore(&up->port.lock, flags); - return lsr & UART_LSR_TEMT ? TIOCSER_TEMT : 0; + return (lsr & BOTH_EMPTY) == BOTH_EMPTY ? TIOCSER_TEMT : 0; } static unsigned int serial8250_get_mctrl(struct uart_port *port) @@ -1850,8 +1853,6 @@ static void serial8250_break_ctl(struct uart_port *port, int break_state) spin_unlock_irqrestore(&up->port.lock, flags); } -#define BOTH_EMPTY (UART_LSR_TEMT | UART_LSR_THRE) - /* * Wait for transmitter & holding register to empty */ -- GitLab