From 43dd1f9a5b05d6db2cb258354a01ace63baa5c0b Mon Sep 17 00:00:00 2001 From: Dave Martin Date: Thu, 21 May 2015 16:37:49 +0100 Subject: serial/amba-pl011: Unconditionally poll for FIFO space before each TX char Commit 734745caeb9f155ab58918834a8c70e83fa6afd3 serial/amba-pl011: (Activate TX IRQ passively) introduces a race which causes the driver sometimes to attempt to write a character to the TX FIFO when the FIFO is already full. The PL011 does not guarantee its behaviour when the FIFO is overfilled. In practice, this can cause duplicate and/or dropped characters to be output on the wire. The problem is common enough to be readily observable on the ARM Juno platform when the PL011 UART is used as the console and DMA is not in use. This patch fixes this problem by always polling for space before each character is written to the FIFO. This will be amended to a less brute-force approach in a later commit, but this patch should help ensure correct behaviour for now. Signed-off-by: Dave Martin Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/amba-pl011.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) (limited to 'drivers/tty') diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c index 6f5a0720a8c8..763eb20fe321 100644 --- a/drivers/tty/serial/amba-pl011.c +++ b/drivers/tty/serial/amba-pl011.c @@ -1249,20 +1249,19 @@ __acquires(&uap->port.lock) /* * Transmit a character - * There must be at least one free entry in the TX FIFO to accept the char. * - * Returns true if the FIFO might have space in it afterwards; - * returns false if the FIFO definitely became full. + * Returns true if the character was successfully queued to the FIFO. + * Returns false otherwise. */ static bool pl011_tx_char(struct uart_amba_port *uap, unsigned char c) { + if (readw(uap->port.membase + UART01x_FR) & UART01x_FR_TXFF) + return false; /* unable to transmit character */ + writew(c, uap->port.membase + UART01x_DR); uap->port.icount.tx++; - if (likely(uap->tx_irq_seen > 1)) - return true; - - return !(readw(uap->port.membase + UART01x_FR) & UART01x_FR_TXFF); + return true; } static bool pl011_tx_chars(struct uart_amba_port *uap) @@ -1296,7 +1295,8 @@ static bool pl011_tx_chars(struct uart_amba_port *uap) return false; if (uap->port.x_char) { - pl011_tx_char(uap, uap->port.x_char); + if (!pl011_tx_char(uap, uap->port.x_char)) + goto done; uap->port.x_char = 0; --count; } -- cgit v1.2.3