From cfde770d945f63b8d66eef0246209cea985f0913 Mon Sep 17 00:00:00 2001 From: Phil Elwell Date: Wed, 28 Jun 2017 10:41:25 +0100 Subject: [PATCH] serial: 8250: Fix THRE flag usage for CAP_MINI The BCM2835 MINI UART has non-standard THRE semantics. Conventionally the bit means that the FIFO is empty (although there may still be a byte in the transmit register), but on 2835 it indicates that the FIFO is not full. This causes interrupts after every byte is transmitted, with the FIFO providing some interrupt latency tolerance. A consequence of this difference is that the usual strategy of writing multiple bytes into the TX FIFO after checking THRE once is unsafe. In the worst case of 7 bytes in the FIFO, writing 8 bytes loses all but the first since by then the FIFO is full. There is an HFIFO ("Hidden FIFO") capability that causes the transmit loop to terminate when both THRE and TEMT are set, i.e. when the TX block is completely idle. This is unnecessarily cautious, potentially causing gaps in transmission. Add a new conditional to the transmit loop, predicated on CAP_MINI, that exits when THRE is no longer set (the FIFO is full). This allows the FIFO to fill quickly but subsequent writes are paced by the transmission rate. Signed-off-by: Phil Elwell Acked-by: Eric Anholt Acked-by: Andy Shevchenko Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250_port.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 4c620bedfbf4..a5fe0e66c607 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1764,6 +1764,10 @@ void serial8250_tx_chars(struct uart_8250_port *up) if ((up->capabilities & UART_CAP_HFIFO) && (serial_in(up, UART_LSR) & BOTH_EMPTY) != BOTH_EMPTY) break; + /* The BCM2835 MINI UART THRE bit is really a not-full bit. */ + if ((up->capabilities & UART_CAP_MINI) && + !(serial_in(up, UART_LSR) & UART_LSR_THRE)) + break; } while (--count > 0); if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS) -- 2.20.1