The previous implementation has had a detrimental effect on devices using
high bitrates (bluetooth), as the fifo being non-empty for a single check
would result in a 10 µs delay.
This patch updates the code such that it will allow 1000 retries to fail
before delaying by 1 µs for the remaining retry limit.
In addition, the maximum number of retries has been increased, both to
cover the decreased delay length, and to cover a new worst-case seen on
the Armada 385 SoC. "dmesg ; resize", filling the buffer with text to
output before reconfiguring, required 13 ms to empty the buffer and avoid
losing characters.
---
drivers/tty/serial/8250/8250_dw.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 823c1dc..843c2a2 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -127,13 +127,20 @@ static void dw8250_check_lcr(struct uart_port *p, int value)
/* Returns once the transmitter is empty or we run out of retries */
static void dw8250_tx_wait_empty(struct uart_port *p, int tries)
{
+ unsigned int delay_threshold = tries - 1000;
unsigned int lsr;
while (tries--) {
lsr = readb (p->membase + (UART_LSR << p->regshift));
if (lsr & UART_LSR_TEMT)
break;
- udelay (10);
+
+ /* The device is first given a chance to empty without delay,
+ * to avoid slowdowns at high bitrates. If after 1000 tries
+ * the buffer has still not emptied, allow more time for low-
+ * speed links. */
+ if (tries < delay_threshold)
+ udelay (1);
}
}
@@ -143,7 +150,7 @@ static void dw8250_serial_out(struct uart_port *p, int offset, int value)
/* Allow the TX to drain before we reconfigure */
if (offset == UART_LCR)
- dw8250_tx_wait_empty(p, 1000);
+ dw8250_tx_wait_empty(p, 20000);
writeb(value, p->membase + (offset << p->regshift));
--
2.16.2
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