The current PL011 driver transmits a dummy character when the UART
is opened, to assert the TX IRQ for the first time
(see pl011_startup()). The UART is put in loopback mode temporarily,
so the receiver presumably shouldn't see anything.
However...
At least some platforms containing a PL011 send characters down the
wire even when loopback mode is enabled. This means that a
spurious NUL character may be seen at the receiver when the PL011 is
opened through the TTY layer.
The current code also temporarily sets the baud rate to maximum and
the character width to the minimum, to that the dummy TX completes
as quickly as possible. If this is seen by the receiver it will
result in a framing error and can knock the receiver out of sync --
turning subsequent output into garbage until synchronisation
is reestablished. (Particularly problematic during boot with systemd.)
To avoid spurious transmissions, this patch removes assumptions about
whether the TX IRQ will fire until the FIFO has been filled.
This should also mean that the driver works on the SBSA Generic
UART[1] (a cut-down PL011) without invasive changes. The Generic
UART lacks some features needed for the dummy TX approach to work
(FIFO disabling and loopback).
[1] Server Base System Architecture (ARM-DEN-0029-v2.3)
http://infocenter.arm.com/
(click-thru required :/)
Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
---
drivers/tty/serial/amba-pl011.c | 119 +++++++++------------------------------
1 file changed, 26 insertions(+), 93 deletions(-)
diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 6f5a072..f5bd842 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -58,7 +58,6 @@
#include <linux/pinctrl/consumer.h>
#include <linux/sizes.h>
#include <linux/io.h>
-#include <linux/workqueue.h>
#define UART_NR 14
@@ -157,9 +156,7 @@ struct uart_amba_port {
unsigned int lcrh_tx; /* vendor-specific */
unsigned int lcrh_rx; /* vendor-specific */
unsigned int old_cr; /* state during shutdown */
- struct delayed_work tx_softirq_work;
bool autorts;
- unsigned int tx_irq_seen; /* 0=none, 1=1, 2=2 or more */
char type[12];
#ifdef CONFIG_DMA_ENGINE
/* DMA stuff */
@@ -1172,15 +1169,14 @@ static void pl011_stop_tx(struct uart_port *port)
pl011_dma_tx_stop(uap);
}
-static bool pl011_tx_chars(struct uart_amba_port *uap);
+static void pl011_tx_chars(struct uart_amba_port *uap, bool from_irq);
/* Start TX with programmed I/O only (no DMA) */
static void pl011_start_tx_pio(struct uart_amba_port *uap)
{
uap->im |= UART011_TXIM;
writew(uap->im, uap->port.membase + UART011_IMSC);
- if (!uap->tx_irq_seen)
- pl011_tx_chars(uap);
+ pl011_tx_chars(uap, false);
}
static void pl011_start_tx(struct uart_port *port)
@@ -1247,87 +1243,54 @@ __acquires(&uap->port.lock)
spin_lock(&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.
- */
-static bool pl011_tx_char(struct uart_amba_port *uap, unsigned char c)
+static bool pl011_tx_char(struct uart_amba_port *uap, unsigned char c,
+ bool from_irq)
{
+ if (unlikely(!from_irq) &&
+ 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)
+static void pl011_tx_chars(struct uart_amba_port *uap, bool from_irq)
{
struct circ_buf *xmit = &uap->port.state->xmit;
- int count;
-
- if (unlikely(uap->tx_irq_seen < 2))
- /*
- * Initial FIFO fill level unknown: we must check TXFF
- * after each write, so just try to fill up the FIFO.
- */
- count = uap->fifosize;
- else /* tx_irq_seen >= 2 */
- /*
- * FIFO initially at least half-empty, so we can simply
- * write half the FIFO without polling TXFF.
-
- * Note: the *first* TX IRQ can still race with
- * pl011_start_tx_pio(), which can result in the FIFO
- * being fuller than expected in that case.
- */
- count = uap->fifosize >> 1;
-
- /*
- * If the FIFO is full we're guaranteed a TX IRQ at some later point,
- * and can't transmit immediately in any case:
- */
- if (unlikely(uap->tx_irq_seen < 2 &&
- readw(uap->port.membase + UART01x_FR) & UART01x_FR_TXFF))
- return false;
+ int count = uap->fifosize >> 1;
if (uap->port.x_char) {
- pl011_tx_char(uap, uap->port.x_char);
+ if (!pl011_tx_char(uap, uap->port.x_char, from_irq))
+ return;
uap->port.x_char = 0;
--count;
}
if (uart_circ_empty(xmit) || uart_tx_stopped(&uap->port)) {
pl011_stop_tx(&uap->port);
- goto done;
+ return;
}
/* If we are using DMA mode, try to send some characters. */
if (pl011_dma_tx_irq(uap))
- goto done;
+ return;
- while (count-- > 0 && pl011_tx_char(uap, xmit->buf[xmit->tail])) {
- xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
- if (uart_circ_empty(xmit))
+ do {
+ if (likely(from_irq) && count-- == 0)
break;
- }
+
+ if (!pl011_tx_char(uap, xmit->buf[xmit->tail], from_irq))
+ break;
+
+ xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
+ } while (!uart_circ_empty(xmit));
if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
uart_write_wakeup(&uap->port);
- if (uart_circ_empty(xmit)) {
+ if (uart_circ_empty(xmit))
pl011_stop_tx(&uap->port);
- goto done;
- }
-
- if (unlikely(!uap->tx_irq_seen))
- schedule_delayed_work(&uap->tx_softirq_work, uap->port.timeout);
-
-done:
- return false;
}
static void pl011_modem_status(struct uart_amba_port *uap)
@@ -1354,28 +1317,6 @@ static void pl011_modem_status(struct uart_amba_port *uap)
wake_up_interruptible(&uap->port.state->port.delta_msr_wait);
}
-static void pl011_tx_softirq(struct work_struct *work)
-{
- struct delayed_work *dwork = to_delayed_work(work);
- struct uart_amba_port *uap =
- container_of(dwork, struct uart_amba_port, tx_softirq_work);
-
- spin_lock(&uap->port.lock);
- while (pl011_tx_chars(uap)) ;
- spin_unlock(&uap->port.lock);
-}
-
-static void pl011_tx_irq_seen(struct uart_amba_port *uap)
-{
- if (likely(uap->tx_irq_seen > 1))
- return;
-
- uap->tx_irq_seen++;
- if (uap->tx_irq_seen < 2)
- /* first TX IRQ */
- cancel_delayed_work(&uap->tx_softirq_work);
-}
-
static irqreturn_t pl011_int(int irq, void *dev_id)
{
struct uart_amba_port *uap = dev_id;
@@ -1414,10 +1355,8 @@ static irqreturn_t pl011_int(int irq, void *dev_id)
if (status & (UART011_DSRMIS|UART011_DCDMIS|
UART011_CTSMIS|UART011_RIMIS))
pl011_modem_status(uap);
- if (status & UART011_TXIS) {
- pl011_tx_irq_seen(uap);
- pl011_tx_chars(uap);
- }
+ if (status & UART011_TXIS)
+ pl011_tx_chars(uap, true);
if (pass_counter-- == 0)
break;
@@ -1639,9 +1578,6 @@ static int pl011_startup(struct uart_port *port)
writew(uap->vendor->ifls, uap->port.membase + UART011_IFLS);
- /* Assume that TX IRQ doesn't work until we see one: */
- uap->tx_irq_seen = 0;
-
spin_lock_irq(&uap->port.lock);
/* restore RTS and DTR */
@@ -1697,8 +1633,6 @@ static void pl011_shutdown(struct uart_port *port)
container_of(port, struct uart_amba_port, port);
unsigned int cr;
- cancel_delayed_work_sync(&uap->tx_softirq_work);
-
/*
* disable all interrupts
*/
@@ -2245,7 +2179,6 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
uap->port.ops = &amba_pl011_pops;
uap->port.flags = UPF_BOOT_AUTOCONF;
uap->port.line = i;
- INIT_DELAYED_WORK(&uap->tx_softirq_work, pl011_tx_softirq);
/* Ensure interrupts from this UART are masked and cleared */
writew(0, uap->port.membase + UART011_IMSC);
--
1.7.10.4
--
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