# HG changeset patch
# User kaf24@xxxxxxxxxxxxxxxxxxxx
# Node ID 6aa5179f24166af8a6866cfd1b42c739ba4491ec
# Parent c947b278a349d4b73d481136cace41714417bc11
Reduce spin-waiting in Xen serial driver:
1. Split the serial port lock into receiver and transmitter locks.
2. In the ns16550 interrupt, only call the generic serial service
routines if there is receive (or transmit) work to do.
3. In the generic transmit ISR, avoid long spin-waits by *trying*
to take the transmitter lock and, if that fails, check again
whether the transmitter is empty. This will allow us to bail
bail quickly if there is a long-term lock holder stuffing lots
of bytes.
Also, gdbstub should be setting its serial handle in synchronous mode,
just for sanity.
Signed-off-by: Keir Fraser <keir@xxxxxxxxxxxxx>
diff -r c947b278a349 -r 6aa5179f2416 xen/common/gdbstub.c
--- a/xen/common/gdbstub.c Thu Mar 23 14:53:52 2006
+++ b/xen/common/gdbstub.c Thu Mar 23 16:17:08 2006
@@ -562,6 +562,7 @@
gdb_ctx->serhnd = serial_parse_handle(opt_gdb);
if ( gdb_ctx->serhnd != -1 )
printk("GDB stub initialised.\n");
+ serial_start_sync(gdb_ctx->serhnd);
}
/*
diff -r c947b278a349 -r 6aa5179f2416 xen/drivers/char/ns16550.c
--- a/xen/drivers/char/ns16550.c Thu Mar 23 14:53:52 2006
+++ b/xen/drivers/char/ns16550.c Thu Mar 23 16:17:08 2006
@@ -121,8 +121,11 @@
while ( !(ns_read_reg(uart, IIR) & IIR_NOINT) )
{
- serial_tx_interrupt(port, regs);
- serial_rx_interrupt(port, regs);
+ char lsr = ns_read_reg(uart, LSR);
+ if ( lsr & LSR_THRE )
+ serial_tx_interrupt(port, regs);
+ if ( lsr & LSR_DR )
+ serial_rx_interrupt(port, regs);
}
}
diff -r c947b278a349 -r 6aa5179f2416 xen/drivers/char/serial.c
--- a/xen/drivers/char/serial.c Thu Mar 23 14:53:52 2006
+++ b/xen/drivers/char/serial.c Thu Mar 23 16:17:08 2006
@@ -7,6 +7,7 @@
*/
#include <xen/config.h>
+#include <xen/delay.h>
#include <xen/init.h>
#include <xen/irq.h>
#include <xen/keyhandler.h>
@@ -15,8 +16,8 @@
#include <xen/serial.h>
static struct serial_port com[2] = {
- { .lock = SPIN_LOCK_UNLOCKED },
- { .lock = SPIN_LOCK_UNLOCKED }
+ { .rx_lock = SPIN_LOCK_UNLOCKED, .tx_lock = SPIN_LOCK_UNLOCKED },
+ { .rx_lock = SPIN_LOCK_UNLOCKED, .tx_lock = SPIN_LOCK_UNLOCKED }
};
void serial_rx_interrupt(struct serial_port *port, struct cpu_user_regs *regs)
@@ -25,7 +26,7 @@
serial_rx_fn fn = NULL;
unsigned long flags;
- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&port->rx_lock, flags);
if ( port->driver->getc(port, &c) )
{
@@ -39,7 +40,7 @@
port->rxbuf[MASK_SERIAL_RXBUF_IDX(port->rxbufp++)] = c;
}
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->rx_lock, flags);
if ( fn != NULL )
(*fn)(c & 0x7f, regs);
@@ -50,7 +51,19 @@
int i;
unsigned long flags;
- spin_lock_irqsave(&port->lock, flags);
+ local_irq_save(flags);
+
+ /*
+ * Avoid spinning for a long time: if there is a long-term lock holder
+ * then we know that they'll be stuffing bytes into the transmitter which
+ * will therefore not be empty for long.
+ */
+ while ( !spin_trylock(&port->tx_lock) )
+ {
+ if ( !port->driver->tx_empty(port) )
+ return;
+ cpu_relax();
+ }
if ( port->driver->tx_empty(port) )
{
@@ -63,7 +76,7 @@
}
}
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->tx_lock, flags);
}
static void __serial_putc(struct serial_port *port, char c)
@@ -117,7 +130,7 @@
if ( (handle == -1) || !port->driver || !port->driver->putc )
return;
- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&port->tx_lock, flags);
if ( (c == '\n') && (handle & SERHND_COOKED) )
__serial_putc(port, '\r');
@@ -129,7 +142,7 @@
__serial_putc(port, c);
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->tx_lock, flags);
}
void serial_puts(int handle, const char *s)
@@ -141,7 +154,7 @@
if ( (handle == -1) || !port->driver || !port->driver->putc )
return;
- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&port->tx_lock, flags);
while ( (c = *s++) != '\0' )
{
@@ -156,7 +169,7 @@
__serial_putc(port, c);
}
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->tx_lock, flags);
}
char serial_getc(int handle)
@@ -168,27 +181,28 @@
if ( (handle == -1) || !port->driver || !port->driver->getc )
return '\0';
- do {
+ do {
for ( ; ; )
{
- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&port->rx_lock, flags);
if ( port->rxbufp != port->rxbufc )
{
c = port->rxbuf[MASK_SERIAL_RXBUF_IDX(port->rxbufc++)];
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->rx_lock, flags);
break;
}
if ( port->driver->getc(port, &c) )
{
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->rx_lock, flags);
break;
}
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->rx_lock, flags);
cpu_relax();
+ udelay(100);
}
} while ( ((handle & SERHND_LO) && (c & 0x80)) ||
((handle & SERHND_HI) && !(c & 0x80)) );
@@ -241,7 +255,7 @@
if ( handle == -1 )
return;
- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&port->rx_lock, flags);
if ( port->rx != NULL )
goto fail;
@@ -265,11 +279,11 @@
port->rx = fn;
}
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->rx_lock, flags);
return;
fail:
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->rx_lock, flags);
printk("ERROR: Conflicting receive handlers for COM%d\n",
handle & SERHND_IDX);
}
@@ -277,8 +291,13 @@
void serial_force_unlock(int handle)
{
struct serial_port *port = &com[handle & SERHND_IDX];
- if ( handle != -1 )
- port->lock = SPIN_LOCK_UNLOCKED;
+
+ if ( handle == -1 )
+ return;
+
+ port->rx_lock = SPIN_LOCK_UNLOCKED;
+ port->tx_lock = SPIN_LOCK_UNLOCKED;
+
serial_start_sync(handle);
}
@@ -290,7 +309,7 @@
if ( handle == -1 )
return;
- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&port->tx_lock, flags);
if ( port->sync++ == 0 )
{
@@ -303,7 +322,7 @@
}
}
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->tx_lock, flags);
}
void serial_end_sync(int handle)
@@ -314,11 +333,11 @@
if ( handle == -1 )
return;
- spin_lock_irqsave(&port->lock, flags);
+ spin_lock_irqsave(&port->tx_lock, flags);
port->sync--;
- spin_unlock_irqrestore(&port->lock, flags);
+ spin_unlock_irqrestore(&port->tx_lock, flags);
}
int serial_tx_space(int handle)
diff -r c947b278a349 -r 6aa5179f2416 xen/include/xen/serial.h
--- a/xen/include/xen/serial.h Thu Mar 23 14:53:52 2006
+++ b/xen/include/xen/serial.h Thu Mar 23 16:17:08 2006
@@ -42,7 +42,7 @@
char rxbuf[SERIAL_RXBUFSZ];
unsigned int rxbufp, rxbufc;
/* Serial I/O is concurrency-safe. */
- spinlock_t lock;
+ spinlock_t rx_lock, tx_lock;
};
struct uart_driver {
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