On Tue, 18 Dec 2007 16:23:11 +0100
"Remy Bohmer" <[EMAIL PROTECTED]> wrote:
> Preempt-RT now absolutely requires my (4th) IRQ_NODELAY patch, because
> the spinlock now is always inside the code, and not only in
> theexception path, and thus without my NO_DELAY patch we have a panic
> during boot.
Hmm...perhaps we can eliminate the locking in the status handler
too...? Does anyone see a problem with this patch?
diff --git a/drivers/serial/atmel_serial.c b/drivers/serial/atmel_serial.c
index bce5d2a..fac37dc 100644
--- a/drivers/serial/atmel_serial.c
+++ b/drivers/serial/atmel_serial.c
@@ -152,6 +152,7 @@ struct atmel_uart_port {
struct tasklet_struct tasklet;
unsigned int irq_pending;
unsigned int irq_status;
+ unsigned int irq_status_prev;
struct circ_buf rx_ring;
};
@@ -585,12 +586,19 @@ atmel_handle_status(struct uart_port *port, unsigned int
pending,
{
struct atmel_uart_port *atmel_port = (struct atmel_uart_port *)port;
- spin_lock(&port->lock);
-
- atmel_port->irq_pending |= pending;
+ /*
+ * Try to avoid locking here since it may cause problems on
+ * -rt. This may cause bits to re-appear in irq_pending due to
+ * a race with atmel_tasklet_func(), but since the previous
+ * status is tracked explicitly, this will only mean that the
+ * tasklet will do a bit more work than strictly necessary.
+ *
+ * We must make sure that status is written before pending,
+ * hence the barrier.
+ */
atmel_port->irq_status = status;
-
- spin_unlock(&port->lock);
+ smp_mb();
+ atmel_port->irq_pending |= pending;
tasklet_schedule(&atmel_port->tasklet);
}
@@ -750,33 +758,32 @@ static void atmel_tasklet_func(unsigned long data)
{
struct uart_port *port = (struct uart_port *)data;
struct atmel_uart_port *atmel_port = (struct atmel_uart_port *)port;
- unsigned long flags;
unsigned int status;
+ unsigned int status_change;
unsigned int pending;
- spin_lock_irqsave(&port->lock, flags);
+ pending = xchg(&atmel_port->irq_pending, 0);
- if (atmel_port->irq_pending) {
- pending = atmel_port->irq_pending;
+ if (pending) {
+ /* must read/update pending before reading status */
+ smp_mb();
status = atmel_port->irq_status;
- atmel_port->irq_pending = 0;
-
- spin_unlock_irqrestore(&port->lock, flags);
+ status_change = status ^ atmel_port->irq_status_prev;
/* TODO: All reads to CSR will clear these interrupts! */
- if (pending & ATMEL_US_RIIC)
+ if (status_change & ATMEL_US_RI)
port->icount.rng++;
- if (pending & ATMEL_US_DSRIC)
+ if (status_change & ATMEL_US_DSR)
port->icount.dsr++;
- if (pending & ATMEL_US_DCDIC)
+ if (status_change & ATMEL_US_DCD)
uart_handle_dcd_change(port, !(status & ATMEL_US_DCD));
- if (pending & ATMEL_US_CTSIC)
+ if (status_change & ATMEL_US_CTS)
uart_handle_cts_change(port, !(status & ATMEL_US_CTS));
- if (pending & (ATMEL_US_RIIC | ATMEL_US_DSRIC
- | ATMEL_US_DCDIC | ATMEL_US_CTSIC))
+ if (status_change & (ATMEL_US_RI | ATMEL_US_DSR
+ | ATMEL_US_DCD | ATMEL_US_CTS))
wake_up_interruptible(&port->info->delta_msr_wait);
- } else {
- spin_unlock_irqrestore(&port->lock, flags);
+
+ atmel_port->irq_status_prev = status;
}
if (atmel_use_dma_rx(port))
--
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