serial: amba-pl011: use cpu_relax when polling registers

Busy loops that poll on a register should call cpu_relax().  On some
architectures, it can lower CPU power consumption or yield to a
hyperthreaded twin processor.  It also serves as a compiler barrier,
so it can replace barrier() calls.

Signed-off-by: Timur Tabi <timur@codeaurora.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index de846027ba47ac933ec3e6a5ae85282f51d08928..618763b2539adaa0e56af79c96429ddb58de1482 100644 (file)
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -1167,7 +1167,7 @@ static void pl011_dma_shutdown(struct uart_amba_port *uap)
 
        /* Disable RX and TX DMA */
        while (pl011_read(uap, REG_FR) & UART01x_FR_BUSY)
-               barrier();
+               cpu_relax();
 
        spin_lock_irq(&uap->port.lock);
        uap->dmacr &= ~(UART011_DMAONERR | UART011_RXDMAE | UART011_TXDMAE);
@@ -1611,7 +1611,7 @@ static void pl011_put_poll_char(struct uart_port *port,
            container_of(port, struct uart_amba_port, port);
 
        while (pl011_read(uap, REG_FR) & UART01x_FR_TXFF)
-               barrier();
+               cpu_relax();
 
        pl011_write(ch, uap, REG_DR);
 }
@@ -2150,7 +2150,7 @@ static void pl011_console_putchar(struct uart_port *port, int ch)
            container_of(port, struct uart_amba_port, port);
 
        while (pl011_read(uap, REG_FR) & UART01x_FR_TXFF)
-               barrier();
+               cpu_relax();
        pl011_write(ch, uap, REG_DR);
 }
 
@@ -2158,7 +2158,7 @@ static void
 pl011_console_write(struct console *co, const char *s, unsigned int count)
 {
        struct uart_amba_port *uap = amba_ports[co->index];
-       unsigned int status, old_cr = 0, new_cr;
+       unsigned int old_cr = 0, new_cr;
        unsigned long flags;
        int locked = 1;
 
@@ -2188,9 +2188,8 @@ pl011_console_write(struct console *co, const char *s, unsigned int count)
         *      Finally, wait for transmitter to become empty
         *      and restore the TCR
         */
-       do {
-               status = pl011_read(uap, REG_FR);
-       } while (status & UART01x_FR_BUSY);
+       while (pl011_read(uap, REG_FR) & UART01x_FR_BUSY)
+               cpu_relax();
        if (!uap->vendor->always_enabled)
                pl011_write(old_cr, uap, REG_CR);
 
@@ -2302,13 +2301,13 @@ static struct console amba_console = {
 static void pl011_putc(struct uart_port *port, int c)
 {
        while (readl(port->membase + UART01x_FR) & UART01x_FR_TXFF)
-               ;
+               cpu_relax();
        if (port->iotype == UPIO_MEM32)
                writel(c, port->membase + UART01x_DR);
        else
                writeb(c, port->membase + UART01x_DR);
        while (readl(port->membase + UART01x_FR) & UART01x_FR_BUSY)
-               ;
+               cpu_relax();
 }
 
 static void pl011_early_write(struct console *con, const char *s, unsigned n)