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serial: amba-pl011: use cpu_relax when polling registers

PATCH New Timur Tabi 1 participant 1 message Oct 22, 2015

Subsystems (2)
TTY layer and serial drivers Arm primecell UART PL010 and PL011 drivers
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Timur Tabi

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PATCH 1/1 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 <[email protected]>
---
 drivers/tty/serial/amba-pl011.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)
drivers/tty/serial/amba-pl011.c +6 -5
index fd27e98..4b5014b 100644
@@ -1076,7 +1076,7 @@ static void pl011_dma_shutdown(struct uart_amba_port *uap)
 
 	/* Disable RX and TX DMA */
 	while (readw(uap->port.membase + UART01x_FR) & UART01x_FR_BUSY)
-		barrier();
+		cpu_relax();
 
 	spin_lock_irq(&uap->port.lock);
 	uap->dmacr &= ~(UART011_DMAONERR | UART011_RXDMAE | UART011_TXDMAE);

@@ -1520,7 +1520,7 @@ static void pl011_put_poll_char(struct uart_port *port,
 	    container_of(port, struct uart_amba_port, port);
 
 	while (readw(uap->port.membase + UART01x_FR) & UART01x_FR_TXFF)
-		barrier();
+		cpu_relax();
 
 	writew(ch, uap->port.membase + UART01x_DR);
 }

@@ -2053,7 +2053,7 @@ static void pl011_console_putchar(struct uart_port *port, int ch)
 	    container_of(port, struct uart_amba_port, port);
 
 	while (readw(uap->port.membase + UART01x_FR) & UART01x_FR_TXFF)
-		barrier();
+		cpu_relax();
 	writew(ch, uap->port.membase + UART01x_DR);
 }
 

@@ -2093,6 +2093,7 @@ pl011_console_write(struct console *co, const char *s, unsigned int count)
 	 */
 	do {
 		status = readw(uap->port.membase + UART01x_FR);
+		cpu_relax();
 	} while (status & UART01x_FR_BUSY);
 	if (!uap->vendor->always_enabled)
 		writew(old_cr, uap->port.membase + UART011_CR);

@@ -2205,10 +2206,10 @@ 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();
 	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)
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