[PATCH 1/2] serial: amba-pl011: use cpu_relax when polling registers
From: Timur Tabi <hidden>
Date: 2015-11-03 15:01:06
Also in:
linux-serial
Subsystem:
arm primecell uart pl010 and pl011 drivers, the rest, tty layer and serial drivers · Maintainers:
Russell King, Linus Torvalds, Greg Kroah-Hartman, Jiri Slaby
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 <redacted> --- drivers/tty/serial/amba-pl011.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 005de8c..164bd65 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c@@ -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) - barrier(); + cpu_relax(); writel(c, port->membase + UART01x_DR); while (!(readl(port->membase + UART01x_FR) & UART011_FR_TXFE)) - barrier(); + cpu_relax(); } static void pl011_early_write(struct console *con, const char *s, unsigned n)
--
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