[PATCH] [v2] serial: amba-pl011: use cpu_relax when polling registers
From: Dave.Martin@arm.com (Dave Martin)
Date: 2016-01-19 12:05:07
Also in:
linux-arm-msm, linux-serial
On Fri, Jan 15, 2016 at 02:32:20PM -0600, Timur Tabi wrote:
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.
I remember noticing this before, but didn't write any patch for it at the time... cpu_relax() is indeed the write thing to do in such polling loops.
Signed-off-by: Timur Tabi <redacted>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
quoted hunk ↗ jump to hunk
--- v2: rebased to latest code drivers/tty/serial/amba-pl011.c | 17 ++++++++--------- 1 file changed, 8 insertions(+), 9 deletions(-)diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c index c0da0cc..69b5acd 100644 --- 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)-- Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel at lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel