From mboxrd@z Thu Jan 1 00:00:00 1970 From: Timur Tabi Subject: [PATCH 1/2] serial: amba-pl011: use cpu_relax when polling registers Date: Tue, 3 Nov 2015 09:01:06 -0600 Message-ID: <1446562867-5693-1-git-send-email-timur@codeaurora.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: linux-serial@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Andrew.Jackson@arm.com, andre.przywara@arm.com, jun.nie@linaro.org, peter@hurleysoftware.com, Linus Walleij , Greg Kroah-Hartman , jslaby@suse.com List-Id: linux-serial@vger.kernel.org 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 --- 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) -- Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.