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 <timur@xxxxxxxxxxxxxx>
Reviewed-by: Dave Martin <Dave.Martin@xxxxxxx>
> ---
>
> 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.
>
>
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