On 08/11/2015 08:40 AM, Russell King - ARM Linux wrote:

On Mon, Aug 10, 2015 at 08:31:03PM -0400, Peter Hurley wrote:

quoted

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 2af09ab..fd54991 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c

@@ -2348,13 +2348,10 @@ static struct console amba_console = {
 
 static void pl011_putc(struct uart_port *port, int c)
 {
-	struct uart_amba_port *uap =
-	    container_of(port, struct uart_amba_port, port);
-
-	while (pl011_readw(uap, REG_FR) & UART01x_FR_TXFF)
+	while (readl(port->membase + UART01x_FR) & UART01x_FR_TXFF)
 		;
-	pl011_writeb(uap, c, REG_DR);
-	while (pl011_readw(uap, REG_FR) & uap->fr_busy)
+	writeb(c, port->membase + UART01x_DR);
+	while (readl(port->membase + UART01x_FR) & UART01x_FR_BUSY)
 		;

These loops are wrong.  Kernel coding style is that loops like this
will use cpu_relax() in them, just like the driver used to.  This
was introduced by:

commit 0d3c673e7881e691991b2a4745bd4f149603baa2
Author: Rob Herring [off-list ref]
Date:   Fri Apr 18 17:19:57 2014 -0500

    tty/serial: pl011: add generic earlycon support

Also, the above readl()s should be readl_relaxed(), we don't need
barriered reads or to hit the L2 cache on those reads in the above code.

This patch just reverts pl011 earlycon back to the existing mainline
state.

The amba-pl011 driver has many places where cpu_relax() is not used,
so that seems like a good patch for -next during the 4.3-rc cycle.

Similarly for read_relaxed() use in earlycon.

Regards,
Peter Hurley