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Re: [PATCH] serial driver PMC MSP71xx, kernel linux-mips.git master

To: Marc St-Jean <>
Subject: Re: [PATCH] serial driver PMC MSP71xx, kernel linux-mips.git master
From: Sergei Shtylyov <>
Date: Thu, 15 Feb 2007 23:31:32 +0300
In-reply-to: <>
Organization: MontaVista Software Inc.
Original-recipient: rfc822;
References: <>
User-agent: Mozilla/5.0 (X11; U; Linux i686; rv:1.7.2) Gecko/20040803

Marc St-Jean wrote:

There are three different fixes:
1. Fix for DesignWare APB THRE errata:
In brief, this is a non-standard 16550 in that the THRE interrupt
will not re-assert itself simply by disabling and re-enabling the
THRI bit in the IER, it is only re-enabled if a character is actually
sent out.
It appears that the "8250-uart-backup-timer.patch" in the "mm" tree also
fixes it so we have dropped our initial workaround.
This patch now needs to be applied on top of that "mm" patch.

   This patch has hit the mainline kernel already.

2. Fix for Busy Detect on LCR write:
The DesignWare APB UART has a feature which causes a new Busy Detect
interrupt to be generated if it's busy when the LCR is written. This
fix saves the value of the LCR and rewrites it after clearing the

3. Workaround for interrupt/data concurrency issue:
The SoC needs to ensure that writes that can cause interrupts to be
cleared reach the UART before returning from the ISR. This fix reads
a non-destructive register on the UART so the read transaction
completion ensures the previously queued write transaction has
also completed.

The differences from the last attempt is dropping the call to
in_irq() and including more detailed description of the fixes.

The difference part would better be under the "---" cutoff line, along with diffstat.
This way it gets auto-removed by quilt/git when applying the patch.

Signed-off-by: Marc St-Jean <>

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 3d91bfc..bfaacc5 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -308,6 +308,7 @@ static unsigned int serial_in(struct uar
                return inb(up->port.iobase + 1);
case UPIO_MEM:
+       case UPIO_DWAPB:
                return readb(up->port.membase + offset);
case UPIO_MEM32:
@@ -333,6 +334,8 @@ #endif
 static void
 serial_out(struct uart_8250_port *up, int offset, int value)
+       /* Save the offset before it's remapped */
+       int save_offset = offset;
        offset = map_8250_out_reg(up, offset) << up->port.regshift;
switch (up->port.iotype) {

I've just got an idea how to "beautify" this code -- rename the 'offset' arg to something like reg, and then declare/init 'offset' as local variable.
Would certainly look better (and worth doing in all serial_* accessros).

@@ -359,6 +362,18 @@ #endif
                        writeb(value, up->port.membase + offset);
+ case UPIO_DWAPB:
+               /* Save the LCR value so it can be re-written when a
+                * Busy Detect interrupt occurs. */
+               if (save_offset == UART_LCR)
+                       up->lcr = value;
+               writeb(value, up->port.membase + offset);
+               /* Read the IER to ensure any interrupt is cleared before
+                * returning from ISR. */
+               if (save_offset == UART_TX || save_offset == UART_IER)
+                       value = serial_in(up, UART_IER);
+               break;
                outb(value, up->port.iobase + offset);
@@ -2454,8 +2485,8 @@ int __init serial8250_start_console(stru
add_preferred_console("ttyS", line, options);
        printk("Adding console on ttyS%d at %s 0x%lx (options '%s')\n",
-               line, port->iotype == UPIO_MEM ? "MMIO" : "I/O port",
-               port->iotype == UPIO_MEM ? (unsigned long) port->mapbase :
+               line, port->iotype >= UPIO_MEM ? "MMIO" : "I/O port",
+               port->iotype >= UPIO_MEM ? (unsigned long) port->mapbase :
                    (unsigned long) port->iobase, options);
        if (!(serial8250_console.flags & CON_ENABLED)) {
                serial8250_console.flags &= ~CON_PRINTBUFFER;

I've remembered why I decided not to fix it: this code only gets called from 8250__eraly.c which can't handle anything beyond UPIO_MEM anyway.

WBR, Sergei

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