Subject: Re: [PATCH V4] workaround for mpc52xx erratum #364 (serial may not be reset in break state)

From: René Bürgel <[email-masked]>
Date: Thu, 06 Nov 2008 09:11:07 +0100

This patch is a workaround for bug #364 found in the MPC52xx processor.
The errata document can be found under 
http://www.freescale.com/files/32bit/doc/errata/MPC5200E.pdf?fpsp=1&WT_TYPE=Errata&WT_VENDOR=FREESCALE&WT_FILE_FORMAT=pdf&WT_ASSET=Documentation 

When a device with a low baudrate is connected to the serial port, but 
the processor "listens" on a higher baudrate, it might falsely receive 
breaks from the controller. During a break, the serial controller may 
not be reset. The appended patch provides a workaround for that 
situation by lowering the baudrate without resetting the controller and 
waiting until no break is received anymore.

This is v4 if the patch, just reformatted to fit the linux kernel coding 
style without functional changes.

Wolfram Sang schrieb:
> Hi René,
>
> On Tue, Nov 04, 2008 at 08:40:09PM +0100, René Bürgel wrote
>> But there's still one thing, that bothers me a bit - if there is REALLY  
>> a break on the line, closing the driver may take until it's gone. I  
>> don't know whether this is really satisfying, but i think it's better  
>> than the alternative: no serial connection until the next reboot.
>>     
>
> I think we should CC linux-serial to get some opinions about this. At
> least, if it stays like this, it should be mentioned in the source.

What's the opinion from the linux-serial folks about this issue?

-- 
René Bürgel
Software Engineer
Unicontrol Systemtechnik GmbH
OT Dittersbach
Sachsenburger Weg 34
09669 Frankenberg
 
Tel.: 03 72 06/ 88 73 - 19
Fax: 03 72 06/ 88 73 - 60
E-Mail: [email-masked]
Internet: www.unicontrol.de
 
Unicontrol Systemtechnik GmbH
Geschäftsführer: Dipl.-Ing. Siegfried Heinze
Sitz der Gesellschaft: Frankenberg
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diff --git a/drivers/serial/mpc52xx_uart.c b/drivers/serial/mpc52xx_uart.c
index 6117d3d..ae539b5 100644
--- a/drivers/serial/mpc52xx_uart.c
+++ b/drivers/serial/mpc52xx_uart.c
@@ -496,6 +496,74 @@ mpc52xx_uart_break_ctl(struct uart_port *port, int ctl)
 	spin_unlock_irqrestore(&port->lock, flags);
 }
 
+/*
+ * This is a workaround for processor bug #364
+ * described in MPC5200 (L25R) Errata.
+ * The bug is still present in MPC5200B,
+ * but currently not listed in its errata sheet.
+ *
+ * The workaround resets the baudrate to the slowest possible,
+ * waits for a stable state and resets break state repeatedly if necessary.
+ * Optionally it can release the lock while waiting.
+ *
+ * That baudrate is roughly port->uartclk / (1000 * 1000)
+ * The minimum wait time for the first try has to include
+ * the time to wait for stop-bits and a character.
+ * We wait for 2 chars to be sure.
+ * Consecutive waits must just receive one character.
+ */
+
+#ifdef CONFIG_PPC_MPC52xx
+static void reset_errors_and_wait(struct uart_port *port, bool unlock,
+				  unsigned long flags, unsigned int delay)
+{
+	struct mpc52xx_psc __iomem *psc = PSC(port);
+	out_8(&psc->command, MPC52xx_PSC_RST_ERR_STAT);
+	if (unlock) {
+		disable_irq(port->irq);
+		spin_unlock_irqrestore(&port->lock, flags);
+	}
+	mdelay(delay);
+	if (unlock) {
+		spin_lock_irqsave(&port->lock, flags);
+		enable_irq(port->irq);
+	}
+}
+#endif
+
+static void mpc52xx_uart_reset_rx(struct uart_port *port, bool unlock,
+				  unsigned long flags)
+{
+#ifdef CONFIG_PPC_MPC52xx
+	struct mpc52xx_psc __iomem *psc = PSC(port);
+
+	/*
+	 * One character on the serial port may consist of up to 12 bits.
+	 * So the time to receive one char is
+	 * 12  / (port->uartclk / (1000 * 1000) ) * 1000,
+	 * (bits)                 (MHz -> Hz)      (s -> ms)
+	 */
+	unsigned int one_char_receive_duration =
+			(12 * 1000) / (port->uartclk / (1000 * 1000));
+
+	/*
+	 * CT=0xFFFF sets the serial line to the minimal possible baudrate
+	 * (depending on the uartclk).
+	 */
+	out_8(&psc->ctur, 0xFF);
+	out_8(&psc->ctlr, 0xFF);
+
+	reset_errors_and_wait(port, unlock, flags,
+			      one_char_receive_duration * 2);
+
+	while ((in_be16(&psc->mpc52xx_psc_status)) & MPC52xx_PSC_SR_RB) {
+		reset_errors_and_wait(port, unlock, flags,
+				      one_char_receive_duration);
+	}
+#endif
+	out_8(&psc->command, MPC52xx_PSC_RST_RX);
+}
+
 static int
 mpc52xx_uart_startup(struct uart_port *port)
 {
@@ -510,7 +578,7 @@ mpc52xx_uart_startup(struct uart_port *port)
 		return ret;
 
 	/* Reset/activate the port, clear and enable interrupts */
-	out_8(&psc->command, MPC52xx_PSC_RST_RX);
+	mpc52xx_uart_reset_rx(port, false, 0);
 	out_8(&psc->command, MPC52xx_PSC_RST_TX);
 
 	out_be32(&psc->sicr, 0);	/* UART mode DCD ignored */
@@ -529,7 +597,7 @@ mpc52xx_uart_shutdown(struct uart_port *port)
 	struct mpc52xx_psc __iomem *psc = PSC(port);
 
 	/* Shut down the port.  Leave TX active if on a console port */
-	out_8(&psc->command, MPC52xx_PSC_RST_RX);
+	mpc52xx_uart_reset_rx(port, false, 0);
 	if (!uart_console(port))
 		out_8(&psc->command, MPC52xx_PSC_RST_TX);
 
@@ -588,9 +656,6 @@ mpc52xx_uart_set_termios(struct uart_port *port, struct ktermios *new,
 	/* Get the lock */
 	spin_lock_irqsave(&port->lock, flags);
 
-	/* Update the per-port timeout */
-	uart_update_timeout(port, new->c_cflag, baud);
-
 	/* Do our best to flush TX & RX, so we don't loose anything */
 	/* But we don't wait indefinitly ! */
 	j = 5000000;	/* Maximum wait */
@@ -607,9 +672,12 @@ mpc52xx_uart_set_termios(struct uart_port *port, struct ktermios *new,
 			"Some chars may have been lost.\n");
 
 	/* Reset the TX & RX */
-	out_8(&psc->command, MPC52xx_PSC_RST_RX);
+	mpc52xx_uart_reset_rx(port, true, flags);
 	out_8(&psc->command, MPC52xx_PSC_RST_TX);
 
+	/* Update the per-port timeout */
+	uart_update_timeout(port, new->c_cflag, baud);
+
 	/* Send new mode settings */
 	out_8(&psc->command, MPC52xx_PSC_SEL_MODE_REG_1);
 	out_8(&psc->mode, mr1);