[PATCH 1/2 v1] davinci: support disabling modem status interrupts on SOC UARTS

[Michael Williamson]

On the da850/omap-l138/am18x family of SoCs, up to three on chip UARTS may be
configured.  These peripherals support the standard Tx/Rx signals as well as
CTS/RTS hardware flow control signals.  The pins on these SOC's associated with
these signals are multiplexed; e.g., the pin providing UART0_TXD capability
also provides SPI0 chip select line 5 output capability.  The configuration of
the pin multiplexing occurs during platform initialization (or by earlier
bootloader operations).

There is a problem with the multiplexing implementation on these SOCs.  Only
the output and output enable portions of the I/O section of the pin are
multiplexed.  All peripheral input functions remain connected to a given pin
regardless of configuration.

In many configurations of these parts, providing a UART with Tx/Rx capability
is needed, but the HW flow control capability is not.  Furthermore, the pins
associated with the CTS inputs on these UARTS are often configured to support
a different peripheral, and they may be active/toggling during runtime.  This
can result in false modem status (CTS) interrupts being asserted to the 8250
driver controlling the associated Tx/Rx pins, and will impact system
performance.

The 8250 serial driver platform data does not provide a direct mechanism to
tell the driver to disable modem status (i.e., CTS) interrupts for a given
port.  As a work-around, intercept register writes to the IER and disable
modem status interrupts.

This patch was tested using a MityDSP-L138 SOM having UART1 enabled with the
associated CTS pin connected to a clock (configured for the AHCLKX function).

Background / problem reports related to this issue are captured in the links
below:
http://e2e.ti.com/support/dsp/omap_applications_processors/f/42/t/36701.aspx
http://www.mail-archive.com/davinci-linux-open-source@linux.davincidsp.com/msg19524.html

Signed-off-by: Michael Williamson <michael.williamson at criticallink.com>
Tested-by: Michael Williamson <michael.williamson at criticallink.com>
---
This is against the linux-davinci tree.

Changes from original proposed patch:
- instead of overriding set_termios, now override serial_out driver hook
  and intercept writes to the MSR.

An alternate patch was proposed that modified the serial core driver and added a UPF_NO_MSR
flag.  There was resistance to this patch.  The reasoning is that the core 8250 driver code
should not continue to get muddied by platform hacks.

I'm wondering, given this and the original proposed patch, if the set_termios
override might be better?  This patch incurs a test penalty every time the UART
is accessed; whereas the original patch only incurs a penalty on IOCTL calls.  The
set_termios would at least report the proper IOCTL information for CRTSCTS 
when probed, I think, instead of just quietly lying about it...

 arch/arm/mach-davinci/include/mach/serial.h |    2 ++
 arch/arm/mach-davinci/serial.c              |   19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/arch/arm/mach-davinci/include/mach/serial.h b/arch/arm/mach-davinci/include/mach/serial.h
index 8051110..8f7f5e5 100644
--- a/arch/arm/mach-davinci/include/mach/serial.h
+++ b/arch/arm/mach-davinci/include/mach/serial.h
@@ -49,6 +49,8 @@
 struct davinci_uart_config {
 	/* Bit field of UARTs present; bit 0 --> UART1 */
 	unsigned int enabled_uarts;
+	/* Bit field of modem status interrupt disables; bit 0 -> UART1 */
+	unsigned int disable_msi;
 };
 
 extern int davinci_serial_init(struct davinci_uart_config *);
diff --git a/arch/arm/mach-davinci/serial.c b/arch/arm/mach-davinci/serial.c
index 1875740..83d44c0 100644
--- a/arch/arm/mach-davinci/serial.c
+++ b/arch/arm/mach-davinci/serial.c
@@ -31,6 +31,22 @@
 #include <mach/serial.h>
 #include <mach/cputype.h>
 
+static void davinci_serial_out_nomsi(struct uart_port *p, int offset, int value)
+{
+	int lcr, lcr_off;
+
+	if (offset == UART_IER) {
+		lcr_off = UART_LCR << p->regshift;
+		lcr = readb(p->membase + lcr_off);
+		/* don't override DLM setting, or probing operations */
+		if (!(lcr & UART_LCR_DLAB) && p->type != PORT_UNKNOWN)
+			value &= ~UART_IER_MSI;
+	}
+
+	offset <<= p->regshift;
+	writeb(value, p->membase + offset);
+}
+
 static inline unsigned int serial_read_reg(struct plat_serial8250_port *up,
 					   int offset)
 {
@@ -109,6 +125,9 @@ int __init davinci_serial_init(struct davinci_uart_config *info)
 
 		if (p->membase && p->type != PORT_AR7)
 			davinci_serial_reset(p);
+
+		if (info->disable_msi & (1 << i))
+			p->serial_out = davinci_serial_out_nomsi;
 	}
 
 	return platform_device_register(soc_info->serial_dev);
-- 
1.7.0.4