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

[Nori, Sekhar]

Hi Michael,

On Sat, Jan 01, 2011 at 06:41:56, Michael Williamson wrote:
> 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 can impact system
> performance.  This is especially true if the CTS pin is shared with something
> like a clock line as is the case with UART1 CTS and the McASP AHCLKX.
> 
> 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, allow davinci platforms to override set_termios for
> configured UARTS that do not provide a true CTS input and ensure any request
> does not enable HW flow control.
> 
> 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 patch is against davinci-linux.
> 
> I'm open to suggestions for reducing the patch description.
> 
> I'm open to alternatives to the solution below, outside of disabling the
> UARTs in question as is done on the DA850 evm and the hawkboard (both
> of which might be able to use this patch?).

Can you please CC linux-serial on this patch? Folks on that list
will have ideas on how best to work around this issue.

I think setting the UART_BUG_NOMSR in up->bugs for these ports
(as you previously implemented on your tree) is a better patch
since it utilizes an existing established mechanism. 

I understand there is no existing way to pass the bugs through
platform data, but may be that needs to be created (or may be
have a new port type for "DaVinci UART with no flow control" and
then setup up->bugs after checking for this port type).

Thanks,
Sekhar

> 
>  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..db1d208 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 RTS/CTS disable; bit 0 --> UART1 */
> +	unsigned int disable_rtscts;
>  };
>  
>  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..738048e 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_set_termios_noms(struct uart_port *up,
> +				     struct ktermios *new,
> +				     struct ktermios *old)
> +{
> +	/*
> +	 * disabling CLOCAL, or enabling CRTSCTS, will enable the modem status
> +	 * interrupts.  If this routine is being called, the port in question
> +	 * does not have valid CTS/RTS pins (they are pinmuxed to some other
> +	 * function).  Override any requested operation that may enable the
> +	 * interrupts.
> +	 */
> +	new->c_cflag &= ~CRTSCTS;
> +	new->c_cflag |= CLOCAL;
> +	serial8250_do_set_termios(up, new, old);
> +}
> +
>  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_rtscts & (1 << i))
> +			p->set_termios = davinci_set_termios_noms;
>  	}
>  
>  	return platform_device_register(soc_info->serial_dev);
> -- 
> 1.7.0.4
> 
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