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

Michael Williamson [off-list ref] writes:

Hi Kevin...

On 01/05/2011 07:26 AM, Michael Williamson wrote:

quoted

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 at linux.davincidsp.com/msg19524.html

Signed-off-by: Michael Williamson <redacted>
Tested-by: Michael Williamson <redacted>
---
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'd like to withdraw this patch.  It works, but it's inefficient and ugly, and 
I also get the feeling it isn't going to make it in it's current form anyway.

I have another patch that is limited to just the mityomapl138 platform that 
I would like to submit in place of this.  No other board appears to have this
problem, so it makes sense to constrain the fix to platform file that does.

OK

Kevin