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From: Laurent Pinchart <laurentp@cse-semaphore.com>
To: linuxppc-dev@ozlabs.org
Subject: [RFC] GPIO-based flow control in the cpm_uart driver
Date: Tue, 15 Apr 2008 15:22:33 +0200
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Hi everybody,

I'm implementing flow control and modem control lines support in the cpm_ua=
rt=20
driver.

The implementation is based on the GPIO lib. Modem control lines are descri=
bed=20
in the device tree as GPIO resources and accessed through the OF GPIO=20
bindings. The I/O ports have to be initialized as GPIOs in the=20
platform-specific code.

Reading and writing the modem control lines isn't an issue, but activating=
=20
hardware flow control is more complex. The driver needs to turn dedicated=20
functions on and off for the RTS and CTS signals, and the GPIO API doesn't=
=20
provide a way to access the PPAR* registers (which does make sense - althou=
gh=20
arguably - as PPAR* control specific functions, not GPIOs).

Hardcoding RTS and CTS lines control in the driver is not an option I want =
to=20
consider. Extending the GPIO API to handled special functions has been nack=
ed=20
in the past (see=20
http://ozlabs.org/pipermail/linuxppc-dev/2008-February/051241.html). An=20
option would be to export gpio_to_chip from drivers/gpio/gpiolib.c and use=
=20
cpm1/2_set_pin in the cpm_uart driver.

Can anyone think of a better solution ?

Best regards,

=2D-=20
Laurent Pinchart
CSE Semaphore Belgium

Chaussee de Bruxelles, 732A
B-1410 Waterloo
Belgium

T +32 (2) 387 42 59
=46 +32 (2) 387 42 75

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