[PATCH] gpio: mxs: implement get_direction callback

Janusz Użycki j.uzycki at elproma.com.pl
Sat Nov 15 11:29:04 PST 2014

Hello Uwe,

W dniu 2014-11-15 o 00:26, Uwe Kleine-König pisze:
> Hello,
> On Fri, Nov 14, 2014 at 11:27:06PM +0100, Janusz Uzycki wrote:
>> Function gpiod_get_direction() of gpiolib calls get_direction()
>> callback. If chip doesn't implement it EINVAL error is returned.
>> The function doesn't use for returned value shadowed FLAG_IS_OUT
>> bit of gpio_desc.flags field so the callback is required.
> This sounds like a bugfix but "required" is wrong as AFAIR this call is
> optional and hardly used. Or did that change? The only drawback is that
> the debug output for (by Linux) uninitialized gpios is wrong.

Yes, the callback is optional but gpiod_get_direction() doesn't work 
without it.
gpiod_get_direction() doesn't seem optional, 
"A driver can also query the current direction of a GPIO:
          int gpiod_get_direction(const struct gpio_desc *desc)
This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
Be aware that there is no default direction for GPIOs. Therefore, 
**using a GPIO
without setting its direction first is illegal and will result in undefined
There is nothing about EINVAL error. It happens despite direction was
set before. The reason is undefined get_direction callback.

The commit f9e42397d79b6e810437ba1130b0b4b594f5e56c
("serial: mxs-auart: add interrupts for modem control lines")
is based on Richard's commit ab5e4e4108ca5d8326cb6b4b3a21b096a002f68f
("tty/serial: at91: add interrupts for modem control lines").
Both patches use the condition:
"if (gpiod && (gpiod_get_direction(gpiod) == GPIOF_DIR_IN))"

at91 had defined get_direction() callback, mxs platform not.
The condition helps to find gpio-inputs to set them as interrupts.

Likely gpiod_get_direction() was used because 
has no function to read back "dir_out" from mctrl_gpios_desc table.

There were the ways to fix it:
a) add to serial_mctrl_gpio.c function to read the "dir_out" and use it 
instead of
if gpiod_get_direction() still used in the condition:
b) modify gpiod_get_direction() implementation in gpiolib.c:
     if get_direction() callback is not defined use shadow flag 
c) modify drivers/gpio/gpio-generic.c:
    bgpio_init() could assign to get_direction default callback
d) implement get_direction callback in gpio-mxs.c

Solution d) was selected because it is safe for other drivers. Although 
a) and b)
are also nice the patch doesn't modify neither serial_mctrl_gpio.c and
atmel_serial.c nor gpiolib.c. It focuses on mxs platform as needed for
the commit "serial: mxs-auart: add interrupts for modem control lines".

Maybe Richard will be interested in a).

> Also the grammar of you sentence isn't German enough to sound correct.
Right, I was tired, sorry.
What about the following description for the patch?

gpiolib's function gpiod_get_direction() returns the EINVAL error
if get_direction() callback is not defined.
The patch implements the callback for mxs chip as commit
f9e42397d79b ("serial: mxs-auart: add interrupts for modem control lines")
uses the gpiod_get_direction() function.

best regards

> Best regards
> Uwe

More information about the linux-arm-kernel mailing list