52ca7d0f7b
The PCA9552 lines can be used either for driving LEDs or as GPIOs. The
manual states that for LEDs, the operation is open-drain:
The LSn LED select registers determine the source of the LED data.
00 = output is set LOW (LED on)
01 = output is set high-impedance (LED off; default)
10 = output blinks at PWM0 rate
11 = output blinks at PWM1 rate
For GPIOs it suggests a pull-up so that the open-case drives the line
high:
For use as output, connect external pull-up resistor to the pin
and size it according to the DC recommended operating
characteristics. LED output pin is HIGH when the output is
programmed as high-impedance, and LOW when the output is
programmed LOW through the ‘LED selector’ register. The output
can be pulse-width controlled when PWM0 or PWM1 are used.
Now, I have a hardware design that uses the LED controller to control
LEDs. However, for $reasons, we're using the leds-gpio driver to drive
the them. The reasons are here are a tangent but lead to the discovery
of the inversion, which manifested as the LEDs being set to full
brightness at boot when we expected them to be off.
As we're driving the LEDs through leds-gpio, this means wending our way
through the gpiochip abstractions. So with that in mind we need to
describe an active-low GPIO configuration to drive the LEDs as though
they were GPIOs.
The set() gpiochip callback in leds-pca955x does the following:
...
if (val)
pca955x_led_set(&led->led_cdev, LED_FULL);
else
pca955x_led_set(&led->led_cdev, LED_OFF);
...
Where LED_FULL = 255. pca955x_led_set() in turn does:
...
switch (value) {
case LED_FULL:
ls = pca955x_ledsel(ls, ls_led, PCA955X_LS_LED_ON);
break;
...
Where PCA955X_LS_LED_ON is defined as:
#define PCA955X_LS_LED_ON 0x0 /* Output LOW */
So here we have some type confusion: We've crossed domains from GPIO
behaviour to LED behaviour without accounting for possible inversions
in the process.
Stepping back to leds-gpio for a moment, during probe() we call
create_gpio_led(), which eventually executes:
if (template->default_state == LEDS_GPIO_DEFSTATE_KEEP) {
state = gpiod_get_value_cansleep(led_dat->gpiod);
if (state < 0)
return state;
} else {
state = (template->default_state == LEDS_GPIO_DEFSTATE_ON);
}
...
ret = gpiod_direction_output(led_dat->gpiod, state);
In the devicetree the GPIO is annotated as active-low, and
gpiod_get_value_cansleep() handles this for us:
int gpiod_get_value_cansleep(const struct gpio_desc *desc)
{
int value;
might_sleep_if(extra_checks);
VALIDATE_DESC(desc);
value = _gpiod_get_raw_value(desc);
if (value < 0)
return value;
if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
value = !value;
return value;
}
_gpiod_get_raw_value() in turn calls through the get() callback for the
gpiochip implementation, so returning to our get() implementation in
leds-pca955x we find we extract the raw value from hardware:
static int pca955x_gpio_get_value(struct gpio_chip *gc, unsigned int offset)
{
struct pca955x *pca955x = gpiochip_get_data(gc);
struct pca955x_led *led = &pca955x->leds[offset];
u8 reg = pca955x_read_input(pca955x->client, led->led_num / 8);
return !!(reg & (1 << (led->led_num % 8)));
}
This behaviour is not symmetric with that of set(), where the val is
inverted by the driver.
Closing the loop on the GPIO_ACTIVE_LOW inversions,
gpiod_direction_output(), like gpiod_get_value_cansleep(), handles it
for us:
int gpiod_direction_output(struct gpio_desc *desc, int value)
{
VALIDATE_DESC(desc);
if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
value = !value;
else
value = !!value;
return _gpiod_direction_output_raw(desc, value);
}
All-in-all, with a value of 'keep' for default-state property in a
leds-gpio child node, the current state of the hardware will in-fact be
inverted; precisely the opposite of what was intended.
Rework leds-pca955x so that we avoid the incorrect inversion and clarify
the semantics with respect to GPIO.
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Joel Stanley <joel@jms.id.au>
Tested-by: Matt Spinler <mspinler@linux.vnet.ibm.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
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README
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.