How to set value of i2c PCA9570 gpio expander as early as possible during Linux boot? - linux

Question

I have written a working driver for the PCA9570. Its four outputs are set (and read back) via Linux's GPIO subsystem. e.g.
root#armbox:/sys/class/gpio# echo 508 > export
root#armbox:/sys/class/gpio# echo 509 > export
root#armbox:/sys/class/gpio# echo 510 > export
root#armbox:/sys/class/gpio# echo 511 > export
The problem is that the chip starts with its outputs high. https://www.nxp.com/docs/en/data-sheet/PCA9570.pdf section 8.1.
root#armbox:/sys/class/gpio# cat gpio508/value
1
root#armbox:/sys/class/gpio# cat gpio509/value
1
root#armbox:/sys/class/gpio# cat gpio510/value
1
root#armbox:/sys/class/gpio# cat gpio511/value
1
I can manually set them low from userspace, after boot. e.g.
root#armbox:/sys/class/gpio# echo 0 > gpio510/value
root#armbox:/sys/class/gpio# cat gpio510/value
0
How can I set the chip's outputs low as early as possible during the boot sequence?
I can hack my own driver to do this, during pca9570_probe(), but that feels very hacky. pca9570_probe() currently reads the values from the chip.
static int pca9570_probe(struct i2c_client *client,
const struct i2c_device_id *id)
{
...
ret = pca9570_readb(chip, &chip->reg);
if (ret)
goto out_failed;
return 0;
out_failed:
if (chip->client)
i2c_unregister_device(chip->client);
return -1;
}
Is there a correct way in Linux to specify GPIO values during boot, rather than hacking a driver?
P.S. The dts clause is:
pca9570: pca9570#48 {
compatible = "pca9570";
reg = <0x24>;
gpio-controller;
#gpio-cells = <4>;
};

Answer 1

You can use 'GPIO hogging' mechanism as described in DeviceTree gpio binding documentation. Quoting basic information:
GPIO hogging is a mechanism
providing automatic GPIO request and configuration as part of the
gpio-controller's driver probe function.
For example, we have this definition in our device tree for our gpio expander:
gpio_expander: tca6424a#22 {
compatible = "ti,tca6424";
reg = <0x22>;
[...] /* some other gpio expander configuration */
lcd-rst {
gpio-hog;
gpios = <7 GPIO_ACTIVE_LOW>;
output-low;
};
};
You don't need to extend your driver, it is part of the gpio subsystem.