Hi Patryk,
On Tue, Mar 26, 2024 at 01:26:10PM +0100, Patryk wrote:
> Suppose that I want to create a Kernel driver that will handle GPIO
> Expander and Interrupt controller parts whereas the watchdog
> functionality will be handled in Userspace using /sys/class/i2c-dev
>
> Correct me if I'm wrong - but if the kernel driver claims that it
> handles the communication with a particular device, does it
> automatically prevent userspace programs from communicating with this
> device using i2c-dev?
Yes, if a device is claimed by a kernel driver, userspace cannot claim
it again. This prevents accidental usages of I2C devices from userspace.
> So splitting responsibilities between kernel and userspace which both
> handle the same physical device will not work, yes?
No, it can work. For the i2c-dev userspace API there exists the flag
#define I2C_SLAVE_FORCE 0x0706 /* Use this slave address, even if it
is already in use by a driver! */
See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/i2c-dev.h#n29
So userspace can override the safety check and also use the I2C device,
by providing this flag.
> For me it makes sense because accessing the device that is already
> used by a kernel, from userspace might be unexpected by the kernel,
> however, I haven't found any solid confirmation (it may be my poor
> research though)
Yes, you can very easily mess things up, if a kernel driver and a
userspace driver uses the same device at once. It would require
carefully programming and coordination to make it work correctly and
robust. Nevertheless if you really really want it, you can do it.
But why?
> Suppose that I have an I2C device (CPLD MAX10 with custom
> configuration) that implements the following functionalities:
> - GPIO Expander && Interrupt controller
> - Watchdog
It's possible and already done to implement a kernel watchdog driver and
combine all the functionalites (gpio expander, interrupt controller and
watchdog) in a multi-function device (MFD)[1],
Kind regards,
Stefan
[1]: https://bootlin.com/pub/conferences/2015/elce/belloni-mfd-regmap-syscon/belloni-mfd-regmap-syscon.pdf
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