How to debug a driver failing to bind to a device on Linux?

Question

I am trying to figure out why the following device is not setup to its driver on my Creator CI20. For reference I am using a Linux kernel v4.13.0 and doing the compilation locally:

make ARCH=mips ci20_defconfig
make -j8 ARCH=mips CROSS_COMPILE=mipsel-linux-gnu- uImage

From the running system I can see:

ci20@ci20:~# find /sys | grep rng
/sys/firmware/devicetree/base/jz4780-cgu@10000000/rng@d8
/sys/firmware/devicetree/base/jz4780-cgu@10000000/rng@d8/compatible
/sys/firmware/devicetree/base/jz4780-cgu@10000000/rng@d8/name
/sys/bus/platform/drivers/jz4780-rng
/sys/bus/platform/drivers/jz4780-rng/bind
/sys/bus/platform/drivers/jz4780-rng/unbind
/sys/bus/platform/drivers/jz4780-rng/uevent

So the device is seen by the kernel at runtime, now the missing piece is why the driver is never binded ? I would have expected something like this:

/sys/bus/platform/drivers/jz4780-rng/100000d8.rng

I did find some other posts explaining how to debug a running system, such as:

• https://stackoverflow.com/questions/28406776/driver-binding-using-device-tree-without-compatible-string-in-the-driver
• https://stackoverflow.com/questions/35580862/device-tree-mismatch-probe-never-called
• https://stackoverflow.com/questions/41446737/platform-device-driver-autoloading-mechanism
• Is it possible to get the information for a device tree using /sys of a running kernel?

While the information is accurate on those posts, it is not very helpful for me. Since I am building locally my kernel (I added printk in the probe function of jz4780-rng driver), my question is instead:

• what option should I turn on at compile time so that the kernel prints an accurate information on its failure to call the probe function for the jz4780-rng driver ?
• In particular how do I print the complete list of the tested bus/driver for driver_probe_device ?

I am ok to add printk anywhere in the code to debug this. The question is rather: which function is traversing the device tree and calling the probe/init function ?

For reference:

$ dtc -I fs -O dts /sys/firmware/devicetree/base | grep -A 1 rng
              rng@d8 {
                      compatible = "ingenic,jz4780-rng";
              };

compatible string is declared as:

cgu: jz4780-cgu@10000000 {
    compatible = "ingenic,jz4780-cgu", "syscon";
    reg = <0x10000000 0x100>;

    clocks = <&ext>, <&rtc>;
    clock-names = "ext", "rtc";

    #clock-cells = <1>;

    rng: rng@d8 {
        compatible = "ingenic,jz4780-rng";
    };
};

And in the driver as:

static const struct of_device_id jz4780_rng_dt_match[] = {
    {
        .compatible = "ingenic,jz4780-rng",
    },
    { },
};
MODULE_DEVICE_TABLE(of, jz4780_rng_dt_match);

static struct platform_driver jz4780_rng_driver = {
    .driver     = {
        .name   = "jz4780-rng",
        .of_match_table = jz4780_rng_dt_match,
    },
    .probe      = jz4780_rng_probe,
    .remove     = jz4780_rng_remove,
};
module_platform_driver(jz4780_rng_driver);

---

Update1:

When I build my kernel with CONFIG_DEBUG_DRIVER=y, here is what I can see:

# grep driver_probe_device syslog
Sep  6 10:08:07 ci20 kernel: [    0.098280] bus: 'platform': driver_probe_device: matched device 10031000.serial with driver ingenic-uart
Sep  6 10:08:07 ci20 kernel: [    0.098742] bus: 'platform': driver_probe_device: matched device 10033000.serial with driver ingenic-uart
Sep  6 10:08:07 ci20 kernel: [    0.099209] bus: 'platform': driver_probe_device: matched device 10034000.serial with driver ingenic-uart
Sep  6 10:08:07 ci20 kernel: [    0.106945] bus: 'platform': driver_probe_device: matched device 1b000000.nand-controller with driver jz4780-nand
Sep  6 10:08:07 ci20 kernel: [    0.107282] bus: 'platform': driver_probe_device: matched device 134d0000.bch with driver jz4780-bch
Sep  6 10:08:07 ci20 kernel: [    0.107470] bus: 'platform': driver_probe_device: matched device 16000000.dm9000 with driver dm9000
Sep  6 10:08:07 ci20 kernel: [    0.165618] bus: 'platform': driver_probe_device: matched device 10003000.rtc with driver jz4740-rtc
Sep  6 10:08:07 ci20 kernel: [    0.166177] bus: 'platform': driver_probe_device: matched device 10002000.jz4780-watchdog with driver jz4740-wdt
Sep  6 10:08:07 ci20 kernel: [    0.170930] bus: 'platform': driver_probe_device: matched device 1b000000.nand-controller with driver jz4780-nand

But only:

# grep rng syslog
Sep  6 10:08:07 ci20 kernel: [    0.166842] bus: 'platform': add driver jz4780-rng
Sep  6 10:08:42 ci20 kernel: [   54.584451] random: crng init done

As a side note, the rng toplevel node: cgu is not referenced here, but there is a jz4780-cgu driver.

---

Update2:

If I move the rng node declaration outside the toplevel cgu node, I can at least see some binding happening at last:

# grep rng /var/log/syslog 
Sep  6 10:30:57 ci20 kernel: [    0.167017] bus: 'platform': add driver jz4780-rng
Sep  6 10:30:57 ci20 kernel: [    0.167033] bus: 'platform': driver_probe_device: matched device 10000000.rng with driver jz4780-rng
Sep  6 10:30:57 ci20 kernel: [    0.167038] bus: 'platform': really_probe: probing driver jz4780-rng with device 10000000.rng
Sep  6 10:30:57 ci20 kernel: [    0.167050] jz4780-rng 10000000.rng: no pinctrl handle
Sep  6 10:30:57 ci20 kernel: [    0.167066] devices_kset: Moving 10000000.rng to end of list
Sep  6 10:30:57 ci20 kernel: [    0.172774] jz4780-rng: probe of 10000000.rng failed with error -22
Sep  6 10:31:32 ci20 kernel: [   54.802794] random: crng init done

Using:

    rng: rng@100000d8 {
        compatible = "ingenic,jz4780-rng";
    };

I can also verify:

# find /sys/ | grep rng
/sys/devices/platform/10000000.rng
/sys/devices/platform/10000000.rng/subsystem
/sys/devices/platform/10000000.rng/driver_override
/sys/devices/platform/10000000.rng/modalias
/sys/devices/platform/10000000.rng/uevent
/sys/devices/platform/10000000.rng/of_node
/sys/firmware/devicetree/base/rng@100000d8
/sys/firmware/devicetree/base/rng@100000d8/compatible
/sys/firmware/devicetree/base/rng@100000d8/status
/sys/firmware/devicetree/base/rng@100000d8/reg
/sys/firmware/devicetree/base/rng@100000d8/name
/sys/bus/platform/devices/10000000.rng
/sys/bus/platform/drivers/jz4780-rng
/sys/bus/platform/drivers/jz4780-rng/bind
/sys/bus/platform/drivers/jz4780-rng/unbind
/sys/bus/platform/drivers/jz4780-rng/uevent

Solution

A working solution to get the driver to bind to the device is:

cgublock: jz4780-cgublock@10000000 {
    compatible = "simple-bus", "syscon";

    #address-cells = <1>;
    #size-cells = <1>;

    reg = <0x10000000 0x100>;
    ranges;

    cgu: jz4780-cgu@10000000 {
        compatible = "ingenic,jz4780-cgu";
        reg = <0x10000000 0x100>;

        clocks = <&ext>, <&rtc>;
        clock-names = "ext", "rtc";

        #clock-cells = <1>;
    };

    rng: rng@d8 {
        compatible = "ingenic,jz4780-rng";
        reg = <0x100000d8 0x8>;
    };
};

This was found by staring at other examples. I would prefer a solution where I get a proper diagnosis why the previous attempt is incorrect.