Day72 - Platform Bus Internals¶

Objective¶

Understand how the Linux Platform Bus is built on top of the Linux Driver Model and how Device Tree creates platform devices that are matched against platform drivers.

Topics covered:

platform_bus_type
platform_device
platform_driver
Device Tree integration
of_match_table
Name matching
Compatible matching
MODULE_DEVICE_TABLE(of)
Platform Bus sysfs hierarchy

Lab 1 - Observe Real Platform Bus Devices¶

Inspect Platform Bus¶

ls /sys/bus/platform

Example output:

devices
drivers
drivers_autoprobe
drivers_probe
uevent

Inspect Platform Devices¶

ls /sys/bus/platform/devices | head

Example output:

1000005100.iommu
1000010000.dma
1000120000.pcie
...

Inspect Device Binding¶

readlink /sys/bus/platform/devices/1000120000.pcie/driver

Example output:

../../../../bus/platform/drivers/brcm-pcie

Inspect Driver Side¶

ls -l /sys/bus/platform/drivers/brcm-pcie

Example output:

1000120000.pcie -> ../../../../devices/platform/axi/1000120000.pcie
bind
unbind
uevent

Inspect Device Tree Node¶

ls -l /sys/bus/platform/devices/1000120000.pcie/of_node

Example output:

../../../firmware/devicetree/base/axi/pcie@1000120000

Inspect Compatible String¶

cat \
/sys/bus/platform/devices/1000120000.pcie/of_node/compatible \
| tr '\0' '\n'

Example output:

brcm,bcm2712-pcie

Lab 2 - Minimal Platform Driver¶

Goal¶

Register a platform driver without creating any platform device.

Driver Registration¶

static struct platform_driver demo_driver = {
    .probe = demo_probe,
    .remove = demo_remove,

    .driver = {
        .name = "demo_platform",
    },
};

platform_driver_register(&demo_driver);

Verify Driver Registration¶

sudo insmod platform_driver_demo.ko

Expected log:

[DEMO] init

No probe should occur.

Verify Sysfs¶

ls -l /sys/bus/platform/drivers/demo_platform

Expected:

bind
unbind
module
uevent

No device symlink exists.

Lab 3 - Name Matching¶

Goal¶

Create a platform device manually and verify name-based matching.

Create Platform Device¶

demo_pdev =
    platform_device_register_simple(
        "demo_platform",
        -1,
        NULL,
        0);

Register Driver¶

.driver = {
    .name = "demo_platform",
}

Verify Probe¶

Expected log:

[DEMO] probe

Verify Binding¶

Driver side:

ls -l /sys/bus/platform/drivers/demo_platform

Device side:

readlink \
/sys/bus/platform/devices/demo_platform/driver

Registration Order Test¶

Driver First¶

platform_driver_register()
    ↓
platform_device_register_simple()
    ↓
probe()

Device First¶

platform_device_register_simple()
    ↓
platform_driver_register()
    ↓
probe()

Result:

Registration order does not affect matching.

Lab 4 - Device Tree Compatible Matching¶

Goal¶

Verify Device Tree based matching using of_match_table.

Device Tree Overlay¶

/dts-v1/;
/plugin/;

/ {
    compatible = "brcm,bcm2712";

    fragment@0 {
        target-path = "/";

        __overlay__ {

            demo_platform: demo-platform {
                compatible = "demo,platform-bus";
                status = "okay";
            };
        };
    };
};

Driver Match Table¶

static const struct of_device_id demo_of_match[] = {
    { .compatible = "demo,platform-bus" },
    {}
};

MODULE_DEVICE_TABLE(of, demo_of_match);

Driver Definition¶

.driver = {
    .name = "demo_platform",
    .of_match_table = demo_of_match,
}

Load Driver First¶

sudo insmod platform_device_driver_demo.ko

Expected:

[DEMO] init begin
[DEMO] init end

Apply Overlay¶

sudo dtoverlay demo-platform.dtbo

Expected:

[DEMO] probe

Remove Overlay¶

sudo dtoverlay -r demo-platform

Expected:

[DEMO] remove

Verify Device Tree Node¶

ls -l \
/sys/bus/platform/devices/demo-platform/of_node

Expected:

../../../firmware/devicetree/base/demo-platform

Verify Compatible¶

cat \
/sys/bus/platform/devices/demo-platform/of_node/compatible \
| tr '\0' '\n'

Expected:

demo,platform-bus

Lab 5 - Compatible Match Failure¶

Goal¶

Verify that matching fails when compatible strings differ.

Modify Driver¶

static const struct of_device_id demo_of_match[] = {
    { .compatible = "demo,wrong-device" },
    {}
};

Load Driver¶

sudo insmod platform_device_driver_demo.ko

Apply Overlay¶

sudo dtoverlay demo-platform.dtbo

Expected:

[DEMO] init begin
[DEMO] init end

No probe should occur.

Conclusion¶

Successful registration does not imply successful binding.

Lab 6 - MODULE_DEVICE_TABLE(of)¶

Goal¶

Inspect generated OF aliases.

Inspect Module Metadata¶

modinfo platform_device_driver_demo.ko

Example output:

alias: of:N*T*Cdemo,platform-busC*
alias: of:N*T*Cdemo,platform-bus

Conclusion¶

of_match_table
    ↓
Kernel runtime matching

MODULE_DEVICE_TABLE(of)
    ↓
Module alias generation

Result¶

Verified:

Real Raspberry Pi 5 Platform Bus hierarchy
Platform driver registration
Name-based matching
Device Tree compatible matching
Match failure behavior
Registration order independence
Platform Bus sysfs hierarchy
Device Tree integration
MODULE_DEVICE_TABLE(of) behavior