Day 73 - Platform Driver Development¶

Today's Goal¶

Understand how a platform driver acquires hardware resources, stores private driver state, and manages resource lifetime using devm APIs.

Topics:

A platform driver does not access hardware addresses directly.

Hardware resources are described by Device Tree and converted into platform resources during device creation.

The driver acquires these resources through:

Common resource types:

Each platform device instance owns its own private driver state.

Instead of using global variables, Linux drivers allocate instance-specific data:

struct demo_priv {
    struct resource *mem_res;
    int irq;
};

The private data is attached to the platform device:

platform_set_drvdata()

and retrieved later through:

platform_get_drvdata()

Interrupt information is obtained from Device Tree.

The platform driver retrieves the mapped Linux IRQ number using:

platform_get_irq()

The returned IRQ number may differ from the interrupt specifier defined in Device Tree because irqdomain mapping occurs inside the kernel.

The devm framework simplifies resource lifetime management.

Examples:

Resources allocated through devm APIs are automatically released when the device is removed.

A GPIO interrupt may generate multiple triggers due to signal bouncing.

A common Linux driver solution is:

IRQ
 ↓
mod_delayed_work()
 ↓
Debounce timeout
 ↓
Worker execution

Using mod_delayed_work() automatically refreshes the debounce timeout when additional interrupts arrive.

Implemented a platform driver using a Device Tree Overlay.

Verified:

Implemented:

Device Tree
    ↓
platform_device
    ↓
resource[]
    ↓
platform_get_resource()

GPIO23
    ↓
rp1_gpio irqdomain
    ↓
platform_get_irq()
    ↓
Linux IRQ number

probe()
    ↓
devm_kzalloc()
    ↓
platform_set_drvdata()
    ↓
platform_get_drvdata()

Successfully verified:

This completes the fundamental platform driver initialization flow.

Day 74: