Day74 - MMIO Register Access¶

Objective¶

Understand how Linux platform drivers access hardware registers through Memory-Mapped I/O (MMIO).

This lab follows the complete path from Device Tree resources to register access patterns commonly used in Linux drivers.

Lab Environment¶

Platform:

Raspberry Pi 5
Kernel 6.12.x
Debian 13

Driver:

demo_mmio_resource

Lab1 - MMIO Resource Inspection¶

Device Tree¶

demo_mmio: demo-mmio@10000000 {
    compatible = "demo,mmio-resource";
    reg = <0x0 0x10000000 0x1000>;
};

Resource Retrieval¶

res = platform_get_resource(
        pdev,
        IORESOURCE_MEM,
        0);

Result¶

start=0x10000000
end=0x10000fff
size=0x1000
flags=0x200

Verification¶

Device Tree reg
    ↓
struct resource

Lab2 - devm_ioremap_resource()¶

Test¶

base = devm_ioremap_resource(
            dev,
            res);

Result¶

failed to ioremap MMIO resource: -EBUSY

Observation¶

devm_ioremap_resource()

=
request_mem_region()
+
ioremap()
+
devm cleanup

The helper performs ownership checks before mapping.

Lab3 - Real MMIO Resource Inspection¶

Device Tree Nodes¶

gpio@7d508500
gpio@7d517c00

pinctrl@7d504100
pinctrl@7d510700

Device Tree Reg¶

gpio@7d508500
base = 0x7d508500
size = 0x40

/proc/iomem¶

107d508500-107d50853f

Observation¶

Device Tree address
!=
CPU physical address

Address translation occurs through Device Tree bus mappings.

Lab4 - Device Tree ranges Translation¶

Inspection¶

hexdump -Cv /proc/device-tree/soc@107c000000/ranges

Result¶

child address = 0x00000000

parent physical address
= 0x0000001000000000

size
= 0x80000000

Translation¶

CPU physical address

=
0x1000000000
+
bus address

Example:

7d508500
    ↓
107d508500

Verification¶

reg
    ↓
ranges
    ↓
resource
    ↓
/proc/iomem

Lab5 - __iomem¶

Test¶

sizeof(void *)
sizeof(void __iomem *)

Result¶

8
8

Observation¶

__iomem

is an address-space annotation used by kernel tooling.

It does not change pointer size.

Lab6 - Register Offset Model¶

Register Layout¶

#define DEMO_REG_CTRL        0x00
#define DEMO_REG_STATUS      0x04
#define DEMO_REG_DATA        0x08
#define DEMO_REG_IRQ_STATUS  0x0c

Observation¶

Base Address
+
Offset
=
Register

Lab7 - Simulated Register Access¶

Register Block¶

struct demo_regs {
    u32 ctrl;
    u32 status;
    u32 data;
    u32 irq_status;
};

Helper Functions¶

static u32 demo_readl(void *addr)
{
    return *(u32 *)addr;
}

static void demo_writel(u32 val, void *addr)
{
    *(u32 *)addr = val;
}

Result¶

STATUS=0x22
CTRL=0x12345678

Verification¶

Register Access

=
Base
+
Offset

Lab8 - Bit Operations¶

Definitions¶

#define CTRL_ENABLE    BIT(0)
#define CTRL_IRQ_EN    BIT(1)
#define CTRL_RESET     BIT(2)
#define CTRL_DMA_EN    BIT(3)

Initial Value¶

CTRL = 0x0A

Binary:

Enable Bit¶

ctrl |= CTRL_ENABLE;

Result:

0x0B

Disable Bit¶

ctrl &= ~CTRL_DMA_EN;

Result:

0x03

Verification¶

Read
Modify
Write

pattern.

Lab9 - Register Field Operations¶

Field Definitions¶

#define CTRL_MODE_MASK    GENMASK(1, 0)
#define CTRL_DIV_MASK     GENMASK(4, 2)
#define CTRL_DRV_MASK     GENMASK(7, 5)

Write Fields¶

reg |= FIELD_PREP(CTRL_MODE_MASK, 2);
reg |= FIELD_PREP(CTRL_DIV_MASK, 5);
reg |= FIELD_PREP(CTRL_DRV_MASK, 3);

Read Fields¶

FIELD_GET(CTRL_MODE_MASK, reg);
FIELD_GET(CTRL_DIV_MASK, reg);
FIELD_GET(CTRL_DRV_MASK, reg);

Result¶

MODE=2
DIV=5
DRV=3

Verification¶

GENMASK()
    ↓
FIELD_PREP()
    ↓
FIELD_GET()

Key Learnings¶

Device Tree reg
        ↓
platform_get_resource()
        ↓
struct resource
        ↓
MMIO Resource
        ↓
__iomem
        ↓
Register Offset
        ↓
readl()/writel()
        ↓
BIT()
        ↓
Read-Modify-Write
        ↓
GENMASK()/FIELD_PREP()/FIELD_GET()

This workflow forms the foundation of most Linux platform drivers.