Day74 - MMIO Access and Register Operations¶

Date: 2026-06-10

Summary¶

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

Starting from the Device Tree reg property, I followed the complete resource path:

Device Tree
    reg
      ↓
platform_get_resource()
      ↓
struct resource
      ↓
MMIO resource
      ↓
__iomem pointer
      ↓
readl() / writel()

I also investigated how Device Tree bus addresses are translated into CPU physical addresses through the ranges property and examined real MMIO resources on Raspberry Pi 5.

In addition, I built a simulated register block to understand register offsets, bit operations, read-modify-write patterns, and Linux register field helpers.

Topics Learned¶

Memory-Mapped I/O (MMIO)
Device Tree reg property
platform_get_resource()
IORESOURCE_MEM
struct resource
Device Tree ranges translation
/proc/iomem
__iomem
ioremap()
devm_ioremap_resource()
Register offset model
readl() / writel()
BIT()
Read-Modify-Write (RMW)
GENMASK()
FIELD_PREP()
FIELD_GET()

Experiments¶

Lab1 - MMIO Resource Inspection¶

Created a platform driver that retrieves an MMIO resource from Device Tree using:

platform_get_resource(pdev, IORESOURCE_MEM, 0);

Verified:

reg
    ↓
struct resource
    ↓
start / end / size

Lab2 - devm_ioremap_resource()¶

Attempted to map a fake MMIO resource using:

devm_ioremap_resource()

Observed:

-EBUSY

Learned that the helper performs both resource ownership checks and MMIO mapping.

Lab3 - Real MMIO Resource Investigation¶

Inspected Raspberry Pi 5 Device Tree nodes:

gpio@7d508500
gpio@7d517c00
pinctrl@7d504100
pinctrl@7d510700

Verified corresponding entries in:

/proc/iomem

Lab4 - Device Tree Address Translation¶

Examined:

ranges
#address-cells
#size-cells

Verified that Device Tree bus addresses are translated into CPU physical addresses before becoming kernel resources.

Lab5 - __iomem¶

Verified:

sizeof(void *)
sizeof(void __iomem *)

Learned that __iomem is an address-space annotation used by kernel tooling rather than a different pointer type.

Lab6 - Register Offset Model¶

Built a fake register map and verified:

Base Address
    +
Register Offset
    =
Register

Lab7 - Simulated readl() / writel()¶

Implemented helper functions to simulate register access:

demo_readl()
demo_writel()

Verified register reads and writes using offset-based addressing.

Lab8 - Bit Operations¶

Practiced:

BIT()

and read-modify-write patterns:

reg |= BIT(...)
reg &= ~BIT(...)

Lab9 - Register Fields¶

Practiced:

GENMASK()
FIELD_PREP()
FIELD_GET()

for encoding and decoding register fields.

Key Takeaways¶

Device Tree reg describes bus addresses, not necessarily CPU physical addresses.
MMIO resources are represented by struct resource.
devm_ioremap_resource() performs resource ownership checks in addition to mapping.
Linux drivers access registers using:

readl()
writel()

rather than direct pointer dereferencing.

Register access is fundamentally:

MMIO Base
    +
Register Offset

Most Linux drivers rely heavily on:
BIT()
Read-Modify-Write
GENMASK()
FIELD_PREP()
FIELD_GET()

Next Step¶

Day75 - GPIO Controller Architecture

Planned topics:

GPIO controller structure
GPIO register blocks
Direction and data registers
GPIO lines and numbering
Relationship between GPIO subsystem and MMIO registers