Understanding SPI Driver Patterns with the spi_stm32 Driver
In Linux, many SPI-based drivers use two well-known design patterns: the Driver Data Pattern and the Controller Pattern. In this article, we’ll explore these patterns through the lens of the spi_stm32 driver, illustrating how memory is allocated in one contiguous block, how pointer arithmetic retrieves private data, and why this design is efficient and maintainable.
Memory Allocation and Pointer Arithmetic
When a driver allocates a new SPI controller, it often reserves extra space for driver-specific data. For instance, in the spi_stm32 driver, you might see a call like this:
controller = spi_alloc_master(&pdev->dev, sizeof(*priv));
Here, spi_alloc_master reserves a contiguous block of memory that includes:
How Pointer Arithmetic Works
The helper function spi_controller_get_devdata calculates the offset to the driver’s private data. Pseudo-code might look like this:
void *spi_controller_get_devdata(struct spi_controller *ctlr) {
// Calculate offset to private data:
// sizeof(struct spi_controller) + sizeof(struct device)
return (char *)ctlr + sizeof(struct spi_controller) + sizeof(struct device);
}
After this calculation, casting the returned pointer gives you the private data:
struct spi_stm32_data *priv = (struct spi_stm32_data *)spi_controller_get_devdata(controller);
Visual Memory Layout
Imagine the memory block looks like this:
+--------------------------+
| spi_controller structure | <-- controller pointer
+--------------------------+
| device structure | <-- part of spi_controller
+--------------------------+
| spi_stm32_data structure | <-- private data pointer (priv)
+--------------------------+
For example, if:
Then:
The use of pointer arithmetic guarantees that each segment is correctly aligned and accessible without multiple memory allocations.
The Driver Data Pattern
The Driver Data Pattern encapsulates all device-specific information within a dedicated structure. In the context of spi_stm32, this might be defined as:
struct spi_stm32_data {
struct spi_controller *controller;
// Additional fields for spi_stm32 configuration, buffers, etc.
int mode;
u32 max_speed_hz;
// ... other device-specific settings
};
How It’s Used in the Probe Function
During the probe (initialization) of the spi_stm32 driver, the allocated memory block is split between controller structures and the spi_stm32 private data. A typical probe function might look like:
static int spi_stm32_probe(struct platform_device *pdev)
{
struct spi_stm32_data *priv;
struct spi_controller *controller;
// Allocate memory for the controller plus private data
controller = spi_alloc_master(&pdev->dev, sizeof(*priv));
if (!controller)
return -ENOMEM;
// Retrieve pointer to private data using pointer arithmetic
priv = spi_controller_get_devdata(controller);
priv->controller = controller;
priv->mode = SPI_MODE_0;
priv->max_speed_hz = 500000;
// Associate the private data with the platform device
platform_set_drvdata(pdev, priv);
// Additional configuration and registration steps...
return 0;
}
This pattern ensures:
The Controller Pattern
The Controller Pattern is used to manage the SPI hardware controller itself. The spi_controller structure holds information about the SPI bus, chip select lines, and other hardware parameters:
struct spi_controller {
struct device dev; // Core device information
int bus_num; // SPI bus number
u16 num_chipselect; // Number of chip selects available
// ... other fields for the SPI hardware controller
};
Configuring the Controller
After allocation, the driver needs to initialize the controller’s parameters before registration:
static int spi_stm32_configure(struct spi_stm32_data *priv)
{
struct spi_controller *controller = priv->controller;
controller->bus_num = 0; // Example bus number
controller->num_chipselect = 1; // Typically, spi_stm32 works with one chip select
// Register the controller with the SPI subsystem
return spi_register_controller(controller);
}
This approach standardizes how hardware resources are managed and ensures that the lifecycle of the controller is properly maintained.
Integration of Both Patterns in spi_stm32
In spi_stm32, both the Driver Data and Controller patterns work hand in hand. During initialization:
The result is a clean, efficient, and easily extendable driver implementation.
Benefits and Best Practices
Memory Management
Encapsulation and Maintainability
Error Handling and Extensibility
Conclusion
By using pointer arithmetic to partition a single memory allocation, the spi_stm32 driver effectively employs both the Driver Data Pattern and the Controller Pattern. This design is not only memory efficient but also enhances code clarity and maintainability—principles that are critical in Linux kernel development.
Understanding these patterns through a common driver like spi_stm32 can help developers design robust and scalable drivers for various hardware components.
In Linux, many SPI-based drivers use two well-known design patterns: the Driver Data Pattern and the Controller Pattern. In this article, we’ll explore these patterns through the lens of the spi_stm32 driver, illustrating how memory is allocated in one contiguous block, how pointer arithmetic retrieves private data, and why this design is efficient and maintainable.
Memory Allocation and Pointer Arithmetic
When a driver allocates a new SPI controller, it often reserves extra space for driver-specific data. For instance, in the spi_stm32 driver, you might see a call like this:
controller = spi_alloc_master(&pdev->dev, sizeof(*priv));
Here, spi_alloc_master reserves a contiguous block of memory that includes:
How Pointer Arithmetic Works
The helper function spi_controller_get_devdata calculates the offset to the driver’s private data. Pseudo-code might look like this:
void *spi_controller_get_devdata(struct spi_controller *ctlr) {
// Calculate offset to private data:
// sizeof(struct spi_controller) + sizeof(struct device)
return (char *)ctlr + sizeof(struct spi_controller) + sizeof(struct device);
}
After this calculation, casting the returned pointer gives you the private data:
struct spi_stm32_data *priv = (struct spi_stm32_data *)spi_controller_get_devdata(controller);
Visual Memory Layout
Imagine the memory block looks like this:
+--------------------------+
| spi_controller structure | <-- controller pointer
+--------------------------+
| device structure | <-- part of spi_controller
+--------------------------+
| spi_stm32_data structure | <-- private data pointer (priv)
+--------------------------+
For example, if:
Then:
The use of pointer arithmetic guarantees that each segment is correctly aligned and accessible without multiple memory allocations.
The Driver Data Pattern
The Driver Data Pattern encapsulates all device-specific information within a dedicated structure. In the context of spi_stm32, this might be defined as:
struct spi_stm32_data {
struct spi_controller *controller;
// Additional fields for spi_stm32 configuration, buffers, etc.
int mode;
u32 max_speed_hz;
// ... other device-specific settings
};
How It’s Used in the Probe Function
During the probe (initialization) of the spi_stm32 driver, the allocated memory block is split between controller structures and the spi_stm32 private data. A typical probe function might look like:
static int spi_stm32_probe(struct platform_device *pdev)
{
struct spi_stm32_data *priv;
struct spi_controller *controller;
// Allocate memory for the controller plus private data
controller = spi_alloc_master(&pdev->dev, sizeof(*priv));
if (!controller)
return -ENOMEM;
// Retrieve pointer to private data using pointer arithmetic
priv = spi_controller_get_devdata(controller);
priv->controller = controller;
priv->mode = SPI_MODE_0;
priv->max_speed_hz = 500000;
// Associate the private data with the platform device
platform_set_drvdata(pdev, priv);
// Additional configuration and registration steps...
return 0;
}
This pattern ensures:
The Controller Pattern
The Controller Pattern is used to manage the SPI hardware controller itself. The spi_controller structure holds information about the SPI bus, chip select lines, and other hardware parameters:
struct spi_controller {
struct device dev; // Core device information
int bus_num; // SPI bus number
u16 num_chipselect; // Number of chip selects available
// ... other fields for the SPI hardware controller
};
Configuring the Controller
After allocation, the driver needs to initialize the controller’s parameters before registration:
static int spi_stm32_configure(struct spi_stm32_data *priv)
{
struct spi_controller *controller = priv->controller;
controller->bus_num = 0; // Example bus number
controller->num_chipselect = 1; // Typically, spi_stm32 works with one chip select
// Register the controller with the SPI subsystem
return spi_register_controller(controller);
}
This approach standardizes how hardware resources are managed and ensures that the lifecycle of the controller is properly maintained.
Integration of Both Patterns in spi_stm32
In spi_stm32, both the Driver Data and Controller patterns work hand in hand. During initialization:
The result is a clean, efficient, and easily extendable driver implementation.
Benefits and Best Practices
Memory Management
Encapsulation and Maintainability
Error Handling and Extensibility
Conclusion
By using pointer arithmetic to partition a single memory allocation, the spi_stm32 driver effectively employs both the Driver Data Pattern and the Controller Pattern. This design is not only memory efficient but also enhances code clarity and maintainability—principles that are critical in Linux kernel development.
Understanding these patterns through a common driver like spi_stm32 can help developers design robust and scalable drivers for various hardware components.