This post will walk you through how libblock works in RTEMS!

What is libblock?

libblock is RTEMS’s block device management library. Think of it as the middleman between your filesystem (like FatFS) and the actual storage hardware (SD cards, flash memory, etc.). It provides a consistent interface for reading and writing blocks of data, regardless of the underlying storage technology.

The key insight is that libblock operates on blocks rather than individual bytes. This is much more efficient for storage devices, which typically work best when reading/writing larger chunks of data at once.

The Big Picture: How It All Fits Together

Here’s how the pieces connect:

• Your Application wants to read/write files
• FatFS (or another filesystem) manages file organization
• libblock handles the actual block-level I/O operations
• Hardware Driver talks to the physical storage device

Key libblock Concepts

Let me break down the essential libblock functions I encountered while building the FatFS integration:

1. Initialization: rtems_disk_io_initialize()

This is your starting point. It sets up the entire block device subsystem. You only need to call this once in your application, and it prepares RTEMS to manage block devices.

2. Getting Device Information: rtems_disk_fd_get_disk_device()

This function is fascinating! You give it a file descriptor (from opening something like /dev/sda1), and it returns a rtems_disk_device structure that contains all the metadata about your storage device:

• Block size (how big each block is)
• Total size (how many blocks)
• Whether it’s read-only
• And more!

This is like getting a “spec sheet” for your storage device.

3. The Block Buffer Manager: rtems_bdbuf_* Functions

This is where the magic happens. The block buffer manager (bdbuf) is libblock’s smart caching system:

Reading Blocks: rtems_bdbuf_read()

Instead of directly reading from the hardware every time, libblock maintains a cache of recently accessed blocks. When you request a block:

• If it’s already in cache → instant access!
• If not → libblock fetches it from the hardware and caches it

Writing Blocks: rtems_bdbuf_get() and rtems_bdbuf_release_modified()

Writing is a two-step process:

1. Get a buffer (either from cache or allocate a new one)
2. Release as modified (tells libblock this buffer needs to be written back)

The beauty here is that libblock can optimize when the actual hardware write happens!

Releasing Blocks: rtems_bdbuf_release()

When you’re done with a block buffer, you must release it. This tells libblock that other parts of your system can use this buffer.

4. Synchronization: rtems_bdbuf_syncdev()

This forces all pending writes for a device to actually hit the hardware. It’s like the “flush” operation – crucial for ensuring data integrity.

Conclusion

libblock initially seemed intimidating, but it’s actually quite elegant once you understand the patterns. The key insights are:

1. Everything is about blocks, not bytes
2. Caching is automatic and transparent
3. You must manage buffer lifecycles carefully
4. The abstraction layer makes your code more portable

If you’re working with storage in RTEMS, spending time to understand libblock will pay dividends. It’s not just about making things work – it’s about making them work efficiently and reliably.