关于usb-skeleton.c中skel_read()函数重试逻辑的疑问与优化探讨

Great catch on this inconsistency in the usb-skeleton.c's skel_read() function—this is exactly the kind of detail that helps you dig deeper into Linux USB driver design and POSIX I/O semantics. Let’s break down your questions and the proposed modification step by step:

1. Is the current behavior intentional design or a teaching simplification? #

The usb-skeleton.c example is explicitly intended as a minimal teaching framework for USB driver development, not a production-ready implementation. The current "inconsistent" behavior is a deliberate simplification to keep the code focused on core concepts: URB submission, wait queues, and basic user-space data transfer.

By returning immediately after submitting a new URB when data is insufficient, the example avoids complicating the code with retry logic, which would add extra state tracking and edge-case handling. The goal is to show how to get data from a USB device to user space, not to fully implement POSIX-compliant blocking I/O semantics.

2. Are there any unforeseen downsides to adding goto retry? #

Your proposed modification makes the blocking I/O behavior more consistent and POSIX-compliant, but there are a few edge cases to consider:

• Infinite loops with persistent short packets: If the USB device consistently returns short packets (e.g., a sensor that sends small chunks of data periodically), your modified code would keep retrying indefinitely in blocking mode until the full count is satisfied. This might not be desirable for devices that never produce large enough data batches. To fix this, you’d need to add logic to detect when a short packet indicates the end of a data transfer (e.g., checking if the URB’s actual_length is less than the requested length and treating it as an implicit EOF).
• URB state management: You must ensure that usb_do_read_io() correctly sets dev->ongoing_read = 1 when submitting the URB, and that the URB completion callback sets dev->ongoing_read = 0 and wakes the wait queue. If this state isn’t properly managed, the wait_event_interruptible() call could hang indefinitely.
• O_NONBLOCK behavior: With your modification, in O_NONBLOCK mode, if the initial available data is insufficient, the code will submit a new URB and then hit the retry label again. Since dev->ongoing_read is now 1, it will immediately return -EAGAIN—this is actually correct behavior, as non-blocking I/O shouldn’t wait for pending transfers to complete.

Overall, the goto retry change is safe for a learning exercise, but production code would need additional safeguards for the edge cases above.

3. Do USB bulk transfer characteristics require short reads? #

No, USB bulk transfers do not enforce short reads. Short packets are a normal part of USB communication—they can indicate that the device has no more data to send, or that the transfer was truncated due to hardware limitations. However, POSIX semantics for blocking read() calls state that the call should return as much data as possible, and only return less than the requested count if an error occurs, a signal is received, or the end of the file/stream is reached.

In the context of USB drivers, it’s up to you to decide whether short packets should trigger a retry or be treated as the end of a transfer. For example:

• For devices that stream continuous data (e.g., a bulk-mode data logger), retrying to fill the user’s request makes sense.
• For devices that send discrete chunks of data (e.g., a USB keyboard sending keystrokes), returning the short packet immediately is appropriate.

The usb-skeleton example takes the latter approach for simplicity, but your modification aligns it more closely with standard blocking I/O expectations.

Final Thoughts on Your Proposed Modification #

Your fix is a great improvement for making the driver’s behavior more consistent and intuitive, especially for anyone learning how blocking I/O should work in Linux drivers. If you’re extending the skeleton for a specific device, you’ll just need to add logic to handle cases where short packets should terminate the read instead of triggering a retry.

内容的提问来源于stack exchange，提问作者const