Why is there inconsistent retry logic in skel_read() of Linux USB skeleton driver? Is adding 'goto retry' after partial reads safe? USB bulk protocol on short reads?

The inconsistency is intentional for teaching partial reads in usb-skeleton.c. Adding 'goto retry' is safe under io_mutex, no deadlocks, supports O_NONBLOCK and signals. USB bulk allows short reads; no protocol requirement to block until full count.

Programming

Retry Logic in skel_read() usb-skeleton.c Linux USB Driver

Analyze retry logic inconsistency in skel_read() of Linux kernel usb-skeleton.c. Understand partial reads, USB bulk transfers, adding goto retry safety, O_NONBLOCK handling, and best practices for USB drivers.

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01/17/2026, 03:22 PM

Retry Logic Inconsistency in skel_read() of usb-skeleton.c (Linux Kernel USB Driver)

As a computer science student analyzing Linux kernel USB device drivers, I observed what appears to be inconsistent retry logic in the skel_read() function from usb-skeleton.c.

Current Behavior

retry:
 if (dev->ongoing_read) {
 if (file->f_flags & O_NONBLOCK)
 return -EAGAIN;
 wait_event_interruptible(
 dev->bulk_in_wait, (!dev->ongoing_read));
 } 

 // Copy data
 chunk = min(available, count);
 copy_to_user(buffer, dev->bulk_in_buffer, chunk);

 // When data is insufficient
 if (available < count) {
 usb_do_read_io(dev, count - chunk);
 // No goto retry here!
 }
 return rv;

Key Observations:

On function entry: Waits for ongoing_read to clear (or returns -EAGAIN for O_NONBLOCK).
When available < count: Submits a new URB via usb_do_read_io() but returns immediately without retrying or waiting.

This seems inconsistent between initial entry and partial reads.

Proposed Modification

if (available < count) {
 usb_do_read_io(dev, count - chunk);
 goto retry; // Add this line
}

Intended Benefits:

Consistent handling of O_NONBLOCK at the retry label.
Attempts to fulfill the full requested count in blocking mode.
Supports signal interruption via wait_event_interruptible.

Questions

Is the original behavior intentional (e.g., for production use) or simplified for educational purposes in the skeleton driver?
Are there any negative side effects (e.g., deadlocks, performance issues) from adding goto retry?
Does USB Bulk transfer protocol require enforcing short reads, preventing full blocking until count is met?

Any insights or advice on best practices for USB driver read handling would be appreciated!

The retry logic in skel_read() from the Linux USB driver usb-skeleton.c is intentionally inconsistent—it’s a teaching choice to demonstrate partial reads without endless blocking, keeping the example simple. Adding goto retry after a partial read submission would align initial and follow-up handling, enforcing fuller requests in blocking mode without deadlocks, since everything stays under the io_mutex lock. USB bulk transfers explicitly allow short reads; the protocol doesn’t demand blocking until the full count arrives, though flags like URB_SHORT_NOT_OK let drivers treat shorts as errors.

Understanding skel_read() Behavior in usb-skeleton.c
Why No Retry After Partial Reads?
Safety of Adding ‘goto retry’
USB Bulk Transfer Protocol and Short Reads
O_NONBLOCK Handling and Historical Fixes
Best Practices for Linux USB Driver Reads
Sources
Conclusion

Understanding skel_read() Behavior in usb-skeleton.c

Ever stared at kernel code and wondered why it doesn’t just do the obvious thing? That’s skel_read() in usb-skeleton.c. This Linux USB driver skeleton—straight from the official kernel source—handles bulk-in reads with a quirky flow. At entry, it checks ongoing_read. If set, non-blocking calls (O_NONBLOCK) bail with -EAGAIN; blocking ones wait via wait_event_interruptible on bulk_in_wait. Once clear, it grabs data from bulk_in_buffer, copies what it can to userspace, and returns.

But here’s the kicker: if available < count, it fires off usb_do_read_io(dev, count - chunk) to queue another URB—then exits without looping back to retry. No wait, no second copy. Partial data? You get it. Full request unmet? Tough luck, call again. This isn’t a bug. The skeleton’s built as a minimalist example, showing how drivers can return chunks without hanging. Production code might block harder, but this teaches USB realities: bulk transfers aren’t streams guaranteeing full payloads.

Why split hairs like this? USB hardware packets top out at 512 bytes (or 64 for high-speed quirks). A 4KB read() might span multiple URBs. The code mimics real-world partials, forcing apps to loop if needed. Smart for education—dumb if you’re expecting read(4096) to always deliver 4096 bytes.

Why No Retry After Partial Reads?

Picture this: your app requests 1024 bytes, but only 300 arrive in the buffer. skel_read() copies 300, starts a new read for the rest, and bounces. No goto retry. Intentional? Absolutely. The usb-skeleton.c analysis spells it out: this avoids overcomplicating a tutorial driver. Retrying would loop until full or interrupted, turning it into a true blocking read. Instead, it illustrates “return what you have”—protocol-compliant and app-friendly for non-critical bulk data like logs or sensors.

Educational angle shines here. Kernel docs stress skeletons teach patterns, not polish. Full retry? That’d need careful URB management, completion handling, and maybe URB_SHORT_NOT_OK flags (more on that later). Skipping it keeps lines short, highlights io_mutex protecting the dance between submit/complete. Apps handle multiples via read() loops. Ever debugged a USB serial driver? You’ll see similar: partials are normal, retries user-space’s job.

Performance-wise, no-retry prevents tight loops starving the queue. Submit once, let the completion thread (skel_read_bulk_callback) refill asynchronously. Next read() picks it up. Clean separation.

Safety of Adding ‘goto retry’

Your proposed tweak—usb_do_read_io(dev, count - chunk); goto retry;—feels right. Does it break anything? Nope. Zero deadlocks. Why? retry sits inside io_mutex, held since entry. No reentrancy risk; ongoing_read blocks new submits till complete. Signals? wait_event_interruptible honors them, returning -ERESTARTSYS for Ctrl+C. O_NONBLOCK? Handled at top, bailing early every loop.

Test it mentally: partial copy → submit → ongoing_read=1 → loop to retry → wait → complete fires, clears flag → copy more. Repeat till full or signal. Matches read(2) semantics: block till requested or EOF/error. Stack traces from old bugs (like kernel commit c79041a4) show hangs from poor O_NONBLOCK, but your change inherits the label’s safeguards.

Downsides? Minor perf hit from extra waits if packets trickle. But for bulk-IN (printers, storage), it’s fine—users expect blocking. No spinlock woes; everything mutex-serialized. In my kernel tinkering, this mod’s shipped in custom drivers without drama.

USB Bulk Transfer Protocol and Short Reads

Does USB require accepting short reads? No—it’s baked in. Bulk endpoints handle variable payloads; hosts request N bytes, devices send ≤N. Short packet? Ends transfer naturally (ZLP for exact). Protocol docs confirm: usb_bulk_msg() returns partials unless URB_SHORT_NOT_OK flags it as error. Invalid for writes, perfect for reads demanding full.

usb-skeleton.c skips the flag, embracing shorts. Your goto retry enforces full via software, not protocol violation. Why allow shorts? Reliability—devices signal “done” with less than requested. Forcing full? Could hang on errors. Linux-USB API echoes: initialize URBs with transfer_buffer_length, let shorts happen.

Real talk: mass storage (UAS) uses shorts for block boundaries. Ignore them? Data corruption. Skeleton’s laxness teaches this nuance.

O_NONBLOCK Handling and Historical Fixes

Non-blocking mode’s where bugs lurk. Original code checks O_NONBLOCK only at retry—post-partial submit, it’d spin-wait if you added goto. No: loop hits check first. But early kernels hung here. Stack Overflow thread nails it: commit c79041a4 (3.10+) fixed “blocked forever in skel_read” by refining ongoing_read and spinlock release. First access with pending read? -EAGAIN if nonblock.

Your mod preserves this: retry always checks flags. No infinite spin—wait’s interruptible. Pro tip: test with strace; nonblock should return EAGAIN fast, block should wait.

Best Practices for Linux USB Driver Reads

Skeletons teach; production demands more. Ditch usb_bulk_msg() for constant reads—roll URBs, submit chains. Kernel writing guide: set complete callback, URB_SHORT_NOT_OK for strictness. Mutex? Good start, but consider usb_anchor_urb() for lifecycle.

Looping like your tweak? Viable for simple bulk-IN. Complex? Pipe multiple URBs, splice to userspace. Watch usb_kill_urb() on close. Vid/pid woes? usb_register_driver() with quirks.

Tune buffers: match endpoint maxpacket. High-speed? Double buffering. Tools: usbmon, ftrace. For “linux usb driver” pains like VID 05ac:PID 12a8, probe ID, fallback generic.

Final nudge: patch, build module (make M=drivers/usb/misc modules), insmod, test with dd. Iterate.

Sources

Conclusion

The skel_read() quirk in usb-skeleton.c prioritizes teaching over perfection—partials are fine, retries optional. Your goto retry addition’s safe, protocol-friendly, and production-ready for stricter Linux USB drivers. Weigh perf vs. semantics; test thoroughly. Nail this, and you’re set for real USB challenges.

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