Return queued buffers on stream start failure

[grawis]

If start_streaming() fails after buffers have been queued to the driver, vb2 requires the driver to return all owned buffers.

Previously, vcam_start_streaming() returned immediately when kthread_create() failed, leaving buffers in the active queue. This can violate the vb2 start_streaming() failure contract.

Drain the active queue under the driver lock and return each buffer with VB2_BUF_STATE_QUEUED before propagating the kthread_create() error.

Build-tested with Linux 6.17.0-23-generic headers.

---

Summary by cubic

Return queued buffers when start_streaming() fails to create the submitter thread, complying with the vb2 failure contract. Previously we returned -ECANCELED and left buffers in the active queue; now we drain the queue, complete buffers as VB2_BUF_STATE_QUEUED, and propagate the actual kthread_create() error.

• Introduces vcam_return_queued_buffers() to drain vcam_out_vidq.active under out_q_slock.
• Uses IS_ERR()/PTR_ERR() for kthread_create(), sets sub_thr_id to NULL, and returns the real error code.
• Affects only the failure path before wake_up_process(); normal start behavior is unchanged.
• Callers may now receive a driver error (e.g., -ENOMEM, -EINTR) instead of -ECANCELED.

^{Written for commit 081d978. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}