reject format changes while buffers are allocated

[grawis]

Summary

Reject VIDIOC_S_FMT with EBUSY while videobuf2 buffers are allocated.

Previously, VIDIOC_S_FMT updated dev->output_format even when existing
buffers had been allocated using the previous format's sizeimage. This
could leave the current format and buffer sizes inconsistent, causing a
later VIDIOC_QBUF call to fail.

The change checks vb2_is_busy() before applying the new format. Applications
must release existing buffers with VIDIOC_REQBUFS count=0 before changing
the format.

Test results

Before:

Initial format: 640x480 YUYV, bytesperline=1280, sizeimage=614400
Allocated buffers: 3
Buffer 0 length: 614400
S_FMT after REQBUFS: succeeded
Current format: 640x480 RGB3, bytesperline=1920, sizeimage=921600
QBUF after S_FMT: failed: Invalid argument

After:

 Initial format: 640x480 YUYV, bytesperline=1280, sizeimage=614400
 Allocated buffers: 3
 Buffer 0 length: 614400
 S_FMT after REQBUFS: failed: Device or resource busy
 Current format: 640x480 YUYV, bytesperline=1280, sizeimage=614400
 QBUF after S_FMT: succeeded

V4L2 compliance:

Total: 50, Succeeded: 50, Failed: 0, Warnings: 0

Fixes #52

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Summary by cubic

Reject VIDIOC_S_FMT while videobuf2 buffers are allocated, returning -EBUSY. This keeps the current format consistent with allocated buffer sizes and prevents VIDIOC_QBUF errors.

Applications must release buffers with VIDIOC_REQBUFS count=0 before changing the format.

^{Written for commit 9e4afcd. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}