Claude review: drm: zynqmp_dp: Fix uninitialized variable in debugfs()

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Analysis of the bug:**

`simple_write_to_buffer()` writes to `to + pos` where `pos = *ppos`. If `*ppos` is non-zero, the beginning of the stack-allocated buffer is never initialized. This affects both functions:

1. **`zynqmp_dp_pattern_write()`** — the uninitialized prefix bytes in `buf` are followed by `buf[ret] = '\0'` and then `sysfs_match_string()`. The match would operate on garbage bytes at the start of the string.

2. **`zynqmp_dp_custom_write()`** — `memcpy(dp->test.custom, buf, ret)` copies from the beginning of `buf`, which includes uninitialized bytes when `*ppos` was non-zero. This leaks stack contents into `dp->test.custom`, which is then written to hardware via `zynqmp_dp_set_test_pattern()`.

**The fix:**

```c
if (*ppos != 0)
    return -EINVAL;
```

Added at the top of both write functions. This is the standard kernel pattern for debugfs write handlers that expect a single complete write. Both `file_operations` structs use `noop_llseek`, confirming these are not designed for seekable I/O.

**Review comments:**

- **Correct and complete.** Both affected write handlers are fixed.
- **Appropriate error code.** `-EINVAL` is the right choice for rejecting an invalid ppos.
- **Behavioral note:** After a successful write, `simple_write_to_buffer()` advances `*ppos`, so a second `write()` on the same fd will now return `-EINVAL` instead of the previous behavior (returning `0` when `*ppos >= available`). This is actually an improvement — `-EINVAL` is more informative than a silent zero-length write.
- **Fixes tag** correctly references the commit that introduced the debugfs interface.

**No issues found.** 

Reviewed-by worthy as-is.

---
Generated by Claude Code Patch Reviewer