Claude review: staging: fbtft: replace -1 with proper error codes

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

Patch Review

**Positive aspects:**
- The commit message is clear and enumerates each error code choice with its rationale.
- The Signed-off-by is present.
- The diff is minimal and focused — no unrelated changes.

**Error code review:**

1. **`fb_ra8875.c:write_spi()` — `-1` → `-ENODEV`**: Correct. The SPI device being NULL means the device is not available.

2. **`fb_ra8875.c:init_display()` — `-1` → `-EINVAL`**: Correct. An unsupported display resolution is an invalid parameter/configuration.

3. **`fbtft-io.c:fbtft_write_spi()` — `-1` → `-ENODEV`**: Correct. Same SPI NULL case as above, consistent choice.

4. **`fbtft-io.c:fbtft_write_gpio16_wr_latched()` — `-1` → `-EOPNOTSUPP`**: Correct. Function is a stub, `-EOPNOTSUPP` is the right code.

5. **`fbtft-bus.c:fbtft_write_vmem8_bus8()` — `-1` → `-EOPNOTSUPP`**: Correct. Same unimplemented stub pattern.

6. **`fbtft-bus.c:fbtft_write_vmem16_bus9()` — `-1` → `-EINVAL`** for `txbuf.buf` being NULL: This is **debatable**. Looking at the code, `txbuf.buf` is allocated via `kzalloc()` during probe in `fbtft-core.c:670`. If it's NULL here at runtime, it's not because the caller passed an invalid argument — it's because the buffer was never allocated (either `txbuflen` was 0, or allocation failed and was silently handled). `-ENOMEM` might be slightly more accurate, but one could also argue this is an internal state error best expressed as `-EINVAL`. It's a minor point and not worth blocking over.

**Nit:** The commit subject could be slightly more specific (e.g., `staging: fbtft: replace bare -1 returns with standard error codes`), but the current subject is acceptable.

**Verdict:** The patch is clean and correct. The only point worth raising to the author is whether `-ENOMEM` would be more appropriate than `-EINVAL` for the `txbuf.buf` NULL case in `fbtft_write_vmem16_bus9()`, since the NULL condition results from a missing buffer allocation rather than an invalid argument from the caller.

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