[PATCH v4 0/2] platform/x86: dell-uart-backlight: handle stray EC byte in responses

[chitranshmandhaniya@gmail.com]

From: Chitransh Mandhaniya <Chitransh_Mandhaniya@dell.com>

On some Dell All-in-One machines the EC sporadically inserts a spurious
byte after the command echo in UART backlight response frames.  This
causes a checksum mismatch and the driver returns -EIO to userspace.

Patch 1 is a preparatory refactor: extract the byte-level response
parsing into a dedicated dell_uart_bl_receive_data() helper with no
functional change.

Patch 2 adds single-attempt stray-byte recovery inside that helper.
When a checksum mismatch occurs on a frame longer than the minimum
size, the suspect byte is dropped, the buffer is shifted down, and
status remains -EBUSY so serdev re-delivers the real checksum byte
naturally.

Tested on Dell OptiPlex AIO 7410 65W (board 0TKWC4) where the EC
inserts a stray byte on every get-brightness response.

v3 -> v4:
  - Perform resync inside dell_uart_bl_receive_data() rather than in
    the wrapper; status stays -EBUSY so serdev re-delivers the
    checksum byte without additional handling in the caller

v2 -> v3:
  - Guard resync with resp_idx == resp_len to prevent memmove on a
    truncated or out-of-range length field

v1 -> v2:
  - Extract dell_uart_bl_receive_data() helper instead of using a
    for-loop with a pass counter
  - Split into two patches: refactor then feature

Chitransh Mandhaniya (2):
  platform/x86: dell-uart-backlight: extract dell_uart_bl_receive_data()
    helper
  platform/x86: dell-uart-backlight: recover from stray EC response byte

 .../platform/x86/dell/dell-uart-backlight.c   | 52 +++++++++++++------
 1 file changed, 37 insertions(+), 15 deletions(-)

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2.43.0