[Report] ns16550: possible missing workaround for NXP erratum A-008006 on T1040/T1042 (DUART in FIFO mode)

[its recursion]

  Hi,

  I would like to report a potential issue
in drivers/serial/ns16550.c concerning NXP QorIQ T1040/T1042  SoCs (e.g.
the T1042D4RDB board).

 * Erratum (public reference)*

  While browsing the NXP community forum, I came across a post discussing
erratum A-008006 — "DUART may miss characters during transmission in FIFO
mode" — and its applicability to T1040/T1042:

  https://community.nxp.com/t5/QorIQ/errata-A-008006-for-T1040/m-p/557567

  The full erratum description is, to my knowledge, only available in NXP's
NDA-protected chip errata document, so I cannot quote or propose the
official workaround.

  *Observation in U-Boot*

  On T1042D4RDB (board/nxp/t104xrdb/, configs/T1042D4RDB_defconfig), the
console UART is driven by drivers/serial/ns16550.c. Relevant points:

  - include/ns16550.h:154 defines

  UART_FCR_DEFVAL = UART_FCR_FIFO_EN | UART_FCR_RXSR | UART_FCR_TXSR i.e.
the default FCR value enables the FIFO.

  - drivers/serial/ns16550.c:249 (ns16550_init) writes this default FCR
into the hardware on init.

  - drivers/serial/ns16550.c:275 (ns16550_putc) transmits through the FIFO
(polls THRE, writes THR).

  A grep of the tree for *A-008006 / A008006 / ERRATUM_A008006* returns no
matches, and no other

  FIFO-related workaround appears to be present in the driver or the
T104xRDB board code.

 * Conclusion*

  The preconditions of erratum A-008006 (FIFO mode enabled, transmission
through the DUART) appear to be met on T1042D4RDB under U-Boot, and no
corresponding workaround seems to be in-tree. This may result in

  occasional missed characters on the U-Boot console for T1040/T1042
platforms.

  I am reporting this for awareness only. I do not have access to the
errata document and cannot propose

  the official workaround. Someone with access to the NXP errata (or an NXP
maintainer) would be better

  placed to evaluate and, if appropriate, send a fix.

  Thanks,