• Sven Eckelmann's avatar
    tty: serial: msm_serial: Deactivate RX DMA for polling support · 7492ffc9
    Sven Eckelmann authored
    The CONSOLE_POLLING mode is used for tools like k(g)db. In this kind of
    setup, it is often sharing a serial device with the normal system console.
    This is usually no problem because the polling helpers can consume input
    values directly (when in kgdb context) and the normal Linux handlers can
    only consume new input values after kgdb switched back.
    
    This is not true anymore when RX DMA is enabled for UARTDM controllers.
    Single input values can no longer be received correctly. Instead following
    seems to happen:
    
    * on 1. input, some old input is read (continuously)
    * on 2. input, two old inputs are read (continuously)
    * on 3. input, three old input values are read (continuously)
    * on 4. input, 4 previous inputs are received
    
    This repeats then for each group of 4 input values.
    
    This behavior changes slightly depending on what state the controller was
    when the first input was received. But this makes working with kgdb
    basically impossible because control messages are always corrupted when
    kgdboc tries to parse them.
    
    RX DMA should therefore be off when CONSOLE_POLLING is enabled to avoid
    these kind of problems. No such problem was noticed for TX DMA.
    
    Fixes: 99693945 ("tty: serial: msm: Add RX DMA support")
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarSven Eckelmann <sven@narfation.org>
    Link: https://lore.kernel.org/r/20211113121050.7266-1-sven@narfation.orgSigned-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    7492ffc9
msm_serial.c 44.6 KB