• Linus Walleij's avatar
    ARM: 9004/1: debug: Split waituart to CTS and TXRDY · 2c50a570
    Linus Walleij authored
    This patch was triggered by a remark from Russell that
    introducing a call to the waituart (needed to fix debug prints
    on the Qualcomm platforms) was dangerous because in some cases
    this will involve waiting for a modem CTS (clear to send)
    signal, and debug messages would maybe not work on platforms
    with no modem connected to the UART port: they will just
    hang waiting for the modem to assert CTS and this might never
    happen.
    
    Looking through all UART debug drivers implementing the waituart
    macro I discovered that all users except two actually use this
    macro to check if the UART is ready for TX, let's call this
    TXRDY.
    
    Only two debug UART drivers actually check for CTS:
    - arch/arm/include/debug/8250.S
    - arch/arm/include/debug/tegra.S
    
    The former is very significant since the 8250 is possibly
    the most common UART on the planet.
    
    We have the following problem: the semantics of waituart are
    ambiguous making it dangerous to introduce the macro to debug
    code fixing debug prints for Qualcomm. To start to pry this
    problem apart, this patch does the following:
    
    - Convert all debug UART drivers to define two macros:
    
      - waituartcts with the clear semantic to wait for CTS
        to be asserted
    
      - waituarttxrdy with the clear semantic to wait for the TX
        capability of the UART to be ready
    
    - When doing this take care to assign the right function to
      each drivers macro, so they now do exactly the above.
    
    - Update the three sites in the kernel invoking the waituart
      macro to call waituartcts/waituarttxrdy in sequence, so that
      the functional impact on the kernel should be zero.
    
    After this we can start to change the code sites using this
    code to do the right thing.
    Signed-off-by: default avatarLinus Walleij <linus.walleij@linaro.org>
    Signed-off-by: default avatarRussell King <rmk+kernel@armlinux.org.uk>
    2c50a570
8250.S 1.09 KB