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After sending data to the uart, the driver was waiting until the TX FIFO was empty (for every single char written). After that, TX was disabled by writing the original TX state to the status register. At that time however, the state machine could still be shifting characters. Not waiting can result in strange hardware states, especially when coupled with calls to cdns_uart_set_termios, whose symptom generally is garbage characters being received from uart or a hang. According to UG585, the TACTIVE bit of the channel status register indicates the shifter operation and we should be waiting for that bit to clear. Sending characters does not require the TX FIFO to be empty, but merely to not be full. So cdns_uart_console_putchar is updated accordingly. During tests with an instrumented kernel and an oscilloscope, we could determine that the chance of a race is reduced by this patch. It is not removed entirely. On the oscilloscope, one can see that disabling the transmitter early can result in the transmission hanging in the middle of a character for a tiny duration. This hiccup is enough to desynchronize with a remote device for a sequence of characters until a data bit doesn't match the start or stop bits anymore. Link: https://www.spinics.net/lists/linux-serial/msg23156.html Link: https://www.spinics.net/lists/linux-serial/msg26139.html Signed-off-by: Helmut Grohne <h.grohne@intenta.de> Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.