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9d3aaceb73
Some Freescale 8250 implementations have the problem that a single long
break results in one irq per character frame time. The code in
fsl8250_handle_irq() that is supposed to handle that uses the BI bit in
lsr_saved_flags to detect such a situation and then skip the second
received character. However it also stores other error bits and so after
a single frame error the character received in the next irq handling is
passed to the upper layer with a frame error, too.
So after a spike on the data line (which is correctly recognized as a
frame error) the following valid character is thrown away, because the
driver reports a frame error for that one, too.
To weaken this problem restrict saving LSR to only the BI bit.
Note however that the handling is still broken:
- lsr_saved_flags is updated using orig_lsr which is the LSR content
for the first received char, but there might be more in the FIFO, so
a character is thrown away that is received later and not necessarily
the one following the break.
- The doubled break might be the 2nd and 3rd char in the FIFO, so the
workaround doesn't catch these, because serial8250_rx_chars() doesn't
handle the workaround.
- lsr_saved_flags might have set UART_LSR_BI at the entry of
fsl8250_handle_irq() which doesn't originate from
fsl8250_handle_irq()'s "up->lsr_saved_flags |= orig_lsr &
UART_LSR_BI;" but from e.g. from serial8250_tx_empty().
- For a long or a short break this isn't about two characters, but more
or only a single one.
Fixes:
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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.
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.