TWx Linux Repository
cfde770d94
The BCM2835 MINI UART has non-standard THRE semantics. Conventionally
the bit means that the FIFO is empty (although there may still be a
byte in the transmit register), but on 2835 it indicates that the FIFO
is not full. This causes interrupts after every byte is transmitted,
with the FIFO providing some interrupt latency tolerance.
A consequence of this difference is that the usual strategy of writing
multiple bytes into the TX FIFO after checking THRE once is unsafe.
In the worst case of 7 bytes in the FIFO, writing 8 bytes loses all
but the first since by then the FIFO is full.
There is an HFIFO ("Hidden FIFO") capability that causes the transmit
loop to terminate when both THRE and TEMT are set, i.e. when the TX
block is completely idle. This is unnecessarily cautious, potentially
causing gaps in transmission.
Add a new conditional to the transmit loop, predicated on CAP_MINI,
that exits when THRE is no longer set (the FIFO is full). This allows
the FIFO to fill quickly but subsequent writes are paced by the
transmission rate.
Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. 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.