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On SMP systems, we see a lot of spurious TX interrupts when a
program generates a steady stream of output to the pl011 UART.
The problem can be easily seen when one CPU generates the output
while another CPU handles the pl011 interrupts, and the rate of
output is low enough not to fill the TX FIFO. The problem seems
to be:
-- CPU a -- -- CPU b --
(take port lock)
pl011_start_tx
pl011_start_tx_pio
enable TXIM in REG_IMSC -> causes uart tx intr (pl011_int)
pl011_tx_chars pl011_int
...tx chars, all done... (wait for port lock)
pl011_stop_tx .
disable TXIM .
(release port lock) -> (take port lock)
check for TXIM, not enabled
(release port lock)
return IRQ_NONE
Enabling the TXIM in pl011_start_tx_pio() causes the interrupt
to be generated and delivered to CPU b, even though pl011_tx_chars()
is able to complete the TX and then disable the tx interrupt.
Fix this by enabling TXIM only after pl011_tx_chars, if it is needed.
pl011_tx_chars will return a boolean indicating whether the TX
interrupts have to be enabled.
Debugged-by: Vijaya Kumar <Vijaya.Kumar@cavium.com>
Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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.