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    serial: add irq handler for Freescale 16550 errata. · 9deaa53a
    Paul Gortmaker 提交于
    Sending a break on the SOC UARTs found in some MPC83xx/85xx/86xx
    chips seems to cause a short lived IRQ storm (/proc/interrupts
    typically shows somewhere between 300 and 1500 events).  Unfortunately
    this renders SysRQ over the serial console completely inoperable.
    
    The suggested workaround in the errata is to read the Rx register,
    wait one character period, and then read the Rx register again.
    We achieve this by tracking the old LSR value, and on the subsequent
    interrupt event after a break, we don't read LSR, instead we just
    read the RBR again and return immediately.
    
    The "fsl,ns16550" is used in the compatible field of the serial
    device to mark UARTs known to have this issue.
    
    Thanks to Scott Wood for providing the errata data which led to
    a much cleaner fix.
    Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
    Acked-by: NAlan Cox <alan@linux.intel.com>
    Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>
    9deaa53a
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