4.4-stable review patch. If anyone has any objections, please let me know.
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From: Vignesh R vigneshr@ti.com
[ Upstream commit 84b40e3b57eef1417479c00490dd4c9f6e5ffdbc ]
Kernel always writes log messages to console via serial8250_console_write()->serial8250_console_putchar() which directly accesses UART_TX register _without_ using DMA.
But, if other processes like systemd using same UART port, then these writes are handled by a different code flow using 8250_omap driver where there is provision to use DMA.
It seems that it is possible that both DMA and CPU might simultaneously put data to UART FIFO and lead to potential loss of data due to FIFO overflow and weird data corruption. This happens when both kernel console and userspace tries to write simultaneously to the same UART port. Therefore, disable DMA on kernel console port to avoid potential race between CPU and DMA.
Signed-off-by: Vignesh R vigneshr@ti.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/serial/8250/8250_omap.c | 4 ++++ 1 file changed, 4 insertions(+)
--- a/drivers/tty/serial/8250/8250_omap.c +++ b/drivers/tty/serial/8250/8250_omap.c @@ -608,6 +608,10 @@ static int omap_8250_startup(struct uart up->lsr_saved_flags = 0; up->msr_saved_flags = 0;
+ /* Disable DMA for console UART */ + if (uart_console(port)) + up->dma = NULL; + if (up->dma) { ret = serial8250_request_dma(up); if (ret) {