This is a note to let you know that I've just added the patch titled
serial: 8250: omap: Disable DMA for console UART
to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: serial-8250-omap-disable-dma-for-console-uart.patch and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From foo@baz Mon Apr 9 17:09:24 CEST 2018
From: Vignesh R vigneshr@ti.com Date: Sat, 22 Apr 2017 18:37:19 +0530 Subject: serial: 8250: omap: Disable DMA for console UART
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 @@ -613,6 +613,10 @@ 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) {
Patches currently in stable-queue which might be from vigneshr@ti.com are
queue-4.9/serial-8250-omap-disable-dma-for-console-uart.patch