On Fri, Apr 7, 2017 at 2:08 PM, Vignesh R <vigneshr@xxxxxx> wrote: > Hi All, > > I seem to be hitting a race condition using 8250_dma (and 8250_omap > specific dma) support: > > Kernel writes log messages to console via > serial8250_console_write()->serial8250_console_putchar() which directly > accesses UART_TX register with port->lock acquired. > > Now, if the same UART instance is being used by systemd/userspace, > characters are written to UART_TX register by serial8250_tx_chars(). The > concurrent access by serial8250_console_write() and > serial8250_tx_chars() is serialized by the use of port->lock spinlock > and hence there is no issue with` non DMA case. > > But when using DMA with 8250 UART, I see that port->lock is held before > scheduling of DMA TX transfer and released as soon as the transfer is > submitted. The lock is not held until the transfer actually completes > See, > uart_start() > ->serial8250_start_tx()-> > __start_tx() > ->up->dma->tx_dma(up) > Or > __dma_tx_complete() in 8250_dma.c that acquires and releases port->lock > once TX DMA transfer is submitted in serial8250_tx_dma() > > So, when the port->lock is released, it is quite possible that DMA is > still transferring data to UART TX FIFO and UART FIFO might be almost full. > I see that when DMA is writing to UART TX FIFO, > serial8250_console_write() may also write kernel log messages to UART TX > FIFO(as port->lock is now free to be acquired), which is leading to > overflow and lose of data. serial8250_console_write() checks for > UART_LSR_THRE to check if Transmit hold register is empty but that may > not be enough as DMA might put data before CPU write. > > It seems that both DMA and CPU might simultaneously put data to UART > FIFO and lead to potential loss of data. > Is the expectation that UART instance used to print kernel log messages > is not intended to use DMA? Or am I missing something? > > > Any help appreciated! I have one patch in my tree for a long time already: https://bitbucket.org/andy-shev/linux/commits/9f86c648e53bd25b8ec374933764577b2a340468?at=topic/uart/rpm Besides that I have patch to disable power management on kernel console (and non-hackish implementation of runtime PM for UART is there in case you are wondering what that repository for). -- With Best Regards, Andy Shevchenko -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
