On Tuesday 18 April 2017 11:46 AM, Andy Shevchenko wrote: > On Mon, Apr 10, 2017 at 11:16 AM, Vignesh R <vigneshr@xxxxxx> wrote: >> On Sunday 09 April 2017 04:37 PM, Andy Shevchenko wrote: >>> 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 >> >> I had similar patch in mind. Do you plan to submit above patch to the >> mailing list? You may also consider to add the issue I mentioned above >> to the commit description. Thanks! > > Yes, I'm planning to do so, but be aware that OMAP has its own DMA > glue layer and thus my patch doesn't affect it. > Yes, I am working on a patch for 8250_omap driver. Thanks! -- Regards Vignesh -- 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
