linux-kernel - Re: [PATCH] serial: 8250: 8250_omap: Fix race b/w dma completion and RX timeout

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Date:   Mon, 19 Jun 2017 10:42:41 +0530
From:   Vignesh R <vigneshr@...com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Jiri Slaby <jslaby@...e.com>,
        Peter Hurley <peter@...leysoftware.com>,
        <linux-serial@...r.kernel.org>, <linux-omap@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH] serial: 8250: 8250_omap: Fix race b/w dma completion and
 RX timeout



On Saturday 17 June 2017 08:07 PM, Andy Shevchenko wrote:
> On Sat, 2017-06-17 at 19:22 +0530, Vignesh R wrote:
>> DMA RX completion handler for UART is called from a tasklet and hence
>> may be delayed depending on the system load. In meanwhile, there may
>> be
>> RX timeout interrupt which can get serviced first before DMA RX
>> completion handler is executed for the completed transfer.
>> omap_8250_rx_dma_flush() which is called on RX timeout interrupt makes
>> sure that the DMA RX buffer is pushed and then the FIFO is drained and
>> also queues a new DMA request. But, when DMA RX completion handler
>> executes, it will erroneously flush the currently queued DMA transfer
>> which sometimes results in data corruption and double queueing of DMA
>> RX
>> requests.
>>
>> Fix this by checking whether RX completion is for the currently queued
>> transfer or not. And also hold port lock when in DMA completion to
>> avoid
>> race wrt RX timeout handler preempting it.
> 
> 
>>  static void __dma_rx_complete(void *param)
>>  {
>> -	__dma_rx_do_complete(param);
>> -	omap_8250_rx_dma(param);
>> +	struct uart_8250_port *p = param;
>> +	struct uart_8250_dma *dma = p->dma;
>> +	unsigned long flags;
>> +
>> +	spin_lock_irqsave(&p->port.lock, flags);
>> +
>> +	/*
>> +	 * If the completion is for the current cookie then handle
>> it,
>> +	 * else a previous RX timeout flush would have already pushed
>> +	 * data from DMA buffers, so exit.
>> +	 */
> 
>> +	if (dma->rx_cookie != dma->rxchan->completed_cookie) {
> 
> Wouldn't be better to call DMAEngine API for that?
> dmaengine_tx_status() I suppose

Yeah, will update the patch. Thanks!



-- 
Regards
Vignesh