Re: [PATCH] dmaengine: omap-dma: Do not call omap_dma_callback() from tx_status()

[Peter Ujfalusi]

On 2016-02-26 03:06, Russell King - ARM Linux wrote:
> On Thu, Feb 25, 2016 at 10:28:59AM +0200, Peter Ujfalusi wrote:
>> When based on the CCR_ENABLE bit the channel is stopped we should not call
>> omap_dma_callback(), only change the return value to DMA_COMPLETE. Client
>> drivers will do the right thing to clean up the channel after the transfer
>> has been completed.
>> Check the CCR_ENABLE only if the channel is not paused since pause in sDMA
>> means that the channel is stopped.
>> This will fix one hard to reproduce race condition when the channel is
>> terminated during transfer (affecting cyclic operation).
>>
>> Fixes: 1a7cf7b26f25 ("dmaengine: omap-dma: Handle cases when the channel is polled for completion")
>>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>
>> ---
>>  drivers/dma/omap-dma.c | 16 ++++++++++------
>>  1 file changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/dma/omap-dma.c b/drivers/dma/omap-dma.c
>> index f6bef0d93998..a6b189fdbbe6 100644
>> --- a/drivers/dma/omap-dma.c
>> +++ b/drivers/dma/omap-dma.c
>> @@ -671,18 +671,22 @@ static enum dma_status omap_dma_tx_status(struct dma_chan *chan,
>>  	struct omap_chan *c = to_omap_dma_chan(chan);
>>  	struct virt_dma_desc *vd;
>>  	enum dma_status ret;
>> -	uint32_t ccr;
>>  	unsigned long flags;
>>  
>> -	ccr = omap_dma_chan_read(c, CCR);
>> -	/* The channel is no longer active, handle the completion right away */
>> -	if (!(ccr & CCR_ENABLE))
>> -		omap_dma_callback(c->dma_ch, 0, c);
>> -
>>  	ret = dma_cookie_status(chan, cookie, txstate);
>>  	if (ret == DMA_COMPLETE || !txstate)
>>  		return ret;
>>  
>> +	if (!c->paused) {
>> +		uint32_t ccr = omap_dma_chan_read(c, CCR);
>> +		/*
>> +		 * The channel is no longer active, set the return value
>> +		 * accordingly
>> +		 */
>> +		if (!(ccr & CCR_ENABLE))
>> +			ret = DMA_COMPLETE;
>> +	}
>> +
> 
> This looks very much like a hack, and surely opens a race condition
> up: what happens when a request submitted and pending but not yet
> started?  If the channel is idle, requesting status will report
> that the request has completed.
> 
> It's also wrong for another reason.  If txstate is NULL...

True, I have fixed these up.

> Your original commit adding the original hack that you're now removing
> above says that this is to support polled operation: I'm not aware of
> DMA engine supporting such a mode.  DMA_PREP_INTERRUPT is a mechanism
> where requests can be queued without an interrupt to allow batching.

Also it is used to suppress DMA interrupts during audio playback for example.
In this case we will run w/o interrupts and the position is polled.

> See the raid5/async_tx code, which queues a set of operations without
> DMA_PREP_INTERRUPT, with the final operation with DMA_PREP_INTERRUPT
> set.

We only allow the interrupts to be disabled in cyclic or memcpy mode. With
slave_sg we have interrupts as it is needed to move to the next SG.

> As the driver is reliant on interrupts to move to the next transfer,
> the patch which causes DMA_PREP_INTERRUPT to influence whether
> interrupts are sent is actually buggy, and will prevent several
> queued DMA operations to fail.

Yes, the omap-dma only allows the interrupts to be actually disabled when it
is save to do so. slave_sg can not work w/o interrupts so there we don't
disable them.

> 


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