DMA: get data count of descriptor in dmaengine

Russell King - ARM Linux linux at
Fri Sep 7 06:35:47 EDT 2012

On Fri, Sep 07, 2012 at 09:59:17AM +0000, Song, Elen wrote:
> Hi All:
> 	When I am developing using dma for uart receive, I found a problem.
> 	I use a cyclic dma transfer and set descriptor transfer size to lk, 
> 	only receive 1k bytes(descriptor transfer done) that will call an interrupt.
> 	If I receive 1 byte, there will happened nothing except my peripherals have timeout function. 
> 	Fortunately, I got timeout function in uart peripheral, in timeout handler, I can receive data from uart.
> 	But I do not know how many data I have receive, so there should be a count register in dma.
> 	I found the count register but I do not found correlative api in dmaengine, I think we should add one.
> 	Even during a single or multi buffer transfer, knowing how many data has been transfer in current descriptor is necessary.
> 	Through that, we will not lose a data.
> 	So there should be an api to get current descriptor receive data count.
> 	Or we could get current descriptor status , the status with count value?
> 	I submit an patch to retrieve descriptor data transfer size.
> 	We use this to get data count .
> 	We use Atmel dma, but it could be a common demand.

See dmaengine_tx_status() - the residue is returned in struct dma_tx_state.
See the PL011 UART driver to see how this is used with a timeout interrupt.

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