Re: [PATCH 5/6] dmaengine: zynqmp_dma: Reject zero-length memcpy transfers

[Golla, Nagendra]

Hi Frank,

On 8/6/2026 9:26 PM, Frank Li wrote:
> On Thu, Aug 06, 2026 at 06:00:13PM +0530, Golla Nagendra wrote:
> > Zero-length prep_memcpy() needlessly consumed a descriptor slot. Reject
> > zero-length memcpy at prep time and document the behaviour.
> >
> > Signed-off-by: Golla Nagendra <nagendra.golla@xxxxxxx>
> > ---
Thanks for the review
> Is it caller problem, which pass down zero len? or are there special usage,
> such as memory barrier.

This is a caller-side issue. In this driver, prep_memcpy() expects a 
non-zero len.
When len is 0, the path allocate/fetch a descriptor and append it to the 
transfer flow, but it represents a zero-byte transaction (no meaningful 
data movement). There is no intended driver-side special use of 
zero-length memcpy (such as a barrier/fence).
So rejecting zero-length requests makes the behavior explicit and avoids 
consuming
descriptor resources for a no-op transfer.

Thanks,
Nagendra
> Frank
>
> >   drivers/dma/xilinx/zynqmp_dma.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/dma/xilinx/zynqmp_dma.c b/drivers/dma/xilinx/zynqmp_dma.c
> > index 2d56ea08ce45..e70874d65123 100644
> > --- a/drivers/dma/xilinx/zynqmp_dma.c
> > +++ b/drivers/dma/xilinx/zynqmp_dma.c
> > @@ -836,7 +836,8 @@ static void zynqmp_dma_synchronize(struct dma_chan *dchan)
> >    * @len: Transfer length
> >    * @flags: transfer ack flags
> >    *
> > - * Return: Async transaction descriptor on success and NULL on failure
> > + * Return: Async transaction descriptor on success and NULL on failure or
> > + *	   zero length transfer
> >    */
> >   static struct dma_async_tx_descriptor *zynqmp_dma_prep_memcpy(
> >   				struct dma_chan *dchan, dma_addr_t dma_dst,
> > @@ -851,6 +852,9 @@ static struct dma_async_tx_descriptor *zynqmp_dma_prep_memcpy(
> >
> >   	chan = to_chan(dchan);
> >
> > +	if (!len)
> > +		return NULL;
> > +
> >   	desc_cnt = DIV_ROUND_UP(len, ZYNQMP_DMA_MAX_TRANS_LEN);
> >
> >   	spin_lock_irqsave(&chan->lock, irqflags);
> > --
> > 2.43.7