RE: [RFC PATCH] dmadev: offload to free source buffer

[Amit Prakash Shukla <amitprakashs@marvell.com>]

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: Wednesday, August 9, 2023 8:19 PM
> To: Amit Prakash Shukla <amitprakashs@marvell.com>; Chengwen Feng
> <fengchengwen@huawei.com>; Kevin Laatz <kevin.laatz@intel.com>; Bruce
> Richardson <bruce.richardson@intel.com>
> Cc: dev@dpdk.org; Jerin Jacob Kollanukkaran <jerinj@marvell.com>;
> conor.walsh@intel.com; Vamsi Krishna Attunuru <vattunuru@marvell.com>;
> g.singh@nxp.com; sachin.saxena@oss.nxp.com; hemant.agrawal@nxp.com;
> cheng1.jiang@intel.com; Nithin Kumar Dabilpuram
> <ndabilpuram@marvell.com>; Anoob Joseph <anoobj@marvell.com>
> Subject: [EXT] RE: [RFC PATCH] dmadev: offload to free source buffer
> 
> External Email
> 
> ----------------------------------------------------------------------
> > From: Amit Prakash Shukla [mailto:amitprakashs@marvell.com]
> > Sent: Wednesday, 9 August 2023 16.27
> >
> > > From: Morten Brørup <mb@smartsharesystems.com>
> > > Sent: Wednesday, August 9, 2023 2:37 PM
> > >
> > > > From: Amit Prakash Shukla [mailto:amitprakashs@marvell.com]
> > > > Sent: Wednesday, 9 August 2023 08.09
> > > >
> > > > This changeset adds support in DMA library to free source DMA
> > > > buffer by hardware. On a supported hardware, application can pass
> > > > on the mempool information as part of vchan config when the DMA
> > > > transfer direction is configured as RTE_DMA_DIR_MEM_TO_DEV.
> > >
> > > Isn't the DMA source buffer a memory area, and what needs to be
> > > freed
> > is
> > > the mbuf holding the memory area, i.e. two different pointers?
> > No, it is same pointer. Assume mbuf created via mempool, mempool needs
> > to be given via vchan config and iova passed to
> > rte_dma_copy/rte_dma_copy_sg's can be any address in mbuf area of
> > given mempool element.
> > For example, mempool element size is S. dequeued buff from mempool is
> > at X. Any address in (X, X+S) can be given as iova to rte_dma_copy.
> 
> So the DMA library determines the pointer to the mbuf (in the given
> mempool) by looking at the iova passed to rte_dma_copy/rte_dma_copy_sg,
> and then calls rte_mempool_put with that pointer?

No. DMA hardware would determine the pointer to the mbuf using iova address and mempool. Hardware will free the buffer, on completion of data transfer.

> 
> >
> > >
> > > I like the concept. Something similar might also be useful for
> > > RTE_DMA_DIR_MEM_TO_MEM, e.g. packet capture. Although such a use
> > > case might require decrementing the mbuf refcount instead of freeing
> > the
> > > mbuf directly to the mempool.
> > This operation is not supported in our hardware. It can be implemented
> > in future if any hardware supports it.
> 
> OK, I didn't expect that - just floating the idea. :-)
> 
> >
> > >
> > > PS: It has been a while since I looked at the DMA library, so ignore
> > > my comments if I got this wrong.