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

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

Hi Morten,

Please find my reply in-line.

Thanks,
Amit Shukla

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: Wednesday, August 9, 2023 2:37 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 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.

> 
> 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.

> 
> PS: It has been a while since I looked at the DMA library, so ignore my
> comments if I got this wrong.