> -----Original Message-----
> From: Morten Brørup <[email protected]>
> Sent: Wednesday, August 9, 2023 8:19 PM
> To: Amit Prakash Shukla <[email protected]>; Chengwen Feng
> <[email protected]>; Kevin Laatz <[email protected]>; Bruce
> Richardson <[email protected]>
> Cc: [email protected]; Jerin Jacob Kollanukkaran <[email protected]>;
> [email protected]; Vamsi Krishna Attunuru <[email protected]>;
> [email protected]; [email protected]; [email protected];
> [email protected]; Nithin Kumar Dabilpuram
> <[email protected]>; Anoob Joseph <[email protected]>
> Subject: [EXT] RE: [RFC PATCH] dmadev: offload to free source buffer
>
> External Email
>
> ----------------------------------------------------------------------
> > From: Amit Prakash Shukla [mailto:[email protected]]
> > Sent: Wednesday, 9 August 2023 16.27
> >
> > > From: Morten Brørup <[email protected]>
> > > Sent: Wednesday, August 9, 2023 2:37 PM
> > >
> > > > From: Amit Prakash Shukla [mailto:[email protected]]
> > > > 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.
