> -----Original Message-----
> From: Jason Gunthorpe <jgg@xxxxxxxx>
> Sent: Tuesday, October 27, 2020 1:00 PM
> To: Xiong, Jianxin <jianxin.xiong@xxxxxxxxx>
> Cc: linux-rdma@xxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx; Doug Ledford <dledford@xxxxxxxxxx>; Leon Romanovsky
> <leon@xxxxxxxxxx>; Sumit Semwal <sumit.semwal@xxxxxxxxxx>; Christian Koenig <christian.koenig@xxxxxxx>; Vetter, Daniel
> <daniel.vetter@xxxxxxxxx>
> Subject: Re: [PATCH v6 1/4] RDMA/umem: Support importing dma-buf as user memory region
>
> On Fri, Oct 23, 2020 at 09:39:58AM -0700, Jianxin Xiong wrote:
> > +/*
> > + * Generate a new dma sg list from a sub range of an existing dma sg list.
> > + * Both the input and output have their entries page aligned.
> > + */
> > +static int ib_umem_dmabuf_sgt_slice(struct sg_table *sgt, u64 offset,
> > + u64 length, struct sg_table *new_sgt) {
> > + struct scatterlist *sg, *new_sg;
> > + u64 start, end, off, addr, len;
> > + unsigned int new_nents;
> > + int err;
> > + int i;
> > +
> > + start = ALIGN_DOWN(offset, PAGE_SIZE);
> > + end = ALIGN(offset + length, PAGE_SIZE);
> > +
> > + offset = start;
> > + length = end - start;
> > + new_nents = 0;
> > + for_each_sgtable_dma_sg(sgt, sg, i) {
> > + len = sg_dma_len(sg);
> > + off = min(len, offset);
> > + len -= off;
> > + len = min(len, length);
> > + if (len)
> > + new_nents++;
> > + length -= len;
> > + offset -= off;
> > + }
> > +
> > + err = sg_alloc_table(new_sgt, new_nents, GFP_KERNEL);
> > + if (err)
> > + return err;
>
> I would really rather not allocate an entirely new table just to take a slice of an existing SGT. Ideally the expoter API from DMA buf would
> prepare the SGL slice properly instead of always giving a whole buffer.
>
> Alternatively making some small edit to rdma_umem_for_each_dma_block() and ib_umem_find_best_pgsz() would let it slice the SGL at
> runtime
>
> You need to rebase on top of this series:
>
> https://patchwork.kernel.org/project/linux-rdma/list/?series=370437
>
> Which makes mlx5 use those new APIs
>
> Jason
Thanks. Will rebase and work on the runtime slicing.
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