Re: [PATCH v3 2/5] IB/core, RDMA RW API: Do not exceed QP SGE send limit

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

On Tue, Jul 19, 2016 at 09:22:03AM -0700, Bart Van Assche wrote:
> For IB and RoCE, the SGE limit for a queue pair is typically lower
> than what is defined by the HCA limits. For iWARP, the RDMA READ SGE
> limit is defined by dev->attrs.max_sge_rd. Modify rdma_rw_max_sge()
> accordingly.

It's not just iWarp - Melannox IB HCAs also have a lower RDMA READ than
WRITE limit.

> -static inline u32 rdma_rw_max_sge(struct ib_device *dev,
> +static inline u32 rdma_rw_max_sge(struct ib_device *dev, struct ib_qp *qp,
>  		enum dma_data_direction dir)
>  {
> +	return dir == DMA_TO_DEVICE ? qp->max_send_sge :
> +		min_t(u32, qp->max_send_sge, dev->attrs.max_sge_rd);
>  }

I'm a bit worried about this implicit and barely documented assumption
that we'll always have a lower RDMA READ than WRITE / SEND limit.

Can we fine tune the patch to add max_write_sge and max_read_sge
to struct ib_qp instead?  That also makes clear they apply to RDMA READ
and WRITE only.  Please also add a comment to ib_verbs.h to document
the exact semantics of this field(s) while you're at it, e.g. that
they only apply to users of the RDMA R/W API.
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