Re: [PATCH rdma-next v1 2/3] RDMA/rw: Support threshold for registration vs scattering to local pages

[Christoph Hellwig]

On Mon, Oct 07, 2019 at 03:36:56PM +0300, Leon Romanovsky wrote:
> > >   if (rdma_protocol_iwarp(dev, port_num) && dir == DMA_FROM_DEVICE)
> > >           return true;
> > > + if (dev->attrs.max_sgl_rd && dir == DMA_FROM_DEVICE &&
> > > +     dma_nents > dev->attrs.max_sgl_rd)
> > > +         return true;
> >
> > This can be simplified to:
> >
> >     if (dir == DMA_FROM_DEVICE &&
> >         (rdma_protocol_iwarp(dev, port_num) ||
> >          (dev->attrs.max_sgl_rd && dma_nents > dev->attrs.max_sgl_rd)))
> >             return true;
> 
> I don't think that it simplifies and wanted to make separate checks to
> be separated. For example, rdma_protocol_iwarp() has nothing to do with
> attrs.max_sgl_rd.

The important bit is to have the DMA_FROM_DEVICE check only once, as
we only do the registration for reads with either parameter.  So if
you want it more verbose the wya would be:

        if (dir == DMA_FROM_DEVICE) {
                if (rdma_protocol_iwarp(dev, port_num))
                        return true;
                if (dev->attrs.max_sgl_rd && dma_nents > dev->attrs.max_sgl_rd)
                        return true;
        }