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

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Date:   Mon, 7 Oct 2019 05:48:31 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        RDMA mailing list <linux-rdma@...r.kernel.org>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Yamin Friedman <yaminf@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        linux-netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH rdma-next v1 2/3] RDMA/rw: Support threshold for
 registration vs scattering to local pages

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;
	}