linux-kernel - Re: [PATCH net] xsk: remove cheap

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Message-ID: <20200708121940.GA19619@lst.de>
Date:   Wed, 8 Jul 2020 14:19:40 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "maximmi@...lanox.com" <maximmi@...lanox.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "jonathan.lemon@...il.com" <jonathan.lemon@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>
Subject: Re: [PATCH net] xsk: remove cheap_dma optimization

On Wed, Jul 08, 2020 at 07:57:23AM +0000, Song Bao Hua (Barry Song) wrote:
> > int dma_map_batch_start(struct device *dev, size_t rounded_len,
> > 	enum dma_data_direction dir, unsigned long attrs, dma_addr_t *addr);
> > int dma_map_batch_add(struct device *dev, dma_addr_t *addr, struct page
> > *page,
> > 		unsigned long offset, size_t size);
> > int dma_map_batch_end(struct device *dev, int ret, dma_addr_t start_addr);
> > 
> 
> Hello Christoph,
> 
> What is the different between dma_map_batch_add() and adding the buffer to sg of dma_map_sg()?

There is not struct scatterlist involved in this API, avoiding the
overhead to allocate it (which is kinda the point).