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Date:   Wed, 8 Jul 2020 14:19:40 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Robin Murphy <robin.murphy@arm.com>,
        =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= <bjorn.topel@intel.com>,
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Subject: Re: [PATCH net] xsk: remove cheap_dma optimization
Message-ID: <20200708121940.GA19619@lst.de>
References: <20200626134358.90122-1-bjorn.topel@gmail.com> <c60dfb5a-2bf3-20bd-74b3-6b5e215f73f8@iogearbox.net> <20200627070406.GB11854@lst.de> <88d27e1b-dbda-301c-64ba-2391092e3236@intel.com> <878626a2-6663-0d75-6339-7b3608aa4e42@arm.com> <20200708065014.GA5694@lst.de> <B926444035E5E2439431908E3842AFD255E99A@DGGEMI525-MBS.china.huawei.com>
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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).