linux-kernel - Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

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Date:   Sat, 12 Jan 2019 12:27:05 -0600
From:   Shiraz Saleem <shiraz.saleem@...el.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Imre Deak <imre.deak@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        Yong Zhi <yong.zhi@...el.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Bingbu Cao <bingbu.cao@...el.com>,
        Tian Shu Qiu <tian.shu.qiu@...el.com>,
        Jian Xu Zheng <jian.xu.zheng@...el.com>,
        Sinclair Yeh <syeh@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

On Fri, Jan 04, 2019 at 10:35:43PM +0000, Jason Gunthorpe wrote:
> Commit 2db76d7c3c6d ("lib/scatterlist: sg_page_iter: support sg lists w/o
> backing pages") introduced the sg_page_iter_dma_address() function without
> providing a way to use it in the general case. If the sg_dma_len is not
> equal to the dma_length callers cannot safely use the
> for_each_sg_page/sg_page_iter_dma_address combination.
> 
> Resolve this API mistake by providing a DMA specific iterator,
> for_each_sg_dma_page(), that uses the right length so
> sg_page_iter_dma_address() works as expected with all sglists. A new
> iterator type is introduced to provide compile-time safety against wrongly
> mixing accessors and iterators.
[..]

>  
> +/*
> + * sg page iterator for DMA addresses
> + *
> + * This is the same as sg_page_iter however you can call
> + * sg_page_iter_dma_address(@dma_iter) to get the page's DMA
> + * address. sg_page_iter_page() cannot be called on this iterator.
> + */
Does it make sense to have a variant of sg_page_iter_page() to get the
page descriptor with this dma_iter? This can be used when walking DMA-mapped
SG lists with for_each_sg_dma_page.

> +struct sg_dma_page_iter {
> +	struct sg_page_iter base;
> +};
> +
>