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From: Christoph Hellwig <hch@lst.de>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Daniel Vetter <daniel@ffwll.ch>, Matthew Wilcox <willy@infradead.org>,
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	Logan Gunthorpe <logang@deltatee.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: Phyr Starter
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References: <YdyKWeU0HTv8m7wD@casper.infradead.org> <20220111004126.GJ2328285@nvidia.com> <CAKMK7uFfpTKQEPpVQxNDi0NeO732PJMfiZ=N6u39bSCFY3d6VQ@mail.gmail.com> <20220111202648.GP2328285@nvidia.com>
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On Tue, Jan 11, 2022 at 04:26:48PM -0400, Jason Gunthorpe wrote:
> What I did in RDMA was make an iterator rdma_umem_for_each_dma_block()
> 
> The driver passes in the page size it wants and the iterator breaks up
> the SGL into that size.
> 
> So, eg on a 16k page size system the SGL would be full of 16K stuff,
> but the driver only support 4k and so the iterator hands out 4 pages
> for each SGL entry.
> 
> All the drivers use this to build their DMA lists and tables, it works
> really well.

The block layer also has the equivalent functionality by setting the
virt_boundary value in the queue_limits.  This is needed for NVMe
PRPs and RDMA drivers.