netdev - Re: [RFC PATCH] net/mlx4: Get rid of page operation after dma_alloc

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Message-ID: <20181218185012.GB16052@lst.de>
Date:   Tue, 18 Dec 2018 19:50:12 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Stephen Warren <swarren@...dotorg.org>,
        Christoph Hellwig <hch@....de>,
        Tariq Toukan <tariqt@...lanox.com>, xavier.huwei@...wei.com,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Doug Ledford <dledford@...hat.com>
Subject: Re: [RFC PATCH] net/mlx4: Get rid of page operation after
 dma_alloc_coherent

On Tue, Dec 18, 2018 at 11:43:08AM -0700, Jason Gunthorpe wrote:
> So the problem here is that on some arches
> 
>  sg_set_buf(sg, dma_coherent_buf, size)
>  p = sg_virt(sg);
>  assert(p == dma_coherent_buf);

dma allocations purely return a virtual address, you must never
call virt_to_page or virt_to_phys on them, which sg_set_buf
will do.  On many architectures this will give your the wrong
result as the coherent DMA address is a vmap or ioremap address.