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

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Date:   Tue, 18 Dec 2018 12:04:12 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christoph Hellwig <hch@....de>
Cc:     Stephen Warren <swarren@...dotorg.org>,
        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 07:50:12PM +0100, Christoph Hellwig wrote:
> 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.

Yes, that is what I gathered - if that is the design then I'd say that
drivers shouldn't be stuffing these DMA coherent virtual addresses
into a sg at all.

Jason