Subject: Re: [RFC PATCH] net/mlx4: Get rid of page operation after dma_alloc_coherent

From: Jason Gunthorpe <[email-masked]>
Date: Tue, 18 Dec 2018 12:04:12 -0700

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