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Date:   Sat, 28 Apr 2018 01:33:47 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Christopher Lameter <cl@linux.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        "Luis R. Rodriguez" <mcgrof@kernel.org>, linux-mm@kvack.org,
        Jan Kara <jack@suse.cz>, matthew@wil.cx, x86@kernel.org,
        luto@amacapital.net, martin.petersen@oracle.com,
        jthumshirn@suse.de, broonie@kernel.org, linux-spi@vger.kernel.org,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        "lsf-pc@lists.linux-foundation.org" 
        <lsf-pc@lists.linux-foundation.org>
Subject: Re: [LSF/MM TOPIC NOTES] x86 ZONE_DMA love
Message-ID: <20180428083347.GC31684@infradead.org>
References: <20180426215406.GB27853@wotan.suse.de>
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On Fri, Apr 27, 2018 at 11:36:23AM -0500, Christopher Lameter wrote:
> On Fri, 27 Apr 2018, Matthew Wilcox wrote:
> 
> > Some devices have incredibly bogus hardware like 28 bit addressing
> > or 39 bit addressing.  We don't have a good way to allocate memory by
> > physical address other than than saying "GFP_DMA for anything less than
> > 32, GFP_DMA32 (or GFP_KERNEL on 32-bit) for anything less than 64 bit".
> >
> > Even CMA doesn't have a "cma_alloc_phys()".  Maybe that's the right place
> > to put such an allocation API.
> 
> The other way out of this would be to require a IOMMU?

Which on many systems doesn't exist.  And even if it exists might not
be usable.