linux-kernel - Re: [PATCH] dell_rbu: stop abusing the DMA API

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Message-ID: <20190207204730.GA21530@wrath>
Date:   Thu, 7 Feb 2019 12:47:30 -0800
From:   Darren Hart <dvhart@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     stuart.w.hayes@...il.com, andy@...radead.org,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mario_Limonciello@...l.com
Subject: Re: [PATCH] dell_rbu: stop abusing the DMA API

On Sat, Feb 02, 2019 at 06:16:59PM +0100, Christoph Hellwig wrote:
> On Fri, Feb 01, 2019 at 03:15:59PM -0800, Darren Hart wrote:
> > On Tue, Jan 29, 2019 at 08:34:09AM +0100, Christoph Hellwig wrote:
> > > For some odd reason dell_rbu actually seems to want the physical and
> > > not a bus address for the allocated buffer.  Lets assume that actually
> > > is correct given that it is BIOS-related and that is a good source
> > > of insanity.  In that case we should not use dma_alloc_coherent with
> > > a NULL device to allocate memory, but use GFP_DMA32 to stay under
> > > the 32-bit BIOS limit.
> > 
> > + Mario re bios related physical address - is Christoph's assumption
> > correct?
> > 
> > Christoph, did you observe a failure? If so, we should probably also
> > tag for stable.
> 
> No, I've been auditing for DMA API (ab-)users that don't pass a
> struct device.  Generally the fix was to just pass a struct device
> that is easily available.  But dell_rbu doesn't actually seem to
> be a "device" in the traditional sense, and the way it uses the
> DMA API is really, really odd - it first does a virt_to_phys on
> memory allocated from the page allocator (so works with physical
> addresses in that case) and the retries with a dma_alloc_coherent
> with a NULL argument, which in no way is guaranteed to you give
> you something else, although for the current x86 implementation
> will give you the equivalent of a GFP_DMA32 page allocator allocation
> plus virt_to_phys.
> 

Thanks Christoph, merged to for-next.


-- 
Darren Hart
VMware Open Source Technology Center