linux-kernel - Re: [PATCH v5 12/13] swiotlb-xen: use xen_alloc/free_coherent

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Message-ID: <20130906141754.GT2590@phenom.dumpdata.com>
Date:	Fri, 6 Sep 2013 10:17:54 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, Ian.Campbell@...rix.com
Subject: Re: [PATCH v5 12/13] swiotlb-xen: use xen_alloc/free_coherent_pages

On Thu, Sep 05, 2013 at 05:50:45PM +0100, Stefano Stabellini wrote:
> On Fri, 30 Aug 2013, Konrad Rzeszutek Wilk wrote:
> > On Thu, Aug 29, 2013 at 07:32:33PM +0100, Stefano Stabellini wrote:
> > > Use xen_alloc_coherent_pages and xen_free_coherent_pages to allocate or
> > > free coherent pages.
> > > 
> > > We need to be careful handling the pointer returned by
> > > xen_alloc_coherent_pages, because on ARM the pointer is not equal to
> > > phys_to_virt(*dma_handle). In fact virt_to_phys on the returned pointer
> > > doesn't return a valid physical address.
> > 
> > Why is it called 'virt_to_phys'? What does it return then?
> 
> virt_to_phys only works for kernel direct mapped RAM memory.
> In this case the virtual address could be an ioremap address, therefore
> passing it to virt_to_phys would give you another physical address that
> doesn't correspond to it.

Ah, so very much ARM (and in some way SPARC) specific. I think you need add some comments
in the code (And git commit) to explain this. Thank you.
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