From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: [PATCH v5 12/13] swiotlb-xen: use xen_alloc/free_coherent_pages Date: Fri, 6 Sep 2013 10:17:54 -0400 Message-ID: <20130906141754.GT2590@phenom.dumpdata.com> References: <1377801154-29215-12-git-send-email-stefano.stabellini@eu.citrix.com> <20130830135333.GL21239@phenom.dumpdata.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: Stefano Stabellini Cc: xen-devel@lists.xensource.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Ian.Campbell@citrix.com List-Id: xen-devel@lists.xenproject.org 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.