linux-kernel - Re: [PATCH 02/10] swiotlb-xen: remove start_dma

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Message-ID: <alpine.DEB.2.21.2005211243060.27502@sstabellini-ThinkPad-T480s>
Date:   Thu, 21 May 2020 20:55:18 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Julien Grall <julien@....org>
cc:     Stefano Stabellini <sstabellini@...nel.org>, jgross@...e.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>
Subject: Re: [PATCH 02/10] swiotlb-xen: remove start_dma_addr

On Thu, 21 May 2020, Julien Grall wrote:
> Hi,
> 
> On 21/05/2020 00:45, Stefano Stabellini wrote:
> > From: Stefano Stabellini <stefano.stabellini@...inx.com>
> > 
> > It is not strictly needed. Call virt_to_phys on xen_io_tlb_start
> > instead. It will be useful not to have a start_dma_addr around with the
> > next patches.
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
> > ---
> >   drivers/xen/swiotlb-xen.c | 5 +----
> >   1 file changed, 1 insertion(+), 4 deletions(-)
> > 
> > diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> > index a42129cba36e..b5e0492b07b9 100644
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -52,8 +52,6 @@ static unsigned long xen_io_tlb_nslabs;
> >    * Quick lookup value of the bus address of the IOTLB.
> >    */
> >   -static u64 start_dma_addr;
> > -
> >   /*
> >    * Both of these functions should avoid XEN_PFN_PHYS because phys_addr_t
> >    * can be 32bit when dma_addr_t is 64bit leading to a loss in
> > @@ -241,7 +239,6 @@ int __ref xen_swiotlb_init(int verbose, bool early)
> >   		m_ret = XEN_SWIOTLB_EFIXUP;
> >   		goto error;
> >   	}
> > -	start_dma_addr = xen_virt_to_bus(xen_io_tlb_start);
> >   	if (early) {
> >   		if (swiotlb_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs,
> >   			 verbose))
> > @@ -389,7 +386,7 @@ static dma_addr_t xen_swiotlb_map_page(struct device
> > *dev, struct page *page,
> >   	 */
> >   	trace_swiotlb_bounced(dev, dev_addr, size, swiotlb_force);
> >   -	map = swiotlb_tbl_map_single(dev, start_dma_addr, phys,
> > +	map = swiotlb_tbl_map_single(dev, virt_to_phys(xen_io_tlb_start),
> > phys,
> 
> xen_virt_to_bus() is implemented as xen_phys_to_bus(virt_to_phys()). Can you
> explain how the two are equivalent?

They are not equivalent. Looking at what swiotlb_tbl_map_single expects,
and also the implementation of swiotlb_init_with_tbl, I think
virt_to_phys is actually the one we want.

swiotlb_tbl_map_single compares the argument with __pa(tlb) which is
__pa(xen_io_tlb_start) which is virt_to_phys(xen_io_tlb_start).