Re: [PATCH 02/10] swiotlb-xen: remove start_dma_addr

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 21 May 2020, Julien Grall wrote:
> Hi,
> 
> On 21/05/2020 00:45, Stefano Stabellini wrote:
> > From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > ---
> >   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).