Re: [Xen-devel] [PATCH v6 12/19] swiotlb: don't assume that io_tlb_start-io_tlb_end is coherent

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 30 Sep 2013, Konrad Rzeszutek Wilk wrote:
> On Fri, Sep 27, 2013 at 05:10:00PM +0100, Stefano Stabellini wrote:
> > The swiotlb code has appropriate calls to dma_mark_clean in place for
> > buffers passed to swiotlb_map_page as an argument. However it assumes
> > that the swiotlb bounce buffer (io_tlb_start-io_tlb_end) is already
> > coherent and doesn't need any calls to dma_mark_clean.
> > 
> > On ARM the swiotlb bounce buffer is not coherent (the memory is
> > writealloc while it should be bufferable) and therefore we need to call
> > dma_mark_clean appropriately on the bounce buffer code paths too.
> > 
> > Note that most architecures have an empty dma_mark_clean implementation
> > anyway.
> 
> The other architecture that uses swiotlb is IA64 and that does have
> an implementation where it touches on page attributes.
> 
> Which means I have to figure out why my HP zx6000 won't boot with 3.11 now :-(
> 

Now this is a very thorny issue.

Honestly I don't like the dma_mark_clean interface very much: it's one
big hammer, when we actually need some finesse to handle coherency.

For example on ARM some devices might not need the dma_mark_clean call,
while others do. Calling it all the times is at the very best
inefficient and incorrect at worst.

I am thinking of calling the original map/unmap_page functions instead
(arm_dma_map_page or arm_coherent_dma_map_page in the arm case).
However in order to do that I would need to add more __get_dma_ops calls in
both lib/swiotlb.c and drivers/xen/swiotlb-xen.c


> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > ---
> >  lib/swiotlb.c |   13 ++++++++++---
> >  1 files changed, 10 insertions(+), 3 deletions(-)
> > 
> > diff --git a/lib/swiotlb.c b/lib/swiotlb.c
> > index 4e8686c..eb45d17 100644
> > --- a/lib/swiotlb.c
> > +++ b/lib/swiotlb.c
> > @@ -515,6 +515,7 @@ found:
> >             io_tlb_orig_addr[index+i] = orig_addr + (i << IO_TLB_SHIFT);
> >     if (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL)
> >             swiotlb_bounce(orig_addr, tlb_addr, size, DMA_TO_DEVICE);
> > +   dma_mark_clean(phys_to_virt(tlb_addr), size);
> >  
> >     return tlb_addr;
> >  }
> > @@ -547,7 +548,10 @@ void swiotlb_tbl_unmap_single(struct device *hwdev, 
> > phys_addr_t tlb_addr,
> >      * First, sync the memory before unmapping the entry
> >      */
> >     if (orig_addr && ((dir == DMA_FROM_DEVICE) || (dir == 
> > DMA_BIDIRECTIONAL)))
> > +   {
> > +           dma_mark_clean(phys_to_virt(tlb_addr), size);
> >             swiotlb_bounce(orig_addr, tlb_addr, size, DMA_FROM_DEVICE);
> > +   }
> >  
> >     /*
> >      * Return the buffer to the free list by setting the corresponding
> > @@ -587,17 +591,20 @@ void swiotlb_tbl_sync_single(struct device *hwdev, 
> > phys_addr_t tlb_addr,
> >  
> >     switch (target) {
> >     case SYNC_FOR_CPU:
> > -           if (likely(dir == DMA_FROM_DEVICE || dir == DMA_BIDIRECTIONAL))
> > +           if (likely(dir == DMA_FROM_DEVICE || dir == DMA_BIDIRECTIONAL)) 
> > {
> > +                   dma_mark_clean(phys_to_virt(tlb_addr), size);
> >                     swiotlb_bounce(orig_addr, tlb_addr,
> >                                    size, DMA_FROM_DEVICE);
> > +           }
> >             else
> >                     BUG_ON(dir != DMA_TO_DEVICE);
> >             break;
> >     case SYNC_FOR_DEVICE:
> > -           if (likely(dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL))
> > +           if (likely(dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL)) {
> >                     swiotlb_bounce(orig_addr, tlb_addr,
> >                                    size, DMA_TO_DEVICE);
> > -           else
> > +                   dma_mark_clean(phys_to_virt(tlb_addr), size);
> > +           } else
> >                     BUG_ON(dir != DMA_FROM_DEVICE);
> >             break;
> >     default:
> > -- 
> > 1.7.2.5
> > 
> 

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