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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Oct 02, 2013 at 06:31:57PM +0100, Stefano Stabellini wrote:
> 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

I think that is OK for the Xen-SWIOTLB case.

For the lib/swiotlb - would that mean that non-Xen-ARM would use the
SWIOTLB? If so, I am OK with that too.

> 
> 
> > > 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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