linux-kernel - Re: [PATCH v6 12/19] swiotlb: don't assume that io_tlb_start-io_tlb

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Message-ID: <20131004132301.GB26673@phenom.dumpdata.com>
Date:	Fri, 4 Oct 2013 09:23:01 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, Ian.Campbell@...rix.com
Subject: Re: [PATCH v6 12/19] swiotlb: don't assume that
 io_tlb_start-io_tlb_end is coherent

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@...citrix.com>
> > > ---
> > >  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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