Re: [PATCH v2] arm64: dma-mapping: Fix dma_mapping_error() when bypassing SWIOTLB

[Michael Zoran <mzoran@crowfest.net>]

On Thu, 2017-01-26 at 12:52 +0000, Will Deacon wrote:
> On Wed, Jan 25, 2017 at 06:31:31PM +0000, Robin Murphy wrote:
> > When bypassing SWIOTLB on small-memory systems, we need to avoid
> > calling
> > into swiotlb_dma_mapping_error() in exactly the same way as we
> > avoid
> > swiotlb_dma_supported(), because the former also relies on SWIOTLB
> > state
> > being initialised.
> > 
> > Under the assumptions for which we skip SWIOTLB,
> > dma_map_{single,page}()
> > will only ever return the DMA-offset-adjusted physical address of
> > the
> > page passed in, thus we can report success unconditionally.
> > 
> > Fixes: b67a8b29df7e ("arm64: mm: only initialize swiotlb when
> > necessary")
> > CC: stable@vger.kernel.org
> > CC: Jisheng Zhang <jszhang@marvell.com>
> > Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
> > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> > ---
> > 
> > v2: Get the return value the right way round this time... After
> > some
> >     careful reasoning it really is that simple.
> > 
> >  arch/arm64/mm/dma-mapping.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-
> > mapping.c
> > index e04082700bb1..1ffb7d5d299a 100644
> > --- a/arch/arm64/mm/dma-mapping.c
> > +++ b/arch/arm64/mm/dma-mapping.c
> > @@ -352,6 +352,13 @@ static int __swiotlb_dma_supported(struct
> > device *hwdev, u64 mask)
> >  	return 1;
> >  }
> >  
> > +static int __swiotlb_dma_mapping_error(struct device *hwdev,
> > dma_addr_t addr)
> > +{
> > +	if (swiotlb)
> > +		return swiotlb_dma_mapping_error(hwdev, addr);
> > +	return 0;
> > +}
> 
> I was about to apply this, but I'm really uncomfortable with the way
> that
> we call into swiotlb without initialising it. For example, if
> somebody
> passes swiotlb=noforce on the command line and all of our memory is
> DMA-able, then we don't call swiotlb_init but we will leave the DMA
> ops
> intact. On a dma_map_page, we then end up in swiotlb_map_page. If,
> for
> some reason or another, dma_capable fails (perhaps the address is out
> of
> range), then we call map_single which will return SWIOTLB_MAP_ERROR
> and subsequently phys_to_dma(dev, io_tlb_overflow_buffer);, which is
> exactly what swiotlb_dma_mapping_error checks for. Except it won't
> get the
> chance, because our swiotlb variable is false.
> 
> I can see three ways to resolve this:
> 
> 1. Revert the hack that skips SWIOTLB initialisation and pay the 64m
> price
>    (but this is configurable on the cmdline).
> 
> 2. Keep the hack, but instead of skipping initialisation altogether,
>    automatically adjust the bounce buffer size to a single entry.
> This
>    shouldn't ever get used, but will allow the error paths to work.
> 
> 3. We bite the bullet and implement some non-swiotlb DMA ops for the
> case
>    when SWIOTLB is not used.
> 
> Thoughts?
> 
> Will
> 

I'm learning about the DMA APIs since I'm new here and just trying to
understand...

On the RPI 3, all the memory is DMA able if I understand.  All the DMA
APIs needs to do is just flush the various caches.  

To keep things as simple as possible, why not just have a seperate dma-
ops table for the simple case where all the functions are no-ops except
for the needed cache flushing?