Re: [PATCH 1/6] swiotlb: Add helper to know if it is in use for a specific device.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Aug 26, 2015 at 03:26:42PM -0400, Jerome Glisse wrote:
> On Wed, Aug 26, 2015 at 03:02:31PM -0400, Konrad Rzeszutek Wilk wrote:
> > On Wed, Aug 26, 2015 at 02:52:02PM -0400, jglisse@redhat.com wrote:
> > > From: Jérôme Glisse <jglisse@redhat.com>
> > > 
> > > Some device like GPU do things differently if swiotlb is in use. We
> > > use to rely on swiotlb_nr_tbl() to know if swiotlb was enabled or not
> > > but this is unreliable. Patch add a simple helpers to check if any of
> > 
> > Why is it unreliable?
> 
> Alex reported on irc that swiotlb_nr_tbl() returns non zero even if swiotlb
> is disabled. This seems to be due to ac2cbab21f318e19bc176a7f38a120cec835220f
> which cleanup swiotlb init and always allocate default size. Which i believe
> is a waste of memory. So we need to add a real helper to know if swiotlb is
> in use or not and we should not rely on expectation of some swiotlb value.

Ah right, that patch. That should have been part of the description
I believe.

> 
> > 
> > > the dma_ops associated with a device points to the swiotlb functions,
> > > making swiotlb check reliable for a device.
> > > 
> > > Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > > Cc: Alex Deucher <alexander.deucher@amd.com>
> > > Cc: Ben Skeggs <bskeggs@redhat.com>
> > > Cc: Dave Airlie <airlied@redhat.com>
> > > Cc: lkml@vger.kernel.org
> > > Cc: Daniel Vetter <daniel.vetter@intel.com>
> > > ---
> > >  include/linux/dma-mapping.h | 18 ++++++++++++++++++
> > >  1 file changed, 18 insertions(+)
> > > 
> > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> > > index ac07ff0..eac911e 100644
> > > --- a/include/linux/dma-mapping.h
> > > +++ b/include/linux/dma-mapping.h
> > > @@ -314,4 +314,22 @@ static inline int dma_mmap_writecombine(struct device *dev,
> > >  #define dma_unmap_len_set(PTR, LEN_NAME, VAL)    do { } while (0)
> > >  #endif
> > >  
> > > +
> > > +#ifdef CONFIG_SWIOTLB
> > > +static inline bool swiotlb_in_use(struct device *dev)
> > > +{
> > > +	struct dma_map_ops *ops = get_dma_ops(dev);
> > > +
> > > +	return (ops->map_sg == swiotlb_map_sg_attrs ||
> > > +		ops->unmap_sg == swiotlb_unmap_sg_attrs ||
> > > +		ops->map_page == swiotlb_map_page);
> > 
> > That won't work. What if we use xen-swiotlb which has different function
> > names?
> 
> I didn't thought about xen, always doing things differently, i think xen is
> just a matter of also testing for the xen function. I just wanted to have
> the helper in common code and only rely on common things, instead of having
> to add a per arch helper.

There has to be a better way. Perhaps you can expand SWIOTLB to actually
check if it is in use?
> 
> Cheers,
> Jérôme
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