Re: [PATCH kernel v4 0/4 repost] powerpc/ioda2: Yet another attempt to allow DMA masks between 32 and 59

[Christoph Hellwig <hch@infradead.org>]

On Mon, Jul 15, 2019 at 06:43:12PM +1000, Alexey Kardashevskiy wrote:
> > e.g. if you have a DMA that supports 40-bit DMA addressing we could
> > always treat it as if supports 32-bit addressing,and I thought the
> > powerpc code does that,
> 
> powerpc does that and this is what the patchset is changing as people
> complained that 2GB DMA window has bad effects on AMD GPUs (cannot allocate
> enough buffers) and 40/100Gbit devices (lower performance), I do not have
> the details handy.

Make sense.  I'm just surprised about the complains from the habalabs
folks, which sounded like a 40something bit DMA mask did not work at all
for them on power9, which did not fit my reading of the code.

> 
> > as the DMA API now relies on that.
> 
> Relies on what precisely? If a device cannot do full 64bit, then it has to
> be no more than just 32bit?

The fact that if say you iommu only supports mode that return up to
32-bit iova and a driver sets a 48 or 64-bit mask you still return
success instead of letting the driver handle the failure and set a
32-bit mask in the fallback code.  As said I think the powerpc code
is fine based on my reading from it.

> >  Did I miss
> > something and it explicitly rejected that (in which case I didn't spot
> > the fix in this series), or is this just an optimization to handle these
> > devices more optimally, in which case maybe the changelog could be
> > improved a bit.
> 
> 
> 4/4 did this essentially:

As long as the above is fine (which I think it is) just make it a little
mor clear that this is a simple optimization, not a bug fix for DMA API
usage.