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

On Mon, Jul 15, 2019 at 06:43:12PM +1000, Alexey Kardashevskiy wrote:

quoted

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.

quoted

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.

quoted

 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.