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Re: [Xen-devel] [RFC PATCH] xen/gntdev: Stop abusing DT of_dma_configure API

On 2019-09-26 11:17 am, Oleksandr Andrushchenko wrote:

On 9/26/19 12:49 PM, Julien Grall wrote:
Hi Rob,

On 9/25/19 10:50 PM, Rob Herring wrote:
As the comment says, this isn't a DT based device. of_dma_configure()
is going to stop allowing a NULL DT node, so this needs to be fixed.

And this can't work on arch not selecting CONFIG_OF and can select

We are lucky enough on x86 because, AFAICT, arch_setup_dma_ops is just
a nop.

No luck is needed as [1] does nothing for those platforms not using

Not sure exactly what setup besides arch_setup_dma_ops is needed...

We probably want to update dma_mask, coherent_dma_mask and

Also, while look at of_configure_dma, I noticed that we consider the
DMA will not be coherent for the grant-table. Oleksandr, do you know
why they can't be coherent?
The main and the only reason to use of_configure_dma is that if we don't
then we
are about to stay with dma_dummy_ops [2]. It effectively means that
operations on dma-bufs
will end up returning errors, like [3], [4], thus not making it possible
for Xen PV DRM and DMA
part of gntdev driver to do what we need (dma-bufs in our use-cases
allow zero-copying
while using graphics buffers and many more).

I didn't find any better way of achieving that, but of_configure_dma...
If there is any better solution which will not break the existing
functionality then
I will definitely change the drivers so we do not abuse DT )
Before that, please keep in mind that merging this RFC will break Xen PV
DMA buf support in gntdev...
Hope we can work out some acceptable solution, so everyone is happy

As I mentioned elsewhere, the recent dma-direct rework means that dma_dummy_ops are now only explicitly installed for the ACPI error case, so - much as I may dislike it - you should get regular (direct/SWIOTLB) ops by default again.

Coherency is trickier - if the guest is allocating buffers for the PV device, which may be shared directly with hardware by the host driver, then the coherency of the PV device should really reflect that of the underlying hardware to avoid potential problems. There are some cases where the stage 2 attributes alone wouldn't be enough to correct a mismatch.


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