Re: [PATCH 4/8] dma-buf: add peer2peer flag

From: Daniel Vetter
Date: Fri Apr 20 2018 - 03:13:25 EST

On Thu, Apr 19, 2018 at 01:16:57AM -0700, Christoph Hellwig wrote:
> On Mon, Apr 16, 2018 at 03:38:56PM +0200, Daniel Vetter wrote:
> > We've broken that assumption in i915 years ago. Not struct page backed
> > gpu memory is very real.
> >
> > Of course we'll never feed such a strange sg table to a driver which
> > doesn't understand it, but allowing sg_page == NULL works perfectly
> > fine. At least for gpu drivers.
> For GPU drivers on x86 with no dma coherency problems, sure. But not
> all the world is x86. We already have problems due to dmabugs use
> of the awkward get_sgtable interface (see the common on
> arm_dma_get_sgtable that I fully agree with), and doing this for memory
> that doesn't have a struct page at all will make things even worse.

x86 dma isn't coherent either, if you're a GPU :-) Flushing gpu caches
tends to be too expensive, so there's pci-e support and chipset support to
forgo it. Plus drivers flushing caches themselves.

The dma_get_sgtable thing is indeed fun, right solution would probably be
to push the dma-buf export down into the dma layer. The comment for
arm_dma_get_sgtable is also not a realy concern, because dma-buf also
abstracts away the flushing (or well is supposed to), so there really
shouldn't be anyone calling the streaming apis on the returned sg table.
That's why dma-buf gives you an sg table that's mapped already.

> > If that's not acceptable then I guess we could go over the entire tree
> > and frob all the gpu related code to switch over to a new struct
> > sg_table_might_not_be_struct_page_backed, including all the other
> > functions we added over the past few years to iterate over sg tables.
> > But seems slightly silly, given that sg tables seem to do exactly what
> > we need.
> It isn't silly. We will have to do some surgery like that anyway
> because the current APIs don't work. So relax, sit back and come up
> with an API that solves the existing issues and serves us well in
> the future.

So we should just implement a copy of sg table for dma-buf, since I still
think it does exactly what we need for gpus?

Yes there's a bit a layering violation insofar that drivers really
shouldn't each have their own copy of "how do I convert a piece of dma
memory into dma-buf", but that doesn't render the interface a bad idea.
Daniel Vetter
Software Engineer, Intel Corporation