Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

[Christian König]

Am 22.06.21 um 17:40 schrieb Oded Gabbay:
> On Tue, Jun 22, 2021 at 6:31 PM Christian König
> <christian.koenig@xxxxxxx> wrote:
> > Am 22.06.21 um 17:28 schrieb Jason Gunthorpe:
> > > On Tue, Jun 22, 2021 at 05:24:08PM +0200, Christian König wrote:
> > >
> > > > > > I will take two GAUDI devices and use one as an exporter and one as an
> > > > > > importer. I want to see that the solution works end-to-end, with real
> > > > > > device DMA from importer to exporter.
> > > > > I can tell you it doesn't. Stuffing physical addresses directly into
> > > > > the sg list doesn't involve any of the IOMMU code so any configuration
> > > > > that requires IOMMU page table setup will not work.
> > > > Sure it does. See amdgpu_vram_mgr_alloc_sgt:
> > > >
> > > >           amdgpu_res_first(res, offset, length, &cursor);
> > >            ^^^^^^^^^^
> > >
> > > I'm not talking about the AMD driver, I'm talking about this patch.
> > >
> > > +             bar_address = hdev->dram_pci_bar_start +
> > > +                             (pages[cur_page] - prop->dram_base_address);
> > > +             sg_dma_address(sg) = bar_address;
> > Yeah, that is indeed not working.
> >
> > Oded you need to use dma_map_resource() for this.
> >
> > Christian.
> Yes, of course.
> But will it be enough ?
> Jason said that supporting IOMMU isn't nice when we don't have struct pages.
> I fail to understand the connection, I need to dig into this.

Question is what you want to do with this?

A struct page is always needed if you want to do stuff like HMM with it, 
if you only want P2P between device I actually recommend to avoid it.

Christian.

> Oded
>
> > > Jason