linux-kernel - Re: [RFC] Make AGP work with IOMMU

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Message-Id: <1248858068.19733.229.camel@macbook.infradead.org>
Date:	Wed, 29 Jul 2009 10:01:08 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Dave Airlie <airlied@...il.com>
Cc:	Zhenyu Wang <zhenyu.z.wang@...el.com>,
	linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [RFC] Make AGP work with IOMMU

On Wed, 2009-07-29 at 18:43 +1000, Dave Airlie wrote:
> On Wed, Jul 29, 2009 at 5:15 PM, David Woodhouse<dwmw2@...radead.org> wrote:
> > On Wed, 2009-07-29 at 16:28 +1000, Dave Airlie wrote:
> >> Yup pretty much we always got lucky, its not like AGP and IOMMU systems
> >> are a huge item, its really only Intel IGPs which use the AGP
> >> subsystem these days.
> >
> > Ah, really? One thing which was bothering me was what happens when I use
> > non-onboard graphics in one of these beasts -- are individual gfx
> > drivers going to need to be fixed too?
> 
> Yes, more than likely. I think radeon and radeon kms drivers dtrt, at
> least they call pci_map_single for the pages they use in their on-chip
> translation units, and the cards have been used in sparc64 and ppc64
> with iommu stuff happening.

It's going to perform a lot better if we use pci_map_sg() to map as many
pages as possible at once.

> nouveau might work when it gets upstream but it may not.
> 
> Other PCI and/or PCIE1x cards still around would be MGA and SiS based,
> and I suspect they would need work, but they might just work.
>
> nvidia/fglrx binary drivers, who knows.

... or cares.

> >
> >> >                cur_gatt = GET_GATT(addr);
> >> >                writel(agp_generic_mask_memory(agp_bridge,
> >> > -                       mem->pages[i], mem->type), cur_gatt+GET_GATT_OFF(addr));
> >> > +                                              phys_to_gart(page_to_phys(mem->pages[i])),
> >>
> >> don't suppose we want page_to_gart or is the double function nicer?
> >
> > I pondered that briefly. But then observed that phys_to_gart() and
> > gart_to_phys() _always_ describe an identity mapping, so perhaps they
> > could just be ditched completely?
> 
> Yeah that could work too, no idea why they were introduced, well before my time.
> 
> >
> >> > @@ -150,8 +150,17 @@ static int agp_backend_initialize(struct agp_bridge_data *bridge)
> >> >                }
> >> >
> >> >                bridge->scratch_page_real = phys_to_gart(page_to_phys(page));
> >> > -               bridge->scratch_page =
> >> > -                   bridge->driver->mask_memory(bridge, page, 0);
> >> > +               bridge->scratch_page = bridge->driver->mask_memory(bridge,
> >> > +                                          phys_to_gart(page_to_phys(page)), 0);
> >> > +
> >> > +               if (bridge->driver->agp_map_page &&
> >> > +                   bridge->driver->agp_map_page(phys_to_virt(page_to_phys(page)),
> >>
> >> and maybe page_to_virt.
> >
> > That's called page_address(), and it (as well as the above construct) is
> > broken with highmem pages. It's actually OK here, since this page is
> > allocated with GFP_DMA32 -- but for cleanliness' sake I should probably
> > switch agp_map_page() to take a 'struct page *' rather than a virtual
> > address.
> 
> Yes that might be best, having a function called map_page taking an
> address seems wrong.

http://git.infradead.org/users/dwmw2/iommu-agp.git?a=commitdiff;h=71fc2ae5

> >
> >> > +       if ((mem->page_count * sizeof(*mem->sg_list)) < 2*PAGE_SIZE)
> >> > +               mem->sg_list = kcalloc(mem->page_count, sizeof(*mem->sg_list),
> >> > +                                      GFP_KERNEL);
> >> > +
> >> > +       if (mem->sg_list == NULL) {
> >> > +               mem->sg_list = vmalloc(mem->page_count * sizeof(*mem->sg_list));
> >> > +               mem->sg_vmalloc_flag = 1;
> >>
> >> Can we drop vmalloc_flag and use is_vmalloc_addr on the free function?
> >
> > I suppose so -- we could eliminate the other vmalloc_flag field in
> > 'struct agp_memory' that way too? Doesn't shrink the structure any --
> > we'd just end up with padding where the flags were.
> 
> Yeah we should drop both of them maybe I can do that as a cleanup
> after this patch.
> 
> >
> >> (aside: yet another place that wants a kmalloc/vmalloc allocator. I suspect
> >> vmalloc here to be slow but I suppose there isn't much we can do.)
> >
> > http://lwn.net/Articles/342915/ ?
> >
> > In fact, can't scatterlists do something like that already?
> 
> no idea, I think I've seen vmalloc used in other places before.

This fixes it for scatterlists:
http://git.infradead.org/users/dwmw2/iommu-agp.git?a=commitdiff;h=89187864

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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