linux-kernel - Re: [PATCH v2 17/17] drm/i915: Properly request PCI BARs

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Date:   Fri, 9 Oct 2020 13:41:54 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Kees Cook <keescook@...omium.org>,
        KVM list <kvm@...r.kernel.org>, Jason Gunthorpe <jgg@...pe.ca>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [PATCH v2 17/17] drm/i915: Properly request PCI BARs

On Fri, Oct 09, 2020 at 12:01:39PM +0200, Daniel Vetter wrote:
> On Fri, Oct 9, 2020 at 11:47 AM Ville Syrjälä
> <ville.syrjala@...ux.intel.com> wrote:
> >
> > On Fri, Oct 09, 2020 at 09:59:34AM +0200, Daniel Vetter wrote:
> > > When trying to test my CONFIG_IO_STRICT_DEVMEM changes I realized they
> > > do nothing for i915. Because i915 doesn't request any regions, like
> > > pretty much all drm pci drivers. I guess this is some very old
> > > remnants from the userspace modesetting days, when we wanted to
> > > co-exist with the fbdev driver. Which usually requested these
> > > resources.
> > >
> > > But makes me wonder why the pci subsystem doesn't just request
> > > resource automatically when we map a bar and a pci driver is bound?
> > >
> > > Knowledge about which pci bars we need kludged together from
> > > intel_uncore.c and intel_gtt.c from i915 and intel-gtt.c over in the
> > > fake agp driver.
> > >
> > > Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> > > Cc: Jason Gunthorpe <jgg@...pe.ca>
> > > Cc: Kees Cook <keescook@...omium.org>
> > > Cc: Dan Williams <dan.j.williams@...el.com>
> > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > Cc: John Hubbard <jhubbard@...dia.com>
> > > Cc: Jérôme Glisse <jglisse@...hat.com>
> > > Cc: Jan Kara <jack@...e.cz>
> > > Cc: Dan Williams <dan.j.williams@...el.com>
> > > Cc: linux-mm@...ck.org
> > > Cc: linux-arm-kernel@...ts.infradead.org
> > > Cc: linux-samsung-soc@...r.kernel.org
> > > Cc: linux-media@...r.kernel.org
> > > Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> > > Cc: linux-pci@...r.kernel.org
> > > ---
> > >  drivers/gpu/drm/i915/intel_uncore.c | 25 +++++++++++++++++++++++--
> > >  1 file changed, 23 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> > > index 54e201fdeba4..ce39049d8919 100644
> > > --- a/drivers/gpu/drm/i915/intel_uncore.c
> > > +++ b/drivers/gpu/drm/i915/intel_uncore.c
> > > @@ -1692,10 +1692,13 @@ static int uncore_mmio_setup(struct intel_uncore *uncore)
> > >       struct pci_dev *pdev = i915->drm.pdev;
> > >       int mmio_bar;
> > >       int mmio_size;
> > > +     int bar_selection;
> >
> > Signed bitmasks always make me uneasy. But looks like
> > that's what it is in the pci api. So meh.
> 
> Yeah it's surprising.
> 
> > > +     int ret;
> > >
> > >       mmio_bar = IS_GEN(i915, 2) ? 1 : 0;
> > > +     bar_selection = BIT (2) | BIT(mmio_bar);
> >                            ^
> > spurious space
> >
> > That's also not correct for gen2 I think.
> >
> > gen2:
> > 0 = GMADR
> > 1 = MMADR
> > 2 = IOBAR
> >
> > gen3:
> > 0 = MMADR
> > 1 = IOBAR
> > 2 = GMADR
> > 3 = GTTADR
> >
> > gen4+:
> > 0+1 = GTTMMADR
> > 2+3 = GMADR
> > 4 = IOBAR
> >
> > Maybe we should just have an explicit list of bars like that in a
> > comment?
> >
> > I'd also suggest sucking this bitmask calculation into a small helper
> > so you can reuse it for the release.
> 
> tbh I just hacked this up for testing. Given how almost no other drm
> driver does this, I'm wondering whether we should or not.
> 
> Also the only reason why I didn't just use the pci_request_regions
> helper is to avoid the vga ioport range, since that's managed by
> vgaarbiter.

VGA io range isn't part of any bar. Or do you mean just the io decode
enable bit in the pci command register? That should be just a matter
or pci_enable_device() vs. pci_enable_device_mem() I think. So nothing
to do with which bars we've requested IIRC.

> 
> So I think if we go for this for real we should:
> - register the vga ioport range in the vgaarbiter
> - have a pci_request_iomem_regions helper that grabs all mem bars
> - roll that out to all drm pci drivers
> 
> Or something like that. The other complication is when we resize the
> iobar. So not really sure what to do here.

We resize it?

-- 
Ville Syrjälä
Intel