Re: [PATCH] PCI: tegra: limit MSI target address to 32-bit

[Thierry Reding <treding@nvidia.com>]

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On Fri, Nov 10, 2017 at 12:11:05AM +0530, Vidya Sagar wrote:
> 
> 
> On Thursday 09 November 2017 11:44 PM, Bjorn Helgaas wrote:
> > [+cc Lorenzo]
> > 
> > On Thu, Nov 09, 2017 at 12:48:14PM +0530, Vidya Sagar wrote:
> > > On Thursday 09 November 2017 02:55 AM, Bjorn Helgaas wrote:
> > > > On Mon, Nov 06, 2017 at 11:33:07PM +0530, Vidya Sagar wrote:
> > > > > limits MSI target address to only 32-bit region to enable
> > > > > some of the PCIe end points where only 32-bit MSIs
> > > > > are supported work properly.
> > > > > One example being Marvel SATA controller
> > > > > 
> > > > > Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
> > > > > ---
> > > > >   drivers/pci/host/pci-tegra.c | 2 +-
> > > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/pci/host/pci-tegra.c b/drivers/pci/host/pci-tegra.c
> > > > > index 1987fec1f126..03d3dcdd06c2 100644
> > > > > --- a/drivers/pci/host/pci-tegra.c
> > > > > +++ b/drivers/pci/host/pci-tegra.c
> > > > > @@ -1531,7 +1531,7 @@ static int tegra_pcie_enable_msi(struct tegra_pcie *pcie)
> > > > >   	}
> > > > >   	/* setup AFI/FPCI range */
> > > > > -	msi->pages = __get_free_pages(GFP_KERNEL, 0);
> > > > > +	msi->pages = __get_free_pages(GFP_DMA, 0);
> > > > >   	msi->phys = virt_to_phys((void *)msi->pages);
> > > > Should this be GFP_DMA32?  See the comment above the GFP_DMA
> > > > definition.
> > > looking at the comments for both GFP_DMA32 and GFP_DMA, I thought GFP_DMA32
> > > is the correct one to use, but, even with that I got >32-bit addresses.
> > > GFP_DMA always gives addresses in <4GB boundary (i.e. 32-bit).
> > > I didn't dig into it to find out why is this the case.
> > This sounds worth looking into (but maybe we don't need the
> > __get_free_pages() at all; see below).  Maybe there's some underlying
> > bug.  My laptop shows this, which looks like it might be related:
> > 
> >    Zone ranges:
> >      DMA      [mem 0x0000000000001000-0x0000000000ffffff]
> >      DMA32    [mem 0x0000000001000000-0x00000000ffffffff]
> >      Normal   [mem 0x0000000100000000-0x00000004217fffff]
> >      Device   empty
> > 
> > What does your machine show?
> I see following in my linux box
>  Zone ranges:
>    DMA      [mem 0x0000000000001000-0x0000000000ffffff]
>    DMA32    [mem 0x0000000001000000-0x00000000ffffffff]
>    Normal   [mem 0x0000000100000000-0x000000106effffff]
>    Device   empty
> 
> and following in my T210 Tegra platform
> Zone ranges:
>   DMA      [mem 0x0000000080000000-0x00000000ffffffff]
>   Normal   [mem 0x0000000100000000-0x000000017fffffff]

This seems to be happening because 64-bit ARM doesn't have the
ZONE_DMA32 Kconfig option, which seems to cause the DMA32 zone
to default to the normal zone (see include/linux/gfp.h).

That's very confusing in conjunction with the kerneldoc comment
for GFP_DMA32 because it isn't actually guaranteed to give you
32-bit addresses for !ZONE_DMA32.

Cc'ing Arnd who knows about these things.

Thierry

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