linux-kernel - Re: [PATCH] pci: tegra: add MSI dependency

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Date:   Wed, 14 Mar 2018 13:13:14 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Manikanta Maddireddy <mmaddireddy@...dia.com>,
        "open list:TEGRA ARCHITECTURE SUPPORT" <linux-tegra@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pci: tegra: add MSI dependency

On Wed, Mar 14, 2018 at 01:06:11PM +0100, Arnd Bergmann wrote:
> On Wed, Mar 14, 2018 at 12:45 PM, Thierry Reding
> <thierry.reding@...il.com> wrote:
> > On Tue, Mar 13, 2018 at 12:52:05PM +0100, Arnd Bergmann wrote:
> >> Building the tegra PCIe host driver without MSI results in a link
> >> failure:
> >>
> >> drivers/pci/host/pci-tegra.o:(.data+0x70): undefined reference to `pci_msi_unmask_irq'
> >> drivers/pci/host/pci-tegra.o:(.data+0x74): undefined reference to `pci_msi_mask_irq'
> >>
> >> This adds the same dependency that everyone else uses.
> >>
> >> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> >> ---
> >>  drivers/pci/host/Kconfig | 1 +
> >>  1 file changed, 1 insertion(+)
> >
> > I'm slightly concerned about the dependency. Not that I doubt its
> > correctness, but because it could mean that PCI_TEGRA is not visible in
> > the default configuration. The only reason why it is currently visible
> > is because PCI_MSI is selected by some symbols that also happen to be
> > enabled. However, what if at some point those symbols are disabled or
> > removed?
> >
> > Some architectures make sure that PCI_MSI is enabled by selecting it,
> > but that's risky, isn't it, because PCI_MSI is user-visible and could
> > therefore easily lead to conflicts.
> >
> > Enabling PCI_MSI in the arm64 defconfig would solve the issue and is
> > good enough for me. I've got a couple of changes to that defconfig in
> > the Tegra tree for v4.17-rc1, I can add a patch to enable PCI_MSI.
> >
> > Unless there are any objections.
> 
> I looked at it again and found that this on ARM64, PCI_MSI is always
> selected indirectly by ARM_GIC&&PCI, so there is no problem.
> 
> The build failure must have been on 32-bit ARM.

Okay, I had assumed that ARM_GIC_V2M (which selects PCI_MSI) was user-
visible and hence could be disabled. But it's not, and always enabled on
ARM64. On 32-bit we already explicitly enable PCI_MSI via the default
configurations.

I withdraw my concerns. I see that Lorenzo already applied the patch, so
just for the record:

Acked-by: Thierry Reding <treding@...dia.com>

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