RE: [PATCH V13 12/12] PCI: tegra: Add Tegra194 PCIe support

[Vidya Sagar <vidyas@nvidia.com>]

> -----Original Message-----
> From: devicetree-owner@vger.kernel.org <devicetree-owner@vger.kernel.org>
> On Behalf Of Bjorn Helgaas
> Sent: Wednesday, July 17, 2019 12:30 AM
> To: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Cc: Vidya Sagar <vidyas@nvidia.com>; robh+dt@kernel.org;
> mark.rutland@arm.com; thierry.reding@gmail.com; Jonathan Hunter
> <jonathanh@nvidia.com>; kishon@ti.com; catalin.marinas@arm.com;
> will.deacon@arm.com; jingoohan1@gmail.com;
> gustavo.pimentel@synopsys.com; digetx@gmail.com; Mikko Perttunen
> <mperttunen@nvidia.com>; linux-pci@vger.kernel.org;
> devicetree@vger.kernel.org; linux-tegra@vger.kernel.org; linux-
> kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org; Krishna Thota
> <kthota@nvidia.com>; Manikanta Maddireddy <mmaddireddy@nvidia.com>;
> sagar.tv@gmail.com
> Subject: Re: [PATCH V13 12/12] PCI: tegra: Add Tegra194 PCIe support
> 
> On Tue, Jul 16, 2019 at 12:22:25PM +0100, Lorenzo Pieralisi wrote:
> > On Sat, Jul 13, 2019 at 12:34:34PM +0530, Vidya Sagar wrote:
> 
> > > > > > So if the link is not up we still go ahead and make probe
> > > > > > succeed. What for ?
> > > > > We may need root port to be available to support hot-plugging of
> > > > > endpoint devices, so, we don't fail the probe.
> > > >
> > > > We need it or we don't. If you do support hotplugging of endpoint
> > > > devices point me at the code, otherwise link up failure means
> > > > failure to probe.
> > > Currently hotplugging of endpoint is not supported, but it is one of
> > > the use cases that we may add support for in future.
> >
> > You should elaborate on this, I do not understand what you mean,
> > either the root port(s) supports hotplug or it does not.
> >
> > > But, why should we fail probe if link up doesn't happen? As such,
> > > nothing went wrong in terms of root port initialization right?  I
> > > checked other DWC based implementations and following are not
> > > failing the probe pci-dra7xx.c, pcie-armada8k.c, pcie-artpec6.c,
> > > pcie-histb.c, pcie-kirin.c, pcie-spear13xx.c, pci-exynos.c,
> > > pci-imx6.c, pci-keystone.c, pci-layerscape.c
> > >
> > > Although following do fail the probe if link is not up.
> > > pcie-qcom.c, pcie-uniphier.c, pci-meson.c
> > >
> > > So, to me, it looks more like a choice we can make whether to fail
> > > the probe or not and in this case we are choosing not to fail.
> >
> > I disagree. I had an offline chat with Bjorn and whether link-up
> > should fail the probe or not depends on whether the root port(s) is
> > hotplug capable or not and this in turn relies on the root port "Slot
> > implemented" bit in the PCI Express capabilities register.
> 
> There might be a little more we can talk about in this regard.  I did bring up the
> "Slot implemented" bit, but after thinking about it more, I don't really think the
> host bridge driver should be looking at that.
> That's a PCIe concept, and it's really *downstream* from the host bridge itself.
> The host bridge is logically a device on the CPU bus, not the PCI bus.
> 
> I'm starting to think that the host bridge driver probe should be disconnected
> from question of whether the root port links are up.
> 
> Logically, the host bridge driver connects the CPU bus to a PCI root bus, so it
> converts CPU-side accesses to PCI config, memory, or I/O port transactions.
> Given that, the PCI core can enumerate devices on the root bus and downstream
> buses.
> 
> Devices on the root bus typically include Root Ports, but might also include
> endpoints, Root Complex Integrated Endpoints, Root Complex Event Collectors,
> etc.  I think in principle, we would want the host bridge probe to succeed so we
> can use these devices even if none of the Root Ports have a link.
> 
> If a Root Port is present, I think users will expect to see it in the "lspci" output,
> even if its downstream link is not up.  That will enable things like manually
> poking the Root Port via "setpci" for debug.  And if it has a connector, the
> generic pciehp should be able to handle hot-add events without any special help
> from the host bridge driver.
> 
> On ACPI systems there is no concept of the host bridge driver probe failing
> because of lack of link on a Root Port.  If a Root Port doesn't have an
> operational link, we still keep the pci_root.c driver, and we'll enumerate the
> Root Port itself.  So I tend to think DT systems should behave the same way, i.e.,
> the driver probe should succeed unless it fails to allocate resources or something
> similar.  I think this is analogous to a NIC or USB adapter driver, where the probe
> succeeds even if there's no network cable or USB device attached.
> 
> Bjorn
Thanks Bjorn for your valuable inputs. I hope we are good here to not power down host
even if there are no endpoints detected.

- Vidya Sagar