Re: [PATCH] PCI: controller: dwc: Make PCI_IMX6 depend on PCIEPORTBUS

[Andrey Smirnov]

On Fri, Dec 7, 2018 at 5:11 AM Niklas Cassel <niklas.cassel@xxxxxxxxxx> wrote:
>
> On Thu, Dec 06, 2018 at 08:55:13PM -0800, Andrey Smirnov wrote:
> > On Thu, Dec 6, 2018 at 2:28 AM Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
> > >
> > > Am Mittwoch, den 05.12.2018, 23:45 -0800 schrieb Andrey Smirnov:
> > > > Building a kernel with CONFIG_PCI_IMX6=y, but CONFIG_PCIEPORTBUS=n
> > > > produces a system where built-in PCIE bridge (16c3:abcd) isn't bound
> > > > to pcieport driver. This, in turn, results in a PCIE bus that is
> > > > capable of enumerating attached PCIE device, but lacks functional
> > > > interrupt support.
> > >
> > > This is odd. AFAIK PCI port services are a totally optional thing and
> > > them being absent should not lead to a non-functional PCI bus. So I
> > > would really like to see some deeper analysis what is going on here.
> > >
> >
> > AFAICT, this is due to pcieport driver enabling MSI of the bridge
> > device (16c3:abcd) via pcie_port_device_register() ->
> > pcie_init_service_irqs() -> pcie_port_enable_irq_vec() -> etc.
> >
> > I did an experiment on a i.MX8MQ/PCIE -> i210 setup I have: I disabled
> > CONFIG_PCIEPORTBUS and hacked igb_main.c enough to make the i210
> > driver believe it should fall back onto legacy interrupts. Even
> > without pcieport present in the system, i210 worked as expected via
> > legacy interrupts, which seems to collaborate my conjecture above.
> >
> > Thanks,
> > Andrey Smirnov
>
> IIUC PCIEPORTBUS should not be needed for MSIs to work,
> it is only needed if you want e.g. PME or AER.
>
> The difference is that if PCIEPORTBUS is enabled, a MSI irq vector will be
> allocated for the Root Complex itself, so that it can send an irq when
> e.g. AER has detected an error.
>
>
> If we disregard that MSI handling is currently broken on DWC PCIe:
> https://marc.info/?l=linux-pci&m=154214986924244&w=2
> It is very possible to have MSIs on dragonboard 820c, which also
> uses the DWC PCIe controller, without having PCIEPORTBUS selected:
>
> # zcat /proc/config.gz | grep -E "PCIE_QCOM|PCIEPORTBUS"
> # CONFIG_PCIEPORTBUS is not set
> CONFIG_PCIE_QCOM=y
>
>
> # lspci -v -s 0000:00:00.0
> 0000:00:00.0 PCI bridge: Qualcomm Device 0104 (prog-if 00 [Normal decode])
>         ...
>         Capabilities: [50] MSI: Enable- Count=1/32 Maskable+ 64bit+
>
> # lspci -v -s 0000:01:00.0
> 0000:01:00.0 Network controller: Qualcomm Atheros QCA6174 802.11ac Wireless Network Adapter (rev 32)
>         ...
>         Capabilities: [50] MSI: Enable+ Count=1/8 Maskable+ 64bit-
>
>
> # cat /proc/interrupts | grep MSI
>  70:       5620          0          0          0   PCI-MSI 524288 Edge      ath10k_pci
>
> So perhaps this is a bug specific to imx6?
>

Yeah, that seems entirely plausible. I reached out to NXP via one of
the support channels to clarify. I'll report if I hear back from them.

Thanks,
Andrey Smirnov