linux-kernel - Re: [PATCH] PCI: controller: dwc: Make PCI

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Date:   Fri, 7 Dec 2018 14:11:13 +0100
From:   Niklas Cassel <niklas.cassel@...aro.org>
To:     Andrey Smirnov <andrew.smirnov@...il.com>
Cc:     Lucas Stach <l.stach@...gutronix.de>, linux-pci@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Chris Healy <cphealy@...il.com>,
        Leonard Crestez <leonard.crestez@....com>,
        Dong Aisheng <aisheng.dong@....com>,
        Richard Zhu <hongxing.zhu@....com>, linux-imx@....com,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI: controller: dwc: Make PCI_IMX6 depend on PCIEPORTBUS

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@...gutronix.de> 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?


Kind regards,
Niklas