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From: Bjorn Helgaas <helgaas@kernel.org>
To: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Cc: thierry.reding@gmail.com, robh+dt@kernel.org,
	mark.rutland@arm.com, jonathanh@nvidia.com,
	lorenzo.pieralisi@arm.com, vidyas@nvidia.com,
	linux-tegra@vger.kernel.org, linux-pci@vger.kernel.org,
	devicetree@vger.kernel.org, Jingoo Han <jingoohan1@gmail.com>,
	Gustavo Pimentel <gustavo.pimentel@synopsys.com>,
	Ley Foon Tan <lftan@altera.com>,
	Michal Simek <michal.simek@xilinx.com>
Subject: Re: [PATCH 22/30] PCI: tegra: Access endpoint config only if PCIe link is up
Date: Mon, 15 Apr 2019 09:04:30 -0500
Message-ID: <20190415140430.GK126710@google.com> (raw)
In-Reply-To: <1039fbf2-24ad-c31c-93d9-663aab74a26a@nvidia.com>

On Mon, Apr 15, 2019 at 05:06:10PM +0530, Manikanta Maddireddy wrote:
> On 12-Apr-19 8:20 PM, Bjorn Helgaas wrote:
> > On Fri, Apr 12, 2019 at 12:30:22PM +0530, Manikanta Maddireddy wrote:
> >> On 12-Apr-19 1:45 AM, Bjorn Helgaas wrote:
> >>> On Thu, Apr 11, 2019 at 10:33:47PM +0530, Manikanta Maddireddy wrote:
> >>>> Add PCIe link up check in config read and write callback functions
> >>>> before accessing endpoint config registers.
> >>>>  static int tegra_pcie_config_read(struct pci_bus *bus, unsigned int devfn,
> >>>>  				  int where, int size, u32 *value)
> >>>>  {
> >>>> +	struct tegra_pcie *pcie = bus->sysdata;
> >>>> +	struct pci_dev *bridge;
> >>>> +	struct tegra_pcie_port *port;
> >>>> +
> >>>>  	if (bus->number == 0)
> >>>>  		return pci_generic_config_read32(bus, devfn, where, size,
> >>>>  						 value);
> >>>>  
> >>>> +	bridge = pcie_find_root_port(bus->self);
> >>>> +
> >>>> +	list_for_each_entry(port, &pcie->ports, list)
> >>>> +		if (port->index + 1 == PCI_SLOT(bridge->devfn))
> >>>> +			break;
> >>>> +
> >>>> +	/* If there is no link, then there is no device */
> >>>> +	if (!tegra_pcie_link_status(port)) {
> >>> This is racy and you should avoid it if possible.  The link could go down
> >>> between calling tegra_pcie_link_status() and issuing the config read/write.
> >>>
> >>> If your driver is to be reliable, it must be able to handle any bad
> >>> consequence of issuing that config read/write anyway, so I think it's
> >>> better if it doesn't even bother checking whether the link is up.
> >> This change is made based on similar check present in dwc driver
> >> dw_pcie_valid_device(), reasons for making this change in Tegra might
> >> differ dwc.
> > Yes, you won't be surprised to learn that I don't like the similar
> > checks in dwc, altera, xilinx, and xilinx-nwl either :)  I raise this
> > issue every time I see it, but I can't remember if I've mentioned dwc
> > specifically.
> >
> > We need to either eradicate this pattern of checking for link up, or
> > include a comment about why it is absolutely necessary.
> 
> This patch is created to address below scenario in our downstream kernel,
> 1) Our platform has WiFi on one slot and GPU in another.
> 2) During WiFi OFF, link is put in L2 and it goes through hot reset
> when turning ON WiFi (since Tegra doesn't support hot-plug).
> 3) Whenever x11 server is started it scans the PCIe bus for video devices.
> Here PCIe configuration registers of all devices are read to find out
> all available video devices.
> 4) If "x11 server" started with WiFi OFF, then we are seeing "response
> decoding error"(Tegra AFI module specific error).

Probably happens with lspci too.  I guess this is when you try to read
the WiFi device config space?

> Best solution we came up with is to have link up check in config access
> callback functions.

Can you check for "response decoding error" in the config accessor and
return 0xffffffff if you see it?

> >> Intention here is to reduce the number of AER errors when device is
> >> falling off the bus or going through hot reset. So racy condition here is
> >> OK
> >
> > I'm not convinced about this.  The issues you mention need to be
> > solved in a generic way, not a tegra-specific way.
> >
> > We don't want to end up with code that silently avoids the config
> > access 99.99% of the time, but once in a blue moon, we lose the race
> > (the device stops responding after we've determined the link is up)
> > and the access causes a mysterious AER error that we have no way to
> > debug.
> >
> >>>> +		*value = 0xffffffff;
> >>>> +		return PCIBIOS_DEVICE_NOT_FOUND;
> >>>> +	}
> >>>> +
> >>>>  	return pci_generic_config_read(bus, devfn, where, size, value);
> >>>>  }

  parent reply index

Thread overview: 106+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-11 17:03 [PATCH 00/30] Enable Tegra PCIe root port features Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 01/30] soc/tegra: pmc: Export tegra_powergate_power_on() Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 02/30] PCI: tegra: Fix PCIe host power up sequence Manikanta Maddireddy
2019-04-15 11:01   ` Thierry Reding
2019-04-15 14:11     ` Manikanta Maddireddy
2019-04-15 14:30       ` Thierry Reding
2019-04-15 18:14         ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 03/30] PCI: tegra: Move REFCLK pad settings out of phy_power_on() Manikanta Maddireddy
2019-04-15 11:06   ` Thierry Reding
2019-04-15 14:20     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 04/30] PCI: tegra: Add PCIe Gen2 link speed support Manikanta Maddireddy
2019-04-15 11:21   ` Thierry Reding
2019-04-15 14:47     ` Manikanta Maddireddy
2019-04-15 15:36       ` Thierry Reding
2019-04-15 15:53         ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 05/30] PCI: tegra: Advertise PCIe Advanced Error Reporting (AER) capability Manikanta Maddireddy
2019-04-15 11:23   ` Thierry Reding
2019-04-15 14:49     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 06/30] PCI: tegra: Program UPHY electrical settings for Tegra210 Manikanta Maddireddy
2019-04-15 11:29   ` Thierry Reding
2019-04-15 14:55     ` Manikanta Maddireddy
2019-04-15 15:38       ` Thierry Reding
2019-04-11 17:03 ` [PATCH 07/30] PCI: tegra: Enable opportunistic update FC and ACK Manikanta Maddireddy
2019-04-15 11:30   ` Thierry Reding
2019-04-11 17:03 ` [PATCH 08/30] PCI: tegra: Disable AFI dynamic clock gating Manikanta Maddireddy
2019-04-15 11:32   ` Thierry Reding
2019-04-11 17:03 ` [PATCH 09/30] PCI: tegra: Process pending DLL transactions before entering L1 or L2 Manikanta Maddireddy
2019-04-15 11:33   ` Thierry Reding
2019-04-11 17:03 ` [PATCH 10/30] PCI: tegra: Enable PCIe xclk clock clamping Manikanta Maddireddy
2019-04-15 11:37   ` Thierry Reding
2019-04-15 14:58     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 11/30] PCI: tegra: Increase the deskew retry time Manikanta Maddireddy
2019-04-15 11:39   ` Thierry Reding
2019-04-15 14:58     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 12/30] PCI: tegra: Add SW fixup for RAW violations Manikanta Maddireddy
2019-04-11 20:01   ` Bjorn Helgaas
2019-04-12  5:59     ` Manikanta Maddireddy
2019-04-15 11:41       ` Thierry Reding
2019-04-15 11:45   ` Thierry Reding
2019-04-15 15:02     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 13/30] PCI: tegra: Update flow control threshold in Tegra210 Manikanta Maddireddy
2019-04-15 11:47   ` Thierry Reding
2019-04-15 15:05     ` Manikanta Maddireddy
2019-04-23  9:27       ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 14/30] PCI: tegra: Set target speed as Gen1 before link up Manikanta Maddireddy
2019-04-11 20:04   ` Bjorn Helgaas
2019-04-12  6:44     ` Manikanta Maddireddy
2019-04-12 14:35       ` Bjorn Helgaas
2019-04-15 10:43         ` Manikanta Maddireddy
2019-04-15 11:52   ` Thierry Reding
2019-04-15 15:12     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 15/30] PCI: tegra: Fix PLLE powerdown issue due to CLKREQ# signal Manikanta Maddireddy
2019-04-15 13:17   ` Thierry Reding
2019-04-15 15:14     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 16/30] PCI: tegra: Program AFI_CACHE* registers only for Tegra20 Manikanta Maddireddy
2019-04-15 13:20   ` Thierry Reding
2019-04-16 10:47     ` Manikanta Maddireddy
2019-04-16 16:11       ` Thierry Reding
2019-04-11 17:03 ` [PATCH 17/30] PCI: tegra: Use switch statements in tegra_pcie_isr() Manikanta Maddireddy
2019-04-15 13:25   ` Thierry Reding
2019-04-15 15:25     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 18/30] PCI: tegra: Change PRSNT_SENSE irq log to debug Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 19/30] PCI: tegra: Use legacy irq for port service drivers Manikanta Maddireddy
2019-04-15 13:35   ` Thierry Reding
2019-04-11 17:03 ` [PATCH 20/30] PCI: tegra: Add AFI_PEX2_CTRL reg offset as part of soc struct Manikanta Maddireddy
2019-04-15 13:31   ` Thierry Reding
2019-04-11 17:03 ` [PATCH 21/30] PCI: tegra: Add "pci" type check before parsing child device tree node Manikanta Maddireddy
2019-04-15 13:37   ` Thierry Reding
2019-04-15 15:30     ` Manikanta Maddireddy
2019-04-15 15:42       ` Thierry Reding
2019-04-11 17:03 ` [PATCH 22/30] PCI: tegra: Access endpoint config only if PCIe link is up Manikanta Maddireddy
2019-04-11 20:15   ` Bjorn Helgaas
2019-04-12  7:00     ` Manikanta Maddireddy
2019-04-12 14:50       ` Bjorn Helgaas
2019-04-15 11:36         ` Manikanta Maddireddy
2019-04-15 13:45           ` Thierry Reding
2019-04-15 13:52             ` Thierry Reding
2019-04-15 14:04           ` Bjorn Helgaas [this message]
2019-04-15 15:43             ` Manikanta Maddireddy
2019-04-23 20:24               ` Bjorn Helgaas
2019-04-11 17:03 ` [PATCH 23/30] dt-bindings: pci: tegra: Document PCIe DPD pinctrl optional prop Manikanta Maddireddy
2019-04-15 14:07   ` Thierry Reding
2019-04-15 15:48     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 24/30] arm64: tegra: Add PEX DPD states as pinctrl properties Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 25/30] PCI: tegra: Put PEX CLK & BIAS pads in DPD mode Manikanta Maddireddy
2019-04-15 14:11   ` Thierry Reding
2019-04-11 17:03 ` [PATCH 26/30] dt-bindings: pci: tegra: Document nvidia,plat-gpios optional prop Manikanta Maddireddy
2019-04-11 20:18   ` Bjorn Helgaas
2019-04-12  7:01     ` Manikanta Maddireddy
2019-04-15 14:16   ` Thierry Reding
2019-04-15 17:58     ` Manikanta Maddireddy
2019-04-16 15:34       ` Thierry Reding
2019-04-17 11:22         ` Manikanta Maddireddy
2019-04-17 15:19           ` Thierry Reding
2019-04-17 18:26             ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 27/30] PCI: tegra: Add support to configure platform GPIOs Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 28/30] dt-bindings: pci: tegra: Document nvidia,rst-gpio optional prop Manikanta Maddireddy
2019-04-15 14:20   ` Thierry Reding
2019-04-15 18:01     ` Manikanta Maddireddy
2019-04-29 18:33     ` Rob Herring
2019-04-11 17:03 ` [PATCH 29/30] PCI: tegra: Add support for GPIO based PCIe reset Manikanta Maddireddy
2019-04-15 14:20   ` Thierry Reding
2019-04-15 18:03     ` Manikanta Maddireddy
2019-04-11 17:03 ` [PATCH 30/30] PCI: tegra: Change link retry log level to INFO Manikanta Maddireddy
2019-04-15 14:23   ` Thierry Reding
2019-04-15 18:05     ` Manikanta Maddireddy

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