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From: Manikanta Maddireddy <mmaddireddy@nvidia.com>
To: Bjorn Helgaas <helgaas@kernel.org>
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 17:06:10 +0530
Message-ID: <1039fbf2-24ad-c31c-93d9-663aab74a26a@nvidia.com> (raw)
In-Reply-To: <20190412145003.GE141472@google.com>


On 12-Apr-19 8:20 PM, Bjorn Helgaas wrote:
> [+cc Jingoo, Gustavo (dwc maintainers), Ley (altera), Michal (xilinx)]
>
> 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).

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

>
>> 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);
>>>>  }

  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 [this message]
2019-04-15 13:45           ` Thierry Reding
2019-04-15 13:52             ` Thierry Reding
2019-04-15 14:04           ` Bjorn Helgaas
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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