Date:   Fri, 6 Jan 2023 09:38:40 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Huacai Chen <chenhuacai@...ngson.cn>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        linux-pci@...r.kernel.org, Jianmin Lv <lvjianmin@...ngson.cn>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Huacai Chen <chenhuacai@...il.com>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] PCI: Add quirk for LS7A to avoid reboot failure

[+cc Rafael, linux-pm, linux-kernel in case you have comments on
whether devices should still be usable after .remove()/.shutdown()]

On Fri, Jan 06, 2023 at 05:51:43PM +0800, Huacai Chen wrote:
> After cc27b735ad3a7557 ("PCI/portdrv: Turn off PCIe services during
> shutdown") we observe poweroff/reboot failures on systems with LS7A
> chipset.
> 
> We found that if we remove "pci_command &= ~PCI_COMMAND_MASTER" in
> do_pci_disable_device(), it can work well. The hardware engineer says
> that the root cause is that CPU is still accessing PCIe devices while
> poweroff/reboot, and if we disable the Bus Master Bit at this time, the
> PCIe controller doesn't forward requests to downstream devices, and also
> does not send TIMEOUT to CPU, which causes CPU wait forever (hardware
> deadlock).
> 
> To be clear, the sequence is like this:
> 
>   - CPU issues MMIO read to device below Root Port
> 
>   - LS7A Root Port fails to forward transaction to secondary bus
>     because of LS7A Bus Master defect
> 
>   - CPU hangs waiting for response to MMIO read
> 
> Then how is userspace able to use a device after the device is removed?
> 
> To give more details, let's take the graphics driver (e.g. amdgpu) as
> an example. The userspace programs call printf() to display "shutting
> down xxx service" during shutdown/reboot, or the kernel calls printk()
> to display something during shutdown/reboot. These can happen at any
> time, even after we call pcie_port_device_remove() to disable the pcie
> port on the graphic card.
> 
> The call stack is: printk() --> call_console_drivers() --> con->write()
> --> vt_console_print() --> fbcon_putcs()
> 
> This scenario happens because userspace programs (or the kernel itself)
> don't know whether a device is 'usable', they just use it, at any time.

Thanks for this background.  So basically we want to call .remove() on
a console device (or a bridge leading to it), but we expect it to keep
working as usual afterwards?

That seems a little weird.  Is that the design we want?  Maybe we
should have a way to mark devices so we don't remove them during
shutdown or reboot?

> This hardware behavior is a PCIe protocol violation (Bus Master should
> not be involved in CPU MMIO transactions), and it will be fixed in new
> revisions of hardware (add timeout mechanism for CPU read request,
> whether or not Bus Master bit is cleared).
> 
> On some x86 platforms, radeon/amdgpu devices can cause similar problems
> [1][2]. Once before I wanted to make a single patch to solve "all of
> these problems" together, but it seems unreasonable because maybe they
> are not exactly the same problem. So, this patch add a new function
> pcie_portdrv_shutdown(), a slight modified copy of pcie_portdrv_remove()
> dedicated for the shutdown path, and then add a quirk just for LS7A to
> avoid clearing Bus Master bit in pcie_portdrv_shutdown(). Leave other
> platforms behave as before.
> 
> [1] https://bugs.freedesktop.org/show_bug.cgi?id=97980
> [2] https://bugs.freedesktop.org/show_bug.cgi?id=98638
> 
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
>  drivers/pci/controller/pci-loongson.c | 17 +++++++++++++++++
>  drivers/pci/pcie/portdrv.c            | 21 +++++++++++++++++++--
>  include/linux/pci.h                   |  1 +
>  3 files changed, 37 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-loongson.c b/drivers/pci/controller/pci-loongson.c
> index 759ec211c17b..641308ba4126 100644
> --- a/drivers/pci/controller/pci-loongson.c
> +++ b/drivers/pci/controller/pci-loongson.c
> @@ -93,6 +93,24 @@ DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
>  DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
>  			DEV_PCIE_PORT_2, loongson_mrrs_quirk);
>  
> +static void loongson_bmaster_quirk(struct pci_dev *pdev)
> +{
> +	/*
> +	 * Some Loongson PCIe ports will cause CPU deadlock if there is
> +	 * MMIO access to a downstream device when the root port disable
> +	 * the Bus Master bit during poweroff/reboot.
> +	 */
> +	struct pci_host_bridge *bridge = pci_find_host_bridge(pdev->bus);
> +
> +	bridge->no_dis_bmaster = 1;
> +}
> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
> +			DEV_PCIE_PORT_0, loongson_bmaster_quirk);
> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
> +			DEV_PCIE_PORT_1, loongson_bmaster_quirk);
> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
> +			DEV_PCIE_PORT_2, loongson_bmaster_quirk);
> +
>  static void loongson_pci_pin_quirk(struct pci_dev *pdev)
>  {
>  	pdev->pin = 1 + (PCI_FUNC(pdev->devfn) & 3);
> diff --git a/drivers/pci/pcie/portdrv.c b/drivers/pci/pcie/portdrv.c
> index 2cc2e60bcb39..96f45c444422 100644
> --- a/drivers/pci/pcie/portdrv.c
> +++ b/drivers/pci/pcie/portdrv.c
> @@ -501,7 +501,6 @@ static void pcie_port_device_remove(struct pci_dev *dev)
>  {
>  	device_for_each_child(&dev->dev, NULL, remove_iter);
>  	pci_free_irq_vectors(dev);
> -	pci_disable_device(dev);
>  }
>  
>  /**
> @@ -727,6 +726,24 @@ static void pcie_portdrv_remove(struct pci_dev *dev)
>  	}
>  
>  	pcie_port_device_remove(dev);
> +
> +	pci_disable_device(dev);
> +}
> +
> +static void pcie_portdrv_shutdown(struct pci_dev *dev)
> +{
> +	struct pci_host_bridge *bridge = pci_find_host_bridge(dev->bus);
> +
> +	if (pci_bridge_d3_possible(dev)) {
> +		pm_runtime_forbid(&dev->dev);
> +		pm_runtime_get_noresume(&dev->dev);
> +		pm_runtime_dont_use_autosuspend(&dev->dev);
> +	}
> +
> +	pcie_port_device_remove(dev);
> +
> +	if (!bridge->no_dis_bmaster)
> +		pci_disable_device(dev);
>  }
>  
>  static pci_ers_result_t pcie_portdrv_error_detected(struct pci_dev *dev,
> @@ -777,7 +794,7 @@ static struct pci_driver pcie_portdriver = {
>  
>  	.probe		= pcie_portdrv_probe,
>  	.remove		= pcie_portdrv_remove,
> -	.shutdown	= pcie_portdrv_remove,
> +	.shutdown	= pcie_portdrv_shutdown,
>  
>  	.err_handler	= &pcie_portdrv_err_handler,
>  
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 3df2049ec4a8..a64dbcb89231 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -573,6 +573,7 @@ struct pci_host_bridge {
>  	unsigned int	ignore_reset_delay:1;	/* For entire hierarchy */
>  	unsigned int	no_ext_tags:1;		/* No Extended Tags */
>  	unsigned int	no_inc_mrrs:1;		/* No Increase MRRS */
> +	unsigned int	no_dis_bmaster:1;	/* No Disable Bus Master */
>  	unsigned int	native_aer:1;		/* OS may use PCIe AER */
>  	unsigned int	native_pcie_hotplug:1;	/* OS may use PCIe hotplug */
>  	unsigned int	native_shpc_hotplug:1;	/* OS may use SHPC hotplug */
> -- 
> 2.31.1
> 

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