linux-kernel - Re: RFC: sysfs node for Secondary PCI bus reset (PCIe Hot Reset)

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Date:   Mon, 1 Mar 2021 14:28:17 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Pali Rohár <pali@...nel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Marek Behún <kabel@...nel.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: RFC: sysfs node for Secondary PCI bus reset (PCIe Hot Reset)

[+cc Alex, reset expert]

On Mon, Mar 01, 2021 at 06:12:21PM +0100, Pali Rohár wrote:
> Hello!
> 
> PCIe card can be reset via in-band Hot Reset signal which can be
> triggered by PCIe bridge via Secondary Bus Reset bit in PCI config
> space.
> 
> Kernel already exports sysfs node "reset" for triggering Functional
> Reset of particular function of PCI device. But in some cases Functional
> Reset is not enough and Hot Reset is required.
> 
> Following RFC patch exports sysfs node "reset_bus" for PCI bridges which
> triggers Secondary Bus Reset and therefore for PCIe bridges it resets
> connected PCIe card.
> 
> What do you think about it?
> 
> Currently there is userspace script which can trigger PCIe Hot Reset by
> modifying PCI config space from userspace:
> 
> https://alexforencich.com/wiki/en/pcie/hot-reset-linux
> 
> But because kernel already provides way how to trigger Functional Reset
> it could provide also way how to trigger PCIe Hot Reset.
> 
> 
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 50fcb62d59b5..f5e11c589498 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -1321,6 +1321,30 @@ static ssize_t reset_store(struct device *dev, struct device_attribute *attr,
>  
>  static DEVICE_ATTR(reset, 0200, NULL, reset_store);
>  
> +static ssize_t reset_bus_store(struct device *dev, struct device_attribute *attr,
> +			       const char *buf, size_t count)
> +{
> +	struct pci_dev *pdev = to_pci_dev(dev);
> +	unsigned long val;
> +	ssize_t result = kstrtoul(buf, 0, &val);
> +
> +	if (result < 0)
> +		return result;
> +
> +	if (val != 1)
> +		return -EINVAL;
> +
> +	pm_runtime_get_sync(dev);
> +	result = pci_bridge_secondary_bus_reset(pdev);
> +	pm_runtime_put(dev);
> +	if (result < 0)
> +		return result;
> +
> +	return count;
> +}
> +
> +static DEVICE_ATTR(reset_bus, 0200, NULL, reset_bus_store);
> +
>  static int pci_create_capabilities_sysfs(struct pci_dev *dev)
>  {
>  	int retval;
> @@ -1332,8 +1356,15 @@ static int pci_create_capabilities_sysfs(struct pci_dev *dev)
>  		if (retval)
>  			goto error;
>  	}
> +	if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE) {
> +		retval = device_create_file(&dev->dev, &dev_attr_reset_bus);
> +		if (retval)
> +			goto error_reset_bus;
> +	}
>  	return 0;
>  
> +error_reset_bus:
> +	device_remove_file(&dev->dev, &dev_attr_reset);
>  error:
>  	pcie_vpd_remove_sysfs_dev_files(dev);
>  	return retval;
> @@ -1414,6 +1445,8 @@ static void pci_remove_capabilities_sysfs(struct pci_dev *dev)
>  		device_remove_file(&dev->dev, &dev_attr_reset);
>  		dev->reset_fn = 0;
>  	}
> +	if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE)
> +		device_remove_file(&dev->dev, &dev_attr_reset_bus);
>  }
>  
>  /**