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PCIe Hot Reset on Linux

Resets in PCI express are a bit complex. There are two main types of resets - conventional reset, and function-level reset. There are also two types of conventional resets, fundamental resets and non-fundamental resets. See the PCI express specification for all of the details.

A 'cold reset' is a fundamental reset that takes place after power is applied to a PCIe device. There appears to be no standard way of triggering a cold reset, save for turning the system off and back on again. On my machines, the /sys/bus/pci/slots directory is empty.

A 'warm reset' is a fundamental reset that is triggered without disconnecting power from the device. There appears to be no standard way of triggering a warm reset.

A 'hot reset' is a conventional reset that is triggered across a PCI express link. A hot reset is triggered either when a link is forced into electrical idle or by sending TS1 and TS2 ordered sets with the hot reset bit set. Software can initiate a hot reset by setting and then clearing the secondary bus reset bit in the bridge control register in the PCI configuration space of the bridge port upstream of the device.

A 'function-level reset' (FLR) is a reset that affects only a single function of a PCI express device. It must not reset the entire PCIe device. Implementing function-level resets is not required by the PCIe specification. A function-level reset is initiated by setting the initiate function-level reset bit in the function's device control register in the PCI express capability structure in the PCI configuration space.

Linux exposes the function-level reset functionality in the form of /sys/bus/pci/devices/ $dev/reset. Writing a 1 to this file will initiate a function-level reset on the corresponding function. Note that this only affects that specific function of the device, not the whole device, and devices are not required to implement function-level resets as per the PCIe specification. I am not aware of any 'nice' method for triggering a hot reset (there is no sysfs entry for that). However, it is possible to use setpci to do so with the following script: <code> #!/bin/bash dev=$ 1

if [ -z “ $dev" ]; then echo "Error: no device specified" exit 1 fi if [ ! -e "/sys/bus/pci/devices/$ dev” ]; then

  dev="0000:$dev"

fi

if [ ! -e “/sys/bus/pci/devices/ $dev" ]; then echo "Error: device$ dev not found”

  exit 1

fi

port= $(basename$ (dirname $(readlink "/sys/bus/pci/devices/$ dev“)))

if [ ! -e ”/sys/bus/pci/devices/ $port" ]; then echo "Error: device$ port not found“

  exit 1

fi

echo “Removing $dev..." echo 1 > "/sys/bus/pci/devices/$ dev/remove”

echo “Performing hot reset of port $port..." bc=$ (setpci -s $port BRIDGE_CONTROL) echo "Bridge control:"$ bc

setpci -s $port BRIDGE_CONTROL=$ (printf ”%04x” $(("0x$ bc“ | 0x40))) sleep 0.01 setpci -s $port BRIDGE_CONTROL=$ bc sleep 0.5

echo “Rescanning bus…”

echo 1 > ”/sys/bus/pci/devices/$port/rescan“

</code>

Ensure that all attached drivers are unloaded before running this script. This script will attempt to remove the PCIe device, then command the upstream switch port to issue a hot reset, then attempt to rescan the PCIe bus. This script has also only been tested on devices with a single function, so it may need some reworking for devices with multiple functions.