PCIe Set Speed

This script uses setpci to change a device's target link speed. This can be useful when using a PCIe protocol analyzer that does not support PCIe gen 3. It can also be helpful for testing a device at a lower bandwidth.

#!/bin/bash
 
dev=$1
speed=$2
 
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
 
pciec=$(setpci -s $dev CAP_EXP+02.W)
pt=$((("0x$pciec" & 0xF0) >> 4))
 
port=$(basename $(dirname $(readlink "/sys/bus/pci/devices/$dev")))
 
if (($pt == 0)) || (($pt == 1)) || (($pt == 5)); then
    dev=$port
fi
 
lc=$(setpci -s $dev CAP_EXP+0c.L)
ls=$(setpci -s $dev CAP_EXP+12.W)
 
max_speed=$(("0x$lc" & 0xF))
 
echo "Link capabilities:" $lc
echo "Max link speed:" $max_speed
echo "Link status:" $ls
echo "Current link speed:" $(("0x$ls" & 0xF))
 
if [ -z "$speed" ]; then
    speed=$max_speed
fi
 
if (($speed > $max_speed)); then
    speed=$max_speed
fi
 
echo "Configuring $dev..."
 
lc2=$(setpci -s $dev CAP_EXP+30.L)
 
echo "Original link control 2:" $lc2
echo "Original link target speed:" $(("0x$lc2" & 0xF))
 
lc2n=$(printf "%08x" $((("0x$lc2" & 0xFFFFFFF0) | $speed)))
 
echo "New target link speed:" $speed
echo "New link control 2:" $lc2n
 
setpci -s $dev CAP_EXP+30.L=$lc2n
 
echo "Triggering link retraining..."
 
lc=$(setpci -s $dev CAP_EXP+10.L)
 
echo "Original link control:" $lc
 
lcn=$(printf "%08x" $(("0x$lc" | 0x20)))
 
echo "New link control:" $lcn
 
setpci -s $dev CAP_EXP+10.L=$lcn
 
sleep 0.1
 
ls=$(setpci -s $dev CAP_EXP+12.W)
 
echo "Link status:" $ls
echo "Current link speed:" $(("0x$ls" & 0xF))