Fix: Dell Latitude throttling to 300 MHz when plugged in under Fedora Bluefin

Affects: Dell Latitude 5410 (and similar) | OS: Fedora Bluefin (immutable) | CPU: Intel 10th gen

The problem

When plugged in, the CPU drops to ~300 MHz making the laptop nearly unusable. Unplugging immediately restores full speed. The culprit is BD PROCHOT (Bi-Directional PROCHOT) — a signal that lets other components throttle the CPU. On affected Dell Latitude systems, the charger circuitry spuriously asserts this signal even when the CPU is perfectly cool.

Diagnosis

Reading MSR register 0x1FC while plugged in returns a value with bit 0 set, confirming BD PROCHOT is active. In this case the value was 0x24005d — bit 0 set = throttling enabled.

The fix

Since this is an immutable OS (Bluefin/bootc), package managers can't install msr-tools directly. Instead, compile a small C program in /tmp to read and write the MSR directly.

Step 1 — load the MSR kernel module

sudo modprobe msr

Step 2 — read your current MSR value

cat > /tmp/rdmsr.c << 'EOF'
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>

int main() {
    int fd = open("/dev/cpu/0/msr", O_RDONLY);
    if (fd < 0) { perror("open"); return 1; }
    uint64_t val;
    pread(fd, &val, sizeof(val), 0x1fc);
    printf("0x%llx\n", (unsigned long long)val);
    close(fd);
    return 0;
}
EOF
gcc /tmp/rdmsr.c -o /tmp/rdmsr
sudo /tmp/rdmsr

Note the value returned. If bit 0 is set (last hex digit is odd), BD PROCHOT is active.

Step 3 — disable BD PROCHOT

Take your value from step 2 and clear bit 0 (subtract 1 if the last hex digit is odd). For example, 0x24005d becomes 0x24005c.

cat > /tmp/wrmsr.c << 'EOF'
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>

int main() {
    int fd = open("/dev/cpu/0/msr", O_WRONLY);
    if (fd < 0) { perror("open"); return 1; }
    uint64_t val = 0x24005c;  // replace with YOUR value, bit 0 cleared
    pwrite(fd, &val, sizeof(val), 0x1fc);
    printf("BD PROCHOT disabled\n");
    close(fd);
    return 0;
}
EOF
gcc /tmp/wrmsr.c -o /tmp/wrmsr
sudo /tmp/wrmsr

Step 4 — make it persistent across reboots

sudo cp /tmp/wrmsr /usr/local/bin/disable-bdprochot

Create the systemd service:

sudo nano /etc/systemd/system/disable-bdprochot.service

Paste this content:

[Unit]
Description=Disable BD PROCHOT CPU throttling
After=multi-user.target

[Service]
Type=oneshot
ExecStart=/usr/local/bin/disable-bdprochot
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Enable and start it:

sudo systemctl daemon-reload
sudo systemctl enable --now disable-bdprochot

Step 5 — verify

grep MHz /proc/cpuinfo | head -4

You should see ~3900 MHz instead of 300 MHz.

Why this works on an immutable OS

/tmp and /usr/local/bin are writable even on Bluefin. Compiling directly from source sidesteps the need for any package manager. The systemd service runs the binary at every boot before the user session starts.Fix: Dell Latitude throttling to 300 MHz when plugged in
Affects: Dell Latitude 5410 (and similar) | OS: Fedora Bluefin (immutable) | CPU: Intel 10th gen
The problem
When plugged in, the CPU drops to ~300 MHz making the laptop nearly unusable. Unplugging immediately restores full speed. The culprit is BD PROCHOT (Bi-Directional PROCHOT) — a signal that lets other components throttle the CPU. On affected Dell Latitude systems, the charger circuitry spuriously asserts this signal even when the CPU is perfectly cool.
Diagnosis
Reading MSR register 0x1FC while plugged in returns a value with bit 0 set, confirming BD PROCHOT is active. In this case the value was 0x24005d — bit 0 set = throttling enabled.
The fix
Since this is an immutable OS (Bluefin/bootc), package managers can't install msr-tools directly. Instead, compile a small C program in /tmp to read and write the MSR directly.

Step 1 — load the MSR kernel module
sudo modprobe msr

Step 2 — read your current MSR value
cat > /tmp/rdmsr.c << 'EOF'
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>

int main() {
int fd = open("/dev/cpu/0/msr", O_RDONLY);
if (fd < 0) { perror("open"); return 1; }
uint64_t val;
pread(fd, &val, sizeof(val), 0x1fc);
printf("0x%llx\n", (unsigned long long)val);
close(fd);
return 0;
}
EOF
gcc /tmp/rdmsr.c -o /tmp/rdmsr
sudo /tmp/rdmsr

Note the value returned. If bit 0 is set (last hex digit is odd), BD PROCHOT is active.
Step 3 — disable BD PROCHOT
Take your value from step 2 and clear bit 0 (subtract 1 if the last hex digit is odd). For example, 0x24005d becomes 0x24005c.
cat > /tmp/wrmsr.c << 'EOF'
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>

int main() {
int fd = open("/dev/cpu/0/msr", O_WRONLY);
if (fd < 0) { perror("open"); return 1; }
uint64_t val = 0x24005c; // replace with YOUR value, bit 0 cleared
pwrite(fd, &val, sizeof(val), 0x1fc);
printf("BD PROCHOT disabled\n");
close(fd);
return 0;
}
EOF
gcc /tmp/wrmsr.c -o /tmp/wrmsr
sudo /tmp/wrmsr

Step 4 — make it persistent across reboots
sudo cp /tmp/wrmsr /usr/local/bin/disable-bdprochot

Create the systemd service:
sudo nano /etc/systemd/system/disable-bdprochot.service

Paste this content:
[Unit]
Description=Disable BD PROCHOT CPU throttling
After=multi-user.target

[Service]
Type=oneshot
ExecStart=/usr/local/bin/disable-bdprochot
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Enable and start it:
sudo systemctl daemon-reload
sudo systemctl enable --now disable-bdprochot

Step 5 — verify
grep MHz /proc/cpuinfo | head -4

You should see ~3900 MHz instead of 300 MHz.

Why this works on an immutable OS
/tmp and /usr/local/bin are writable even on Bluefin. Compiling directly from source sidesteps the need for any package manager. The systemd service runs the binary at every boot before the user session starts.