using linux rt kernel on pi4 is ok using framebuffer gui app or probablz bad ide for user experience? even if had cpu pinned_

It can be totally OK, but it depends on what you mean by “GUI app” and how hard you’re driving the display stack.

When it’s usually fine (good UX possible)

Simple framebuffer rendering (direct /dev/fb0, SDL in fbdev mode, Qt/EGLFS without a full desktop compositor).
Static-ish UI (dashboards, HMI screens, menus, status panels) with modest animations.
You care more about predictable timing than peak graphics throughput.
You pin CPU cores and also isolate them (more below).

In these cases, an RT kernel on a Pi 4 can still feel snappy, especially if you avoid a heavyweight desktop environment.

When it often becomes a UX pain

You run a full desktop + compositor (X11 + GNOME/KDE, or Wayland compositor) and expect “normal PC-like” smoothness.
You rely on GPU acceleration paths that are more finicky on RT + Pi drivers/firmware combos (tearing, odd latencies, occasional stalls).
Your UI needs smooth 60fps animations, video playback, browser-based UI, etc.

A PREEMPT_RT kernel can introduce extra overhead and different scheduling behavior that doesn’t automatically improve “feel” for graphics; sometimes it makes frame pacing less consistent if the graphics stack competes with RT threads.

CPU pinning helps, but it’s not the whole story

Even with a pinned CPU, you can still get jitter/lag from:

GPU/firmware scheduling (Pi’s graphics pipeline can block in ways your RT threads can’t control)
DRM/KMS + vsync/flip waits
Interrupt storms (USB, network, SDIO/Wi-Fi) landing on your “UI core”
Memory bandwidth contention (one busy core can still hurt others)

If you want RT and decent UI, the trick is partitioning:

Practical setup that usually works well

Reserve one core for UI + graphics + IRQs (often Core 0), and isolate the others for RT control loops.
Use:
- isolcpus=, nohz_full=, rcu_nocbs= for RT cores
- keep most interrupts off RT cores via /proc/irq/*/smp_affinity
Put RT threads on isolated cores with SCHED_FIFO, but keep GUI threads SCHED_OTHER or at most modest SCHED_RR (don’t let the GUI preempt hard real-time work).
Prefer DRM/KMS direct scanout where possible; avoid compositors.
Avoid SD card I/O during interaction; use SSD/USB or tmpfs for logs if you’re hammering disk.

Framebuffer specifically: good or bad?

Old-school fbdev (/dev/fb0):

✅ Simple, deterministic-ish
✅ No compositor overhead
❌ Can tear (no proper pageflip/vsync)
❌ CPU-heavy if you repaint large areas
❌ Some modern libs are moving away from fbdev

DRM/KMS (recommended on Pi 4):

✅ Better display pipeline, page flipping, vsync options
✅ Works well with Qt EGLFS / SDL KMSDRM
❌ Slightly more complex, and drivers matter

So: framebuffer can be fine for HMI-style UIs, but for “good UX” you often want DRM/KMS direct rendering rather than raw fbdev.

My rule of thumb

If this is an embedded appliance UI (buttons, graphs, status): RT + (preferably) DRM/KMS direct rendering is fine.
If this is a desktop-like UX (browser, fancy animations, multitasking): RT kernel is often a net negative on perceived smoothness—better to run a standard kernel and isolate the real-time part another way (or even split across processes/MCUs).

If you tell me:

what GUI stack (Qt/SDL/GTK/browser?),
whether you’re on fbdev vs DRM/KMS, and
your real-time workload (rates/latency target),
I’ll give you a concrete “best architecture” for Pi 4 (core layout + kernel params + what to avoid).

why? So: framebuffer can be fine for HMI-style UIs, but for “good UX” you often want DRM/KMS direct rendering rather than raw fbdev.

Good question — the difference is mostly about how frames reach the display and who controls timing.

Here’s what really changes between fbdev (/dev/fb0) and DRM/KMS on a Pi 4.

1️⃣ fbdev: “memory dump to screen”

With fbdev you:

mmap /dev/fb0
Write pixels into that memory
The display controller scans it out continuously

That’s it. Very simple.

Problems for UX

❌ No proper page flipping

You usually draw into the same buffer that the display is currently scanning out.

If you update mid-scan:

Top half of the screen = old frame
Bottom half = new frame
→ tearing

There’s no clean “present this frame at next VSync” mechanism.

You can fake double buffering, but:

It’s not standardized
VSync control is crude or absent
Many drivers don’t expose proper flip sync via fbdev

❌ No real GPU/display scheduling

fbdev doesn’t:

Manage planes
Manage atomic updates
Handle overlay composition
Coordinate timing with hardware

So timing is “best effort”.

On RT kernels this gets worse because:

Your RT threads can delay GUI drawing
GUI drawing can delay scanout preparation
No atomic commit guarantees

❌ High CPU bandwidth usage

Every redraw:

Copies full frame (if not careful)
Competes for memory bandwidth
Can interfere with RT workloads

Pi 4 shares RAM between CPU and GPU — so this matters.

2️⃣ DRM/KMS: real display pipeline control

DRM/KMS is the modern Linux display system.

It gives you:

✅ Real page flipping

You render into buffer A
Display scans out buffer B
At VSync → atomic flip

No tearing. Clean frame boundaries.

✅ VSync-aware presentation

You can:

Block until next VBlank
Queue a page flip
Get event callbacks

This gives you stable frame pacing:

60 fps actually means 16.6 ms cadence
Animations feel smooth
Input-to-display latency becomes predictable

That’s what people perceive as “good UX”.

✅ Atomic updates

Modern KMS supports:

Atomic commits
Multiple planes
Partial updates

That means:

UI elements can be hardware layers
Video overlays don’t require full redraw
Less memory bandwidth
More deterministic timing

✅ Better driver support going forward

fbdev is basically:

“legacy compatibility layer”

On Pi 4 especially:

Full KMS driver is the intended path
fbdev may be emulated on top of DRM
Some fbdev paths are poorly maintained

3️⃣ Why this matters more on RT kernels

RT kernels:

Increase scheduler determinism
Increase locking overhead
Change interrupt behavior

This can:

Add micro-latency to drawing
Increase jitter in naive framebuffer apps

With DRM/KMS:

Frame presentation is hardware-scheduled
Even if your draw thread is slightly late, flip still aligns with VSync
Much more stable visual cadence

With fbdev:

Presentation timing = “whenever you happened to write memory”
Jitter becomes visible

4️⃣ When fbdev is actually fine

fbdev is fine when:

UI mostly static
No animations
No smooth scrolling
No video
Low refresh demands
Industrial HMI with occasional redraws

In those cases:

Simplicity wins
Less stack complexity
Fewer driver moving parts

5️⃣ Why users feel the difference

Humans are extremely sensitive to:

Frame pacing irregularity
Micro stutter
Tearing
Input-to-display latency inconsistency

Even if average FPS is identical:

fbdev	DRM/KMS
60fps average	60fps paced
Random flip timing	VSync aligned
Tearing possible	Tear-free
CPU-tied	Hardware flip

The second feels “polished”.
The first feels “embedded”.

6️⃣ If you want the best combo on Pi4 + RT

For HMI + real-time control:

Use full KMS (vc4-kms-v3d)
Use Qt EGLFS, SDL KMSDRM, or direct GBM
Keep GUI SCHED_OTHER
Isolate RT cores
Leave one core for display + IRQs

That gives:

Deterministic control loop
Tear-free UI
Stable animations
No desktop compositor overhead

Simple summary

fbdev = dumb framebuffer memory
DRM/KMS = hardware display pipeline control

For “it works” → fbdev is fine.
For “feels smooth and modern” → DRM/KMS wins.

If you tell me your GUI stack (Qt? SDL? custom OpenGL?) I can tell you exactly which backend to use on Pi 4 + RT for best results.

what is VSync? im not really familiar with kms.drm

Great — let’s explain this in simple, physical terms first.

What is VSync?

VSync = Vertical Synchronization

It means:

Only change the displayed frame at the exact moment the monitor finishes drawing the current frame.

To understand that, you need to know how a screen refreshes.

How a display actually draws

Even modern LCDs refresh line by line:

It draws row 1
Then row 2
Then row 3
…
Until the bottom
Then it jumps back to the top

On a 60Hz display, this whole process happens 60 times per second.

That means:

One full refresh takes ~16.6ms
The display is constantly scanning

What happens without VSync

If your app writes new pixels while the display is in the middle of scanning:

Top half of screen = old frame
Bottom half = new frame

You see a horizontal split line.

That’s called:

Screen tearing

It looks like the image is cut and misaligned.

This happens a lot with raw framebuffer apps.

What VSync does

With VSync:

Your program renders into a back buffer
The display keeps scanning the current front buffer
When the screen reaches the bottom (vertical blank period)
The system swaps the buffers instantly

So the entire next refresh uses the new frame.

Result:

No tearing
Clean motion
Stable animation

What is "vertical blank"?

After the display finishes drawing the last line,
there is a tiny pause before it jumps back to the top.

That pause is called:

Vertical blank (VBlank)

That is the only safe moment to swap frames.

Where DRM/KMS comes in

Now let’s connect this to Linux.

Old framebuffer (/dev/fb0)

You draw directly into memory the screen is reading
No real VSync control
No guaranteed clean swap
You can’t schedule flips properly

So tearing is common.

DRM/KMS (modern Linux display system)

DRM = Direct Rendering Manager
KMS = Kernel Mode Setting

It controls:

Display resolution
Refresh rate
Buffers
Page flipping
VSync timing

With DRM/KMS you can say:

“Here is my next frame — show it at the next VBlank.”

The kernel schedules that in hardware.

This is how smooth animation is achieved.

Why this matters for UX

Without VSync:

Motion looks unstable
Lines break during movement
Scrolling feels “cheap”

With VSync:

Motion is continuous
Animations feel solid
Feels like a phone or desktop

Even if FPS is the same.

Simple analogy

Imagine changing a billboard:

Without VSync:

You replace half the poster while people are looking at it.

With VSync:

You wait until nobody is looking, then replace it instantly.

Why RT kernel complicates this

Real-time scheduling can:

Delay rendering threads slightly
Change timing behavior

If you don't have proper VSync + page flipping,
those timing differences become visible as tearing or jitter.

With DRM/KMS:

Frame presentation timing is handled by hardware
Much more stable visually

Extremely short summary

VSync = “Only switch frames between refreshes.”

fbdev = no proper control of that
DRM/KMS = full control of that

If you want, I can also explain:

Double buffering
Page flipping
How to check if Pi 4 is using full KMS
Or how to enable proper KMS mode

Just tell me what you’re running on the Pi 🙂

yes maybe some demo with some animation like using imu sensor as an inpput so l can do some virtual leveling?

Nice — that’s actually a perfect use case for DRM/KMS + VSync:

IMU → roll/pitch → animated horizon line (virtual leveling display)

Let’s build this in a practical, Pi-friendly way.

🧭 Goal

Read IMU (roll angle)
Draw artificial horizon
Smooth animation
Tear-free
Works on Pi 4 (no desktop needed)

Best stack for this:

SDL2 + KMSDRM backend
(or Qt EGLFS if you prefer Qt)

I’ll show SDL2 because it’s lightweight and clean.

🧱 Architecture

IMU (I2C/SPI) ↓ Sensor thread (SCHED_FIFO optional) ↓ Shared atomic roll value ↓ Render loop (VSync enabled) ↓ DRM/KMS page flip

Render loop stays normal priority.
Sensor loop can be RT if needed.

🎯 What we’ll render

A simple artificial horizon:

Blue sky
Brown ground
White horizon line
Rotates based on roll angle

🧪 Simple SDL2 Demo (with fake IMU first)

First test animation without hardware.

Compile with:

sudo apt install libsdl2-dev

Run with:

SDL_VIDEODRIVER=kmsdrm ./horizon

Example (C++ SDL2)

#include <SDL2/SDL.h> #include <cmath> #include <atomic> int main() { SDL_Init(SDL_INIT_VIDEO); SDL_Window* window = SDL_CreateWindow( "Virtual Level", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 800, 480, SDL_WINDOW_SHOWN ); SDL_Renderer* renderer = SDL_CreateRenderer( window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC ); std::atomic<float> roll = 0.0f; bool running = true; Uint32 last = SDL_GetTicks(); while (running) { SDL_Event e; while (SDL_PollEvent(&e)) { if (e.type == SDL_QUIT) running = false; } // Fake IMU (slow oscillation) float t = SDL_GetTicks() / 1000.0f; roll = 30.0f * sin(t); SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255); SDL_RenderClear(renderer); int w, h; SDL_GetRendererOutputSize(renderer, &w, &h); SDL_Rect sky = {0, 0, w, h / 2}; SDL_SetRenderDrawColor(renderer, 50, 100, 255, 255); SDL_RenderFillRect(renderer, &sky); SDL_Rect ground = {0, h / 2, w, h / 2}; SDL_SetRenderDrawColor(renderer, 120, 70, 15, 255); SDL_RenderFillRect(renderer, &ground); // Draw horizon line rotated SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255); float angle = roll.load(); float rad = angle * M_PI / 180.0f; int cx = w / 2; int cy = h / 2; int len = 400; int x1 = cx - len * cos(rad); int y1 = cy - len * sin(rad); int x2 = cx + len * cos(rad); int y2 = cy + len * sin(rad); SDL_RenderDrawLine(renderer, x1, y1, x2, y2); SDL_RenderPresent(renderer); } SDL_Quit(); return 0; }

✅ Why this works well on Pi4

SDL_RENDERER_PRESENTVSYNC enables VSync
Using KMSDRM backend → direct DRM page flipping
No X11
No compositor
Tear-free

🧠 Now connect real IMU

If you're using something like:

MPU6050
ICM20948
BNO055

You typically:

1️⃣ Run IMU thread

void imu_thread() { while (true) { float new_roll = read_imu_roll(); roll.store(new_roll, std::memory_order_relaxed); usleep(5000); // 200Hz } }

Run this with:

pthread_setschedparam(... SCHED_FIFO ...)

Pin to RT core.

2️⃣ Render loop stays normal priority

Do NOT make renderer SCHED_FIFO.

You want:

RT sensor deterministic
Renderer cooperative
DRM handles VSync timing

🎛 Optional Improvements

Add pitch (vertical movement of horizon)

Move center line up/down based on pitch:

int pitch_offset = pitch * 5; cy += pitch_offset;

Add smoothing (important for UX)

IMUs are noisy.

Simple low-pass filter:

filtered = filtered * 0.9f + raw * 0.1f;

This massively improves perceived smoothness.

🧩 Why DRM/KMS helps here

If you tried this with raw /dev/fb0:

Rotating line would tear
Motion would look unstable
Frame pacing inconsistent

With KMS:

Each frame swaps at VBlank
Rotation feels solid
Much more “professional”

⚙️ How to verify you’re using full KMS on Pi

In /boot/config.txt:

dtoverlay=vc4-kms-v3d

NOT:

vc4-fkms-v3d

Then reboot.

Check:

lsmod | grep vc4

🏁 If You Want Next Level

I can also show:

OpenGL ES version (GPU accelerated rotation)
Qt version
Bare-metal DRM example without SDL
Proper core isolation setup for RT + UI
Latency measurement method

Tell me:

Which IMU?
C++ or Python?
RT loop frequency target?

And I’ll tailor it exactly to your setup.

im using the bmi160 and should use the hdmi monitor to keep it simple prototype RT is not importatnt but maybe can do a jitter logging so in case install/compile with PREEMPT_RT image can compare?

Yep — that’s a solid prototype plan: BMI160 + HDMI, and then you can log jitter and compare a normal kernel vs a PREEMPT_RT image later.

Below is a practical approach that gives you useful numbers without turning the project into a kernel research rabbit hole.

What to measure (so the comparison is meaningful)

You’ll get the most insight by logging two loops separately:

Sensor loop jitter (how regularly you can read BMI160)
Measure dt_sensor = t_now - t_prev for each IMU read
Log min/avg/max + percentiles
Render loop jitter (frame pacing)
Measure dt_frame between SDL_RenderPresent() calls (or swap-buffers)
This shows how “smooth” the animation timing is

Optional (nice-to-have):
3) Input-to-frame latency estimate

Timestamp when you read IMU sample t_imu
Timestamp when you present a frame using that sample t_present
Log t_present - t_imu (this is what users “feel”)

Recommended stack for HDMI “keep it simple”

Use DRM/KMS path (tear-free) with SDL2.
Run without a full desktop if you want (but can also run under X/Wayland for early dev).

For KMS direct on console:

SDL_VIDEODRIVER=kmsdrm ./app

BMI160: simplest way to read it

Two common paths:

A) Linux IIO driver (nice if your kernel exposes it)

If BMI160 is exposed under /sys/bus/iio/devices/iio:deviceX/ you can read accel/gyro channels from sysfs or buffered IIO. (Buffered is better.)

B) User-space I2C read (fast to prototype)

Talk to it via /dev/i2c-*. This is straightforward and portable, and fine for jitter comparisons as long as you keep it consistent between kernels.

(If you tell me whether you’re using I2C or SPI and which address, I can drop in a ready-to-run BMI160 read snippet.)

Jitter logging recipe (works for both kernels)

Use a monotonic clock

In C/C++:

clock_gettime(CLOCK_MONOTONIC_RAW, ...) (best for jitter measurements)

Log CSV

Write lines like:

t_ns, dt_sensor_ns, dt_frame_ns, roll_deg, latency_ns

Then you can analyze with Python/pandas later.

A minimal measurement structure (C++-ish pseudocode)

Sensor thread at fixed target rate (e.g. 200 Hz)
Render loop at vsync cadence (likely 60 Hz)

Key point: don’t make renderer RT. If you later run PREEMPT_RT, keep RT priorities for sensor/control only.

// pseudo uint64_t now_ns(); std::atomic<float> roll_deg; std::atomic<uint64_t> imu_ts_ns; // timestamp of latest IMU sample // Sensor thread uint64_t t_prev = now_ns(); while (running) { uint64_t t0 = now_ns(); float roll = read_bmi160_roll(); // compute from accel/gyro fusion or accel-only roll_deg.store(roll, std::memory_order_relaxed); imu_ts_ns.store(t0, std::memory_order_relaxed); uint64_t t1 = now_ns(); uint64_t dt = t1 - t_prev; t_prev = t1; log_sensor(dt, roll); // or push into ring buffer for logger thread sleep_until(t0 + 5ms); // for 200 Hz } // Render loop uint64_t f_prev = now_ns(); while (running) { uint64_t f0 = now_ns(); float roll = roll_deg.load(std::memory_order_relaxed); uint64_t imu_t = imu_ts_ns.load(std::memory_order_relaxed); draw_horizon(roll); SDL_RenderPresent(renderer); // vsync if enabled uint64_t f1 = now_ns(); uint64_t dt_frame = f1 - f_prev; f_prev = f1; uint64_t latency = f1 - imu_t; log_frame(dt_frame, latency, roll); }

If you want clean logs, use a ring buffer and a logger thread that flushes to disk (so disk I/O doesn’t distort your timing).

How to compare “normal” vs PREEMPT_RT fairly

Keep these constant:

Same CPU governor (ideally performance)
Same UI stack (SDL2 + same backend)
Same sensor rate, same filtering
Same CPU affinity (if you set it)

Suggested runs:

2 minutes idle desktop
2 minutes with background load (e.g. stress-ng --cpu 4 on non-UI cores)

Then compare:

Sensor dt stddev / max
Frame dt stddev / dropped frames (spikes > 25ms at 60Hz)
Latency distribution (present - imu_ts)

Bonus: quick kernel scheduling jitter baseline

For RT evaluation, cyclictest is the classic baseline. It measures scheduler latency directly (separate from your app). If PREEMPT_RT is helping, cyclictest will show it clearly.

What I need from you (so I can give you a concrete drop-in BMI160 + logger)

Just these two details:

BMI160 is on I2C or SPI?
Are you already getting roll/pitch from a filter, or do you want a simple accel-only roll to start?

If you answer that, I’ll give you:

a minimal BMI160 read loop (I2C or SPI),
a simple roll calculation to start,
and a CSV logger + tiny Python snippet to compute percentiles and plots.