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Hello! Thank you for your detailed description of the project and your clear explanation of the performance challenge.
GPIO access speed on user space (libgpiod):
The main performance bottleneck is generally not in pin or drive strength configuration, but in the inherent latency of user space GPIO frameworks like libgpiod. Each operation (setting address, toggling control lines, reading data) incurs overhead due to:
- User-to-kernel context switches
- Generic, non-optimized access to GPIO controller hardware
On typical Linux platforms (like Verdin iMX8MP), user space GPIO access is slower than direct register access, which limits the rate at which you can use GPIOs for parallel interfaces.
Device Tree Tuning and Its Effects
1. Pad Control Register (Slew Rate, Drive Strength)
- These settings mainly affect signal integrity and electrical characteristics, not the fundamental CPU/software speed of GPIO toggling.
- Fast slew rate and higher drive strength can slightly decrease pin rise/fall times and thus can impact signaling at very high speeds, but for standard GPIO toggling via software, the improvement is marginal compared to software overhead.
- Your register value
0x00000146 can be decoded:
- Bits
0x6 (DSE=6): Indicates a certain drive strength.
- Bit
0x40 (FSE): Fast slew rate enabled.
- These are reasonable electrical settings for fast switching; details here.
2. SION Setting
- If you need to read back data lines, make sure SION (Software Input On) is set if required, but again, this will not increase software-controlled toggling speed.
3. Conflicts/Pinmux
Approaches to Significant Speed Improvement
A. Move to Kernel Space Driver or UIO
- Custom kernel module: By implementing the interface logic in the kernel, you eliminate user-kernel context switching, resulting in orders-of-magnitude faster GPIO access.
- UIO (User-space I/O) or direct memory-mapped GPIO: If your board supports this securely, you can map the GPIO controller registers into user space for direct, faster access.
B. Use a Parallel Bus Peripheral/Interface
- For high-speed data, the Linux GPIO framework is not designed for bus interfaces. Consider using an FPGA, or using an SoC peripheral more suited for parallel data, like an EIM/LCD bus or FlexIO (if available).
C. Hardware Electrical Optimization (as secondary consideration)
- You already have fast slew and high drive.
- Ensure signal traces are short and well-terminated to minimize propagation delay (but this rarely is the bottleneck for software-based GPIO toggling).
Summary Table
| Optimization |
Expected Impact |
How to Apply |
| User-space pinctrl tweak |
Minor |
Confirm fast slew/high drive (done) |
| Kernel-space driver |
Major |
Write kernel module doing read cycles |
| Direct reg. access |
Major |
Possibly via /dev/mem or UIO (if safe/legal in your use case) |
| Use bus peripheral |
Very major |
Re-architect HW/SW to use suitable parallel interface |
Device Tree Example
Your configuration is reasonable:
fsl,pins = <MX8MM_IOMUXC_GPIO1_IO01_GPIO1_IO1 0x146>;
More about pad control fields.
But as stated, this alone will not get you the speed-up you need.
References
Recommendation: To dramatically increase throughput, move your DPRAM control logic from user space to a custom kernel module, or pursue direct hardware (memory-mapped) access to GPIO registers, if possible.
If you need more details on writing a kernel driver or further tuning, please let me know your software requirements.
Best regards,
Toradex AI Assistant.
