multiple i2c_open() calls for same I2C port/address returns valid handles - should be single valid handle for each I2C Port/Address

[PSkeberdis]

Expecting that the first i2c_open for s specific port/address would provide a valid handle, but that a second i2c_open for the same port/address without a close would not provide a valid handle.

First instance of program run well, then second instance stomps the first instance as it also receives a valid handle.

Am testing on Pi4B+ running Ubuntu 21.04: Distributor ID: Ubuntu Description: Ubuntu 21.04 Release: 21.04 Codename: hirsute

Code snippet from lcd_i2c.py: ... import lgpio import time from RPLCD.i2c import CharLCD

from datetime import datetime

address = 0x27

cols = 20 rows = 4

lcd = CharLCD(i2c_expander='PCF8574', address=address, port=1, cols=cols, rows=rows, charmap='A02', # A00, A02 or ST0B backlight_enabled=True) ...

Fix would be to only issue a valid handle for I2C Port/address if a current handle for the same I2C Port/Address did not exist.

[joan2937]

That is by design. If the underlying operating system is happy to have the same device opened multiple times then so is lg.

[PSkeberdis]

I don’t agree with your design philosophy in that regard. The device driver is part of the OS and should be responsible for deconflicting device access.

Here’s a reference for Linux device drivers from https://www.linuxjournal.com/article/2476:

Do I Really Need to Write a Driver?

There are several reasons for writing our own device driver:

• To solve concurrency problems when two or more processes try to access a device at the same time.

• To use hardware interrupts: as the kernel runs in protected mode, the user cannot manage interrupts directly from a program.

• To handle other unusual applications, such as managing a virtual device (a RAM disk or a device simulator).

• To obtain satisfaction as a programmer: writing a driver increases personal motivation as well as control over the computer.

• To learn about the internal parts of the system.

From: joan2937 @.> Sent: Thursday, August 05, 2021 3:30 AM To: joan2937/lg @.> Cc: PSkeberdis @.>; Author @.> Subject: Re: [joan2937/lg] multiple i2c_open() calls for same I2C port/address returns valid handles - should be single valid handle for each I2C Port/Address (#10)

That is by design. If the underlying operating system is happy to have the same device opened multiple times then so is lg.

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