Trjtdtk

What are advantages and disadvantages in accessing I2C and other interfaces directly with /dev/mem (as in this code) instead of using /dev/i2c-1 (as in most examples like this)?

Disadvantages using /dev/mem for accessing the I²C hardware of the Raspberry Pi directly:

• It's not portable to other hardware platforms. It's not portable to additional USB I²C adapters. It's not portable to additional bitbanged I²C.


• You limit yourself to one-application at a time which may use the I²C, while the kernel serializes requests on /dev/i2c-nnn from multiple applications.


• Likewise, you will run into problems when the user wants to use a kernel driver on the same I²C, for example for the handling of an RTC.


• You have to take care of I²C bus expanders in your application code.

Advantages:

• ~--~--~--~ insert tumbleweed ~--~--~--~

In addition, I recommend to use the I2C_RDWR ioctl() instead of read() and write() as you can do multiple transfers in one transaction this way. This is neccessary if multiple applications are accessing the same I²C slave concurrently.

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <inttypes.h>
#include <linux/i2c-dev.h>

int main(int argc, char* argv[]) address > 0x77) 
 fprintf(stderr, "ds7505-readtemp: i2c slave address has to be in the range 0x08..0x77.n");
 return 127;
 

 /* Open I2C device. */
 fd = open(argv[1], O_RDWR);
 if (fd < 0) 
 perror("ds7505-readtemp: i2c device open");
 return 2;
 

 /* Select temperature register. */
 i2c_msgs[0].addr = address;
 i2c_msgs[0].flags = 0;
 i2c_msgs[0].len = 1;
 i2c_msgs[0].buf = "x00";

 i2c_msgs[1].addr = address;
 i2c_msgs[1].flags = I2C_M_RD;
 i2c_msgs[1].len = 2;
 i2c_msgs[1].buf = (char*)&rdata;

 i2c_transfer.msgs = i2c_msgs;
 i2c_transfer.nmsgs = 2;

 if (ioctl(fd, I2C_RDWR, &i2c_transfer) < 0) 
 perror("ds7505-readtemp: i2c temperature read");
 return 2;
 ;

 /* Print result. */
 printf("%.4fn", ((float)((int16_t) (rdata[0] << 8 

In addition to @Janka's answer, more disadvantages:

• No useful interrupt or DMA processing with /dev/mem


• Whatever utility has the permission to interact with /dev/mem, can pretty much have complete control over the system. /dev/i2c would allow access only to I2C bus.

Advantage of /dev/mem is that it can be used for very quick proof of concept, or as help for debugging. I'd advise against using it for anything else.