ARM linux userspace gpio operations using mmap /dev/mem approach (able to write to GPIO registers, but fail to read from them)

Kernel version 3.12.30-AM335x-PD15.1.1 by PHYTEC.

If I use the /sys/class/gpio way, I can see that the button input pin (gpio103 of AM3359) value changes from 0 to 1.

Following the this exercise http://elinux.org/EBC_Exercise_11b_gpio_via_mmap and executing the below command for reading gpio pins usig /dev/mem approach:

`devmem2 0x481ae13c`

(base of gpio bank 3 which is 0x481ae000 + 0x13c dataout offset)

I get the below output regardless of button position.

/dev/mem opened
Memory mapped at address 0xb6fd1000.
Read at address  0x481AE13C (0xb6fd113c): 0x00000000

Also with the c program given below I managed to toggle gpios using /dev/mem; however I could not read them succesfuly.

in the header file:

#define GPIO0_START_ADDRESS (0x44E07000)
#define GPIO1_START_ADDRESS (0x4804C000)
#define GPIO2_START_ADDRESS (0x481AC000)
#define GPIO3_START_ADDRESS (0x481AE000)

#define GPIO0_END_ADDRESS   (0x44e09000)
#define GPIO1_END_ADDRESS   (0x4804E000)
#define GPIO2_END_ADDRESS   (0x481AE000)
#define GPIO3_END_ADDRESS   (0x481B0000)

#define GPIO0_SIZE (GPIO0_END_ADDRESS - GPIO0_START_ADDRESS)
#define GPIO1_SIZE (GPIO1_END_ADDRESS - GPIO1_START_ADDRESS)
#define GPIO2_SIZE (GPIO2_END_ADDRESS - GPIO2_START_ADDRESS)
#define GPIO3_SIZE (GPIO3_END_ADDRESS - GPIO3_START_ADDRESS)

#define GPIO_SETDATAOUT (0x194)
#define GPIO_CLEARDATAOUT (0x190)
#define GPIO_DATAOUT (0x13c)

somewhere in the program:

    int fd = open("/dev/mem", O_RDWR);
    //int fd = open("/dev/mem", O_RDWR|O_SYNC);

    gpio0_address = mmap(0, GPIO0_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO0_START_ADDRESS);
    gpio1_address = mmap(0, GPIO1_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO1_START_ADDRESS);
    gpio2_address = mmap(0, GPIO2_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO2_START_ADDRESS);
    gpio3_address = mmap(0, GPIO3_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO3_START_ADDRESS);

    if(gpio0_address == MAP_FAILED){ printf("unable to map GPIO0 bank.n"); }
    if(gpio1_address == MAP_FAILED){ printf("unable to map GPIO1 bank.n"); }
    if(gpio2_address == MAP_FAILED){ printf("unable to map GPIO2 bank.n"); }
    if(gpio3_address == MAP_FAILED){ printf("unable to map GPIO3 bank.n"); }


    gpio0_dataout_address = gpio0_address + GPIO_DATAOUT;

    gpio0_setdataout_address = gpio0_address + GPIO_SETDATAOUT;

    gpio0_cleardataout_address = gpio0_address + GPIO_CLEARDATAOUT;

    gpio1_dataout_address = gpio1_address + GPIO_DATAOUT;

    gpio1_setdataout_address = gpio1_address + GPIO_SETDATAOUT;

    gpio1_cleardataout_address = gpio1_address + GPIO_CLEARDATAOUT;

    gpio2_dataout_address = gpio2_address + GPIO_DATAOUT;

    gpio2_setdataout_address = gpio2_address + GPIO_SETDATAOUT;

    gpio2_cleardataout_address = gpio2_address + GPIO_CLEARDATAOUT;

    gpio3_dataout_address = gpio3_address + GPIO_DATAOUT;

    gpio3_setdataout_address = gpio3_address + GPIO_SETDATAOUT;

    gpio3_cleardataout_address = gpio3_address + GPIO_CLEARDATAOUT;
    read_GPIO_bank_memory(3, &GPIO3_bank_memory);

read function:

   void read_GPIO_bank_memory(int bank, four_bytes* gpio_bank_dataout)
    {
        switch(bank)
        {
            case 0:

                gpio_bank_dataout->ALL = *gpio0_dataout_address;
                break;
            case 1:

                gpio_bank_dataout->ALL = *gpio1_dataout_address;

                break;
            case 2:

                gpio_bank_dataout->ALL = *gpio2_dataout_address;

                break;
            case 3:

                gpio_bank_dataout->ALL = *gpio3_dataout_address;

                break;
        }
    }

Answer

You should use GPIO_DATAIN Register (offset = 138h) to read from GPIO pin, not GPIO_DATAOUT.

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Answer