[beagleboard] BeagleBone Black GPIO access via mmap is slow

[Andrew P. Lentvorski]

Hi, folks,

I've been poking around to get GPIO to work via /dev/mem and mmap.  After 
finally pushing a lot of things around, I finally managed to pull the 
various flavors of tutorials together into something I now understand 
(mostly).

However, what I *don't* understand is that when I put my scope on the pin, 
I see it toggle at 2.95MHz.  That seems slow.  This is the same frequency 
as here: http://chiragnagpal.com/examples.html

The question is: Why?

I could understand if it was something like 30MHz, but 3MHz seems off by an 
order of magnitude.  And when something seems off in embedded programming, 
it's usually a bug waiting to bite you when you least need it.

I'm on a BeagleBoard Black with Debian Jessie Linux arm 4.1.6-ti-r11 #1 SMP 
PREEMPT Tue Aug 18 21:36:11 UTC 2015 armv7l GNU/Linux

Any suggestions?  Given that the assembly seems to be spot on, *something* 
appears to be stalling the pipeline.

Is the core being throttled?  Is the peripheral bus at some strange 
frequency?  Is the memory mapping hardware inserting cycles?  I'm 
completely at a loss.

Thanks for any advice.


The assembly looks exactly optimal with a ldr/str sequence at maximum 
density (I'm using clang):

.loc 2 83 9                  @ gpi.c:83:9
ldr r1, [sp, #32]
str r0, [r1]
.loc 2 84 2                  @ gpi.c:84:2
ldr r1, [sp, #28]
str r0, [r1]
.loc 2 85 9                  @ gpi.c:85:9
ldr r1, [sp, #32]
str r0, [r1]
.loc 2 86 2                  @ gpi.c:86:2
ldr r1, [sp, #28]
str r0, [r1]



And here's the C code I used to generate the pulse train on P9 Pin23 
(GPIO1[17])



#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h> 

#include <unistd.h>
#include <assert.h>

#define GPIO_OE 0x134
#define GPIO_SETDATAOUT 0x194
#define GPIO_CLEARDATAOUT 0x190


// Hunt these addresses down from ls -al /sys/devices/platform/ocp | grep 
gpio
// You can also pull them from the TI manual (spruh73l.pdf)
#define GPIO0_BASE 0x44E07000
#define GPIO1_BASE 0x4804C000
#define GPIO2_BASE 0x481AC000
#define GPIO3_BASE 0x481AE000

#define GPIO_SIZE  0x00001000


#define PIN_17 ((uint32_t)1<<17)


uint32_t ui32Base[] = {GPIO0_BASE, GPIO1_BASE, GPIO2_BASE, GPIO3_BASE};
uint8_t volatile * bbGPIOMap[] = {0, 0, 0, 0};


int main(int argc, char *argv[])
{
    unsigned int ui;
    
    uint32_t volatile * gpio_oe_addr = NULL;
    uint32_t volatile * gpio_setdataout_addr = NULL;
    uint32_t volatile * gpio_cleardataout_addr = NULL;

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

    for(ui=0; ui<4; ++ui) {
bbGPIOMap[ui] = mmap(0, GPIO_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 
ui32Base[ui]);
assert(bbGPIOMap[ui] != MAP_FAILED);
    }

    
    gpio_oe_addr = (uint32_t volatile *)(bbGPIOMap[1] + GPIO_OE);
    gpio_setdataout_addr = (uint32_t volatile *)(bbGPIOMap[1] + 
GPIO_SETDATAOUT);
    gpio_cleardataout_addr = (uint32_t volatile *)(bbGPIOMap[1] + 
GPIO_CLEARDATAOUT);



    *gpio_oe_addr = *gpio_oe_addr & ~PIN_17;
    
    while(1) {
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
        *gpio_setdataout_addr = PIN_17;
*gpio_cleardataout_addr = PIN_17;
    }
    
    close(fd);
    return 0;
}



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