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Data misalignment in USB Stack code on the iMX6SL (from Baremetal SDK 1.1)

Question asked by Nathan Palmer on Jul 21, 2014
Latest reply on Nov 5, 2014 by Rob_iMX6

I am trying to run the example USB Stack code from the Baremetal SDK on the iMX6SL. I had hoped to use the CDC stack in an application but I cannot get the example code to run:



I cannot get this code to run, does the usb_stack work with the iMX6SL?  If not, is there any example code for the iMX6SL?  I see the the sdk/drivers/usb/src/ files, but that code is not setup to handle an interrupt driven flow and it also is very specific the the HID class of devices. I has hoping to quickly get a CDC class going.


There is a packed structure that is defined in usb_devapi.h:


typedef struct _USB_EP_STRUCT
    uint_8          ep_num;      /* endpoint number         */
    uint_8          type;        /* type of endpoint        */
    uint_8          direction;   /* direction of endpoint   */
    USB_PACKET_SIZE    size __attribute__ ((packed))  ;  /* buffer size of endpoint */
} __attribute__ ((packed))  USB_EP_STRUCT, *USB_EP_STRUCT_PTR;


That is causing bad code to be generated.  Using ARM gcc 4.8.3 creates code in several places that tries to access the 'size' attribute (which is a 16-bit halfword) and causes a misaligned data abort on the iMX6SL


For example, in the reset handler:

usb_class.c:     void USB_Reset_Service (...)



usb_driver.c:     uint_8 _usb_device_init_endpoint (...)


   │0x80086c48 <_usb_device_init_endpoint+88>       ldrb   r3, [r11, #-25]
   │0x80086c4c <_usb_device_init_endpoint+92>       strb   r3, [r11, #-20]  
>│0x80086c50 <_usb_device_init_endpoint+96>       ldrh   r3, [r11, #-28] ;
   │0x80086c54 <_usb_device_init_endpoint+100>      strh   r3, [r11, #-17] ; <------------- This is a half-word instruction on a byte aligned data address: 0x82272697



I have tried to call the following, but I still get the error.

//! @brief Disable aborts on unaligned accesses.
void disable_strict_align_check(void);