Code:

uchar buffer[256];
uchar usbFunctionRead(uchar *data, uchar len)
{
   uchar *r = usbMsgPtr;    //compressed data ptr in flash

                if(firstrun) mydepack(r,buffer);     //decompress needs about 5ms execute time
         
   memcpy(data,buffer+pos,len);
                pos+=len;
   return len;
}

Code:

uchar buffer[256];
uchar usbFunctionRead(uchar *data, uchar len)
{
   uchar *r = usbMsgPtr;    //uncompressed data ptr in flash

                if(firstrun){
                memcpy_P(buffer,r,256); 
                _delay_ms(5);              //to ensure it's not some timing problem.
                }

   memcpy(data,buffer+pos,len);
                pos+=len;
   return len;
}