index

Catalog

I2C low occurrence write/read mismatch due to NOT syncing well in ISR
global char var definition causes reset vector in stack section not aligned by 4

I2C low occurrence write/read mismatch due to NOT syncing well in ISR

1. Find some error while getting input status when BG2 communicating with KG2H.

(Status get from KG2H is not same as real situation, though at a very low occurence.)

2. Try aggressive write/read to debugging I2C. At a rate of about 1/25600, in byte.
3. Use schedtool to debug, find that when app runs on CPU0, never encounter error, CPU1 does.
4. At last, find the bug in I2C driver:

// check if this is the last TXE interrupt for this transaction
if (!bytes_in_fifo) {
	if (bytes_written_to_txfifo[master_id] == bytes_of_write_cmd[master_id]+bytes_of_read_cmd[master_id]) {
		// disable TXE interrupt
		i2c_mask_int(master_id, INTR_TX_EMPTY, MASK);
		// transaction finished
		transaction_done[master_id] = 1;
#ifdef __LINUX_KERNEL__
		if (bytes_of_write_cmd[master_id])
			wake_up_interruptible(&i2c_writeread_wait);
#endif

		return;
	}
}

----------- Original ----------

// check if this is the last TXE interrupt for this transaction
if (!bytes_in_fifo) {
	if (bytes_written_to_txfifo[master_id] == bytes_of_write_cmd[master_id]+bytes_of_read_cmd[master_id]) {
		// disable TXE interrupt
		i2c_mask_int(master_id, INTR_TX_EMPTY, MASK);
		if (bytes_of_write_cmd[master_id]) {
			// transaction finished
			transaction_done[master_id] = 1;
#ifdef __LINUX_KERNEL__
			wake_up_interruptible(&i2c_writeread_wait);
#endif
		}
		return;
	}
}

---------- Fix ----------
5. As we can see above, the bug hide up in the if condition. That is, we can wake up waiting thread only when set transaction_done flag to 1.
As a conclude, we need to ensure each flag and behavior who are bind together can only be done in the same condition.

global char var definition causes reset vector in stack section not aligned by 4

declare a var as global:

unsigned char xxx;

This results in the whole system would not run even 1 line of code, since the stack section not aligned by 4
old linker script

OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(__start)

SECTIONS
{
	. = 0x00000000;
	. = ALIGN(4);
	.text	:
	{
	  *(.start)
	  *(.text)
	}

	.rodata : { *(.rodata) }
	. = ALIGN(4);
	.data : { *(.data) }
	. = ALIGN(4);
	.got : { *(.got) }
	__end = .;

	. = 0x04000000;
	__bss_start = .;
	.bss : { *(.bss) }
	__bss_end = .;

	.stack (NOLOAD)	: { *(.stack) }
}

correct linker script

OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(__start)

SECTIONS
{
	. = 0x00000000;
	. = ALIGN(4);
	.text	:
	{
	  *(.start)
	  *(.text)
	}

	.rodata : { *(.rodata) }
	. = ALIGN(4);
	.data : { *(.data) }
	. = ALIGN(4);
	.got : { *(.got) }
	__end = .;

	. = 0x04000000;
	. = ALIGN(4);
	__bss_start = .;
	.bss : { *(.bss) }
	__bss_end = .;

	. = ALIGN(4);
	.stack (NOLOAD)	: { *(.stack) }
}