[U-Boot] U-boot support for Non Console board

[Jerry Van Baren]

Wolfgang Denk wrote:
> Dear rajeev s,
> 
> In message <843260.87725.qm at web50008.mail.re2.yahoo.com> you wrote:
>> We have a custom board based on coldfire (MCF5484) Similar to MCF5484 Kitlite
>> The board runs Coldfire as PCI agent , 64MB of SDRAM, 4 MB of Bootflash and the PCI bus as a slave (no serial
>> port->no console) 
>> I can flash U-boot using the JTAG . Can you let know how can i pass the Commands over PCI .
>> I do not have a Console to do that ?
> 
> Well, somebody who designed this system must also have had some plan
> how to operate the board?
> 
> Not having a console port sounds like a really stupid thing to me.
> 
> If you have ethenret on your board, you can try netconsole. But it
> will be no fun to port U-Boot to such hardware.
> 
> I suggest you bring the board back to the hardware guys and ask for a
> serial console port, at least for bring-up and debugging.
> 
> Best regards,
> 
> Wolfgang Denk

On the u-boot side, you can write a "fake uart" that...
TX: Takes bytes and puts them in a "TX" queue in shared PCI memory.
RX: Looks at the "RX" queue in shared PCI memory for new characters.

Then on the host side, you write a custom fake uart driver to do the 
opposite (taking advantage of the OS's capabilities and support 
programs) or write a custom terminal program that implements the fake 
uart handling directly (simpler to get running, much less flexible and 
more maintenance long term).

The queue can be a simple /n/ byte array (I would set /n/ to 256 or some 
larger power of 2 since size doesn't matter ;-) with head and tail 
indexes.  The writer puts a byte in queue[head++] and the reader checks 
"if (head != tail) return queue[tail++]".  Note that there is only one 
writer to "head" and one to "tail" so you don't have any race 
condition/locking issues (assuming your r/w access is atomic and you 
don't code any bugs into your algorithm).

Details glossed over:
* head and tail must be incremented modulo the queue length
* You *NEVER* want to set head or tail to be equal to the queue length
     in the shared memory (that would be out-of-bounds on the array).
     "head = (head + 1) % sizeof(queue);" should be save (check the
     assembly code, compiled with optimization!).  If you use
     "head = ++head % sizeof(queue);", the compiler will write head++
     with with wrong value 256 every 256 bytes (see the next bullet)
     and then do the modulo and re-write head with the correct value (0).
* You need to make all the shared variables "volatile"
* You need to prevent head from overtaking tail.  For instance, if
     (head + 1) == tail (check *before* writing the byte to the queue),
     you have an overflow condition and will have to drop the byte.

I have not googled, I'm sure many people have done this before.  Maybe 
you will get lucky and someone will have published source under a GPL 
(compatible) license.

Best regards,
gvb