patch "tty: serial: OMAP: ensure FIFO levels are set correctly in non-DMA" added to tty tree

[Russell King - ARM Linux]

On Fri, Feb 03, 2012 at 11:07:20PM +1100, NeilBrown wrote:
> However what it really shows me is that I was misunderstanding the code
> (If I spent the time to verify every conclusion that I jumped to, I'd never
> get anywhere :-( ).  Better clarify that now.
> 
> So this function - serial_omap_tx_empty() is called to test if the
> tx queue is empty.

Not just the queue.  The documentation for that function is accurate:

  tx_empty(port)
        This function tests whether the transmitter fifo and shifter
        for the port described by 'port' is empty.  If it is empty,
        this function should return TIOCSER_TEMT, otherwise return 0.
        If the port does not support this operation, then it should
        return TIOCSER_TEMT.

So, if this is returning TIOCSER_TEMT while there's still a transmission
in progress, then it's buggy.

> So when I
> 
>    cat somefile
> 
> to the serial console, most of the file comes out fine.  But after 'cat'
> finishes and returns to the shell - while some chars are still in the FIFO - 
> the shell does an 'stty' ioctl to make sure the settings are still OK.
> This ioctl calls serial_omap_tx_empty which calls pm_resume_put() which
> immediately suspends the uart, which seems to stop some clock - even though
> we think it shouldn't.

If there's an on-going transmission, why is the runtime PM count zero,
meaning that the UART can sleep at the point where serial_omap_tx_empty()
is being called - and obviously there's still characters in the FIFO?

I guess this is highlighting a problem with doing runtime PM with
serial ports: you don't know when the port has _actually_ finished
sending its final character, which is even more of a problem if the
port does hardware CTS flow control itself.

It seems that runtime PM should be checking whether the TX FIFO is empty
before shutting the port down - and that probably means a hook into the
idle stuff.

Note though, that serial_omap_tx_empty() can be called via ioctl from
userspace, so this function would still need the runtime callbacks in
it so that the register is readable at _any_ time that the port is open.