CPM_UART: tcdrain() is broken

[Vitaly Bordug]

On Tue, 14 Nov 2006 09:28:00 +0100
Laurent Pinchart <laurent.pinchart at tbox.biz> wrote:

> Hi Dan,
> 
> > > There is, however, a problem left. The SCC controllers have a 32-
> > > byte FIFO between the buffer descriptors and the wires.
> > > cpm_uart_tx_empty() checks if all buffer descriptors have been processed,
> > > but doesn't check if the transmit FIFO is actually empty.
> >
> > There is no way to determine if the hardware FIFO is empty.
> > It's buried under the CPM microcode.
> 
> I was hoping the information would be somewhere in the parameter RAM :-(
> 
> > > .....  I can't figure out how to do so from the datasheet.
> > > Does anyone have an idea ?
> >
> > The original versions of the SCC UART driver used to have a
> > timeout based on the maximum depth of this FIFO and baud rate.
> > After the last BD was empty, it would further wait this amount of
> > time.  Lots of these little details seem to have been lost over
> > the years of making a "better" driver :-)
> 
> That's a solution, but we might wait longer than necessary then. Vitaly, 
> what's your opinion ? We should at least set fifosize to a proper value (see 
> my previous mail). I'll submit a patch after we agree on the need for an 
> additional delay.
> 

Well, I also bothered Dan with the same question some time ago, getting the same answer :)
I think here we aught to have this delay ported, but under Kconfig option, so that those who are happy with current being, do not suffer from additional delays.

Patch appreciated :)
-- 
Sincerely, 
Vitaly