CPM2 (MPC8260) SMC problem. Please help.

[Greg Lopp]

> On Wed, 18 Oct 2006 22:40:31 +0200
> Boris Shteinbock <boris at fabiotec.com> wrote:
>
> > Hi ppl.
> >
> > I have discovered very very odd problem with SMC2 on my 8260 board.
> > kernel version is 2.6.17.
> >
> > The board is 8260 with serial ports on SMC1 and SMC 2 (SMC1 - console)
> >
> > Now if I use SMC2 for communicating with remote device, ( some protocol)
> > I am sending packets and wait for a response.
> > However if the packet I send is SMALLER than 5 bytes, SMC goes
> > completely crazy.
> > write() doesn't return error, but the next read() hangs completely, and the
> > process moves to D state and it is impossible to kill it by any means.
> > It only happens when consequent writes of smaller than 5 bytes.
> > I tried to check the driver, but couldn't find any clues to what goes wrong.
> >
> > I tried both compatibility and platform-based modes with the same results.
> >
> > Any help on the issue would be highliy appreciated.
> >
> >
>
> I think that should be walked along the cpm_uart driver, TX part to see is sending stuff
> do not indicate any error in the CPM registers, or at least discover exact track leading to the endless loop.

HI. I'm new here, but I encounted a very similar problem late last week....

Boris, I am willing to bet that your problem lies with the return
value from cpm_uart_tx_pump().  Here's the last few lines of that
function from 2.6.17 :
693         if (uart_circ_empty(xmit)) {
694                 cpm_uart_stop_tx(port);
695                 return 0;
696         }
697
698         return 1;
699 }
Note that return value is 0 if there is no more data in the tty's
circular queue (uart_circ_empty() returns true)  This is the case when
you send a small packet.  It seems appropriate to turn off the
transmitter (cpm_uart_stop_tx()) in the general case, but the problem
is that you never turned on the transmitter in the first place.....you
called cpm_uart_tx_pump() from cpm_uart_start_tx() like so:
195         if (cpm_uart_tx_pump(port) != 0) {
196                 if (IS_SMC(pinfo)) {
197                         smcp->smc_smcm |= SMCM_TX;
198                         smcp->smc_smcmr |= SMCMR_TEN;
199                 } else {
200                         sccp->scc_sccm |= UART_SCCM_TX;
201                         pinfo->sccp->scc_gsmrl |= SCC_GSMRL_ENT;
202                 }
203         }

When you then attempt to close the port, you'll get stuck here, in
cpm_uart_shutdown():
455                 /* Wait for all the BDs marked sent */
456                 while(!cpm_uart_tx_empty(port)) {
457                         set_current_state(TASK_UNINTERRUPTIBLE);
458                         schedule_timeout(2);
459                 }
Because the transmitter was never turned on, the bd will never be
processed by the CPM and cpm_uart_tx_empty() will always return false.

Recommendation: Change the first block of code above to: "return
(count != 0);"  Now cpm_uart_tx_pump() will return true if data was
placed in a bd, instead of true when there is more data to be
processed later.  This code works for me - but there are folks on this
list who would have a better idea about possible side
effects........anyone?