Ok. Using of c_ispeed equal to 0 was not a good idea, as it has
already a special meaning (the same i_speed and o_speed).
Also use of other arbitary taken value is not good, as it may be not
supported by other tools.
Therefore I've finally decided to steal one of standard values -
B460800.

Unfortunately this value is not passed to the atmel_set_termios
function - instead I get the baud value itself.
So the user program should pass B460800 in the termios structure,
while my version of atmel_set_termios should detect
the integer value "460800".

Hi,

I need to use the USART in AT91SAM9260 working under Linux OS (quite
old 2.6.29.3 version, due to compatibility with some parts of the
system) to communicate with peripheral sending data in synchronous
mode with clock 6MHz. The peripheral provides clock.

The standard atmel_serial.c driver does not seem to support operation
with external clock neither the synchronous mode. (In fact it is still
valid for the newest version: http://lxr.linux.no/linux+v3.0.4/drivers/tty/serial/atmel_serial.c#L1106
).

As the driver is quite complicated (especially considering the fact,
that I need to use DMA to leave CPU power for data processing), I'd
rather  prefer modifying of the atmel_serial.c, than writing my own
driver.

My first idea is to simply modify the  atmel_set_termios function
( http://lxr.linux.no/linux+v2.6.29.3/drivers/serial/atmel_serial.c#L1015
).
E.g. I can use the special value of c_ispeed, equal to zero to switch
the synchronous mode with external clock.
Below is the proposed modification, original code starts here:
http://lxr.linux.no/linux+v2.6.29.3/drivers/serial/atmel_serial.c#L1022

    /* Get current mode register */
       mode = UART_GET_MR(port) & ~(ATMEL_US_USCLKS | ATMEL_US_CHRL
                                        | ATMEL_US_NBSTOP |
ATMEL_US_PAR | ATMEL_US_SYNC );
       /* Added clearing of ATMEL_US_SYNC, not present before, as this
bit was never set
         in the original driver */
      if(termios->c_ispeed) {
        /* original code */
         baud = uart_get_baud_rate(port, termios, old, 0, port-
>uartclk / 16);
         quot = uart_get_divisor(port, baud);
         if (quot > 65535) {     /* BRGR is 16-bit, so switch to
slower clock */
                 quot /= 8;
                 mode |= ATMEL_US_USCLKS_MCK_DIV8;
         }
      } else { /* my code, executed when c_ispeed==0 */
         baud = 6000000 ; /* known in advance, set e.g. for
calculation of timeouts */
                                   /* may be it would be wise to allow
setting of baud via c_ospeed? */
         mode |= ATMEL_US_USCLKS_SCK | ATMEL_US_SYNC ;
      }

I'd appreciate any suggestions regarding the proposed solution.
Is it safe to use c_ispeed==0 as a trigger for setting of synchronous
mode?
How should I manage the SCK pin? Is it enough to set its function in
direction
in the board file (arch/arm/mach-at91/board-my-board.c )?
--
Regards,
Wojtek