Re: [Qemu-devel] [RFC v4 4/5] hw/arm/digic: add UART support

[Peter Maydell]

On 7 September 2013 06:45, Antony Pavlov <address@hidden> wrote:
> But Canon A1100 firmware can use this routine:
>  1. wait TX_RDY bit in status register;
>  2. write TX_RDY and some other (unknown behaviour) bit to status register;
>  3. write byte to TX register.
>
> IMHO the step 2 is bizarre.

Maybe the TX_RDY bit is write-1-to-clear? Some devices
use that kind of thing for a "conditions that happened that
could trigger an interrupt" (and if you're using them for
polling you can just poll the register).

> IMHO the more standart UART outc() routune is:
>    * wait for UART "transmitter is ready to send next char" flag;
>    * write char to TX register.

Yes, you're right. In either case you can see if there's
a uart via:
 * wait for TX_RDY to be set
 * clear TX_RDY
 * write byte to TX
 * immediately look at TX_RDY -- if it's set straight away
   there's almost certainly a FIFO involved; otherwise it
   would have to wait for the char to go out over serial
   before setting it again

This is kind of a sidetrack though, we can model it without a
UART for the moment.

-- PMM