Comtrol eCos - Manual (Page 772)

Writing New Devices - target
arg1
arg2
For some requests it is convenient to pass one or two additional parameters alongside the request code. For
example an ethernet device could define a multicast-all request, with
arg1 controlling whether this mode
should be enabled or disabled. Both arg1 and arg2 should be signed 32-bit integers, and their values are
interpreted only by the device-specific Tcl script.
txdata
txlen
Some I/O operations may involve sending additional data, for example an ethernet packet. Alternatively a
control operation may require many more parameters than can easily be encoded in
arg1 and arg2 , so those
parameters have to be placed in a suitable buffer and extracted at the other end. txdata is an arbitrary buffer
of txlen bytes that should be sent to the host-side. There is no specific upper bound on the number of bytes
that can be sent, but usually it is a good idea to allocate the transmit buffer statically and keep transfers down
to at most several kilobytes.
reply
If the host-side is expected to send a reply message then reply should be a pointer to an integer variable and
will be updated with a reply code, a simple 32-bit integer. The synthetic target HAL code assumes that the
host-side and target-side agree on the protocol being used: if the host-side will not send a reply to this message
then the
reply argument should be a NULL pointer; otherwise the host-side must always send a reply code
and the reply argument must be valid.
rxdata
rxlen
Some operations may involve additional data coming from the host-side, for example an incoming ethernet
packet.
rxdata should be a suitably-sized buffer, and rxlen a pointer to an integer variable that will end up
containing the number of bytes that were actually received. These arguments will only be used if the host-side
is expected to send a reply and hence the
reply argument was not NULL.
max_rxlen
If a reply to this message is expected and that reply may involve additional data, max_rxlen limits the size of
that reply. In other words, it corresponds to the size of the rxdata buffer.
Most I/O operations involve only some of the arguments. For example transmitting an ethernet packet would use the
request , txdata and txlen fields (in addition to device_id which is always required), but would not involve
arg1 or arg2 and no reply would be expected. Receiving an ethernet packet would involve request , rxdata ,
rxlen and max_rxlen ; in addition reply is needed to get any reply from the host-side at all, and could be used
to indicate whether or not any more packets are buffered up. A control operation such as enabling multicast mode
would involve
request and arg1 , but none of the remaining arguments.
Interrupt Handling
Interrupt handling in the synthetic target is much the same as on a real target. An interrupt object is created using
cyg_drv_interrupt_create , attached, and unmasked. The emulated device - in other words the Tcl script run-
ning inside the I/O auxiliary - can raise an interrupt. Subject to interrupts being disabled and the appropriate vector
being masked, the system will invoke the specified ISR function. The synthetic target HAL implementation does
668