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if(( devq = cam_simq_alloc(SIZE) )==NULL) {
error; /* some code to handle the error */
Here `SIZE` is the size of the queue to be allocated, maximal number of requests it could contain. It is the number of requests that the SIM driver can handle in parallel on one SCSI card. Commonly it can be calculated as:
Next we create a descriptor of our SIM:
struct cam_sim *sim;
if(( sim = cam_sim_alloc(action_func, poll_func, driver_name,
softc, unit, mtx, max_dev_transactions,
max_tagged_dev_transactions, devq) )==NULL) {
error; /* some code to handle the error */
Note that if we are not able to create a SIM descriptor we free the `devq` also because we can do nothing else with it and we want to conserve memory.
If a SCSI card has multiple SCSI buses on it then each bus requires its own `cam_sim` structure.
An interesting question is what to do if a SCSI card has more than one SCSI bus, do we need one `devq` structure per card or per SCSI bus? The answer given in the comments to the CAM code is: either way, as the driver's author prefers.
The arguments are:
`action_func` - pointer to the driver's `xxx_action` function.
static void
`poll_func` - pointer to the driver's `xxx_poll()`
static void
driver_name - the name of the actual driver, such as "ncr" or "wds".
`softc` - pointer to the driver's internal descriptor for this SCSI card. This pointer will be used by the driver in future to get private data.
unit - the controller unit number, for example for controller "mps0" this number will be 0
mtx - Lock associated with this SIM. For SIMs that don't know about locking, pass in Giant. For SIMs that do, pass in the lock used to guard this SIM's data structures. This lock will be held when xxx_action and xxx_poll are called.
max_dev_transactions - maximal number of simultaneous transactions per SCSI target in the non-tagged mode. This value will be almost universally equal to 1, with possible exceptions only for the non-SCSI cards. Also the drivers that hope to take advantage by preparing one transaction while another one is executed may set it to 2 but this does not seem to be worth the complexity.
max_tagged_dev_transactions - the same thing, but in the tagged mode. Tags are the SCSI way to initiate multiple transactions on a device: each transaction is assigned a unique tag and the transaction is sent to the device. When the device completes some transaction it sends back the result together with the tag so that the SCSI adapter (and the driver) can tell which transaction was completed. This argument is also known as the maximal tag depth. It depends on the abilities of the SCSI adapter.